Commit e6b80a3b (rcu: Detect illegal rcu dereference in extended
quiescent state) exported the pre-existing rcu_is_cpu_idle() function
using EXPORT_SYMBOL().  However, this is inconsistent with the remaining
exports from RCU, which are all EXPORT_SYMBOL_GPL().  The current state
of affairs means that a non-GPL module could use rcu_is_cpu_idle(),
but in a CONFIG_TREE_PREEMPT_RCU=y kernel would be unable to invoke
rcu_read_lock() and rcu_read_unlock().

This commit therefore makes rcu_is_cpu_idle()'s export be consistent
with the rest of RCU, namely EXPORT_SYMBOL_GPL().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

There is currently no way for kernel code to determine whether it
is safe to enter an RCU read-side critical section, in other words,
whether or not RCU is paying attention to the currently running CPU.
Given the large and increasing quantity of code shared by the idle loop
and non-idle code, the this shortcoming is becoming increasingly painful.

This commit therefore adds __rcu_is_watching(), which returns true if
it is safe to enter an RCU read-side critical section on the currently
running CPU.  This function is quite fast, using only a __this_cpu_read().
However, the caller must disable preemption.
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Because RCU's quiescent-state-forcing mechanism is used to drive the
full-system-idle state machine, and because this mechanism is executed
by RCU's grace-period kthreads, this commit forces these kthreads to
run on the timekeeping CPU (tick_do_timer_cpu).  To do otherwise would
mean that the RCU grace-period kthreads would force the system into
non-idle state every time they drove the state machine, which would
be just a bit on the futile side.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit adds the state machine that takes the per-CPU idle data
as input and produces a full-system-idle indication as output.  This
state machine is driven out of RCU's quiescent-state-forcing
mechanism, which invokes rcu_sysidle_check_cpu() to collect per-CPU
idle state and then rcu_sysidle_report() to drive the state machine.

The full-system-idle state is sampled using rcu_sys_is_idle(), which
also drives the state machine if RCU is idle (and does so by forcing
RCU to become non-idle).  This function returns true if all but the
timekeeping CPU (tick_do_timer_cpu) are idle and have been idle long
enough to avoid memory contention on the full_sysidle_state state
variable.  The rcu_sysidle_force_exit() may be called externally
to reset the state machine back into non-idle state.

For large systems the state machine is driven out of RCU's
force-quiescent-state logic, which provides good scalability at the price
of millisecond-scale latencies on the transition to full-system-idle
state.  This is not so good for battery-powered systems, which are usually
small enough that they don't need to care about scalability, but which
do care deeply about energy efficiency.  Small systems therefore drive
the state machine directly out of the idle-entry code.  The number of
CPUs in a "small" system is defined by a new NO_HZ_FULL_SYSIDLE_SMALL
Kconfig parameter, which defaults to 8.  Note that this is a build-time
definition.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
[ paulmck: Use true and false for boolean constants per Lai Jiangshan. ]
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
[ paulmck: Simplify logic and provide better comments for memory barriers,
  based on review comments and questions by Lai Jiangshan. ]

This commit drops an unneeded ACCESS_ONCE() and simplifies an "our work
is done" check in _rcu_barrier().  This applies feedback from Linus
(https://lkml.org/lkml/2013/7/26/777) that he gave to similar code
in an unrelated patch.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
[ paulmck: Fix comment to match code, reported by Lai Jiangshan. ]

This commit adds an isidle and jiffies argument to force_qs_rnp(),
dyntick_save_progress_counter(), and rcu_implicit_dynticks_qs() to enable
RCU's force-quiescent-state process to check for full-system idle.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
[ paulmck: Use true and false for boolean constants per Lai Jiangshan. ]
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit adds the code that updates the rcu_dyntick structure's
new fields to track the per-CPU idle state based on interrupts and
transitions into and out of the idle loop (NMIs are ignored because NMI
handlers cannot cleanly read out the time anyway).  This code is similar
to the code that maintains RCU's idea of per-CPU idleness, but differs
in that RCU treats CPUs running in user mode as idle, where this new
code does not.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit adds fields to the rcu_dyntick structure that are used to
detect idle CPUs.  These new fields differ from the existing ones in
that the existing ones consider a CPU executing in user mode to be idle,
where the new ones consider CPUs executing in user mode to be busy.
The handling of these new fields is otherwise quite similar to that for
the exiting fields.  This commit also adds the initialization required
for these fields.

So, why is usermode execution treated differently, with RCU considering
it a quiescent state equivalent to idle, while in contrast the new
full-system idle state detection considers usermode execution to be
non-idle?

It turns out that although one of RCU's quiescent states is usermode
execution, it is not a full-system idle state.  This is because the
purpose of the full-system idle state is not RCU, but rather determining
when accurate timekeeping can safely be disabled.  Whenever accurate
timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one
CPU must keep the scheduling-clock tick going.  If even one CPU is
executing in user mode, accurate timekeeping is requires, particularly for
architectures where gettimeofday() and friends do not enter the kernel.
Only when all CPUs are really and truly idle can accurate timekeeping be
disabled, allowing all CPUs to turn off the scheduling clock interrupt,
thus greatly improving energy efficiency.

This naturally raises the question "Why is this code in RCU rather than in
timekeeping?", and the answer is that RCU has the data and infrastructure
to efficiently make this determination.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The rcu_user_enter_after_irq() and rcu_user_exit_after_irq()
functions were intended for use by adaptive ticks, but changes
in implementation have rendered them unnecessary.  This commit
therefore removes them.
Reported-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

When setting up an in-the-future "advanced" grace period, the code needs
to wake up the relevant grace-period kthread, which it currently does
unconditionally.  However, this results in needless wakeups in the case
where the advanced grace period is being set up by the grace-period
kthread itself, which is a non-uncommon situation.  This commit therefore
checks to see if the running thread is the grace-period kthread, and
avoids doing the irq_work_queue()-mediated wakeup in that case.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

If someone does a duplicate call_rcu(), the worst thing the second
call_rcu() could do would be to actually queue the callback the second
time because doing so corrupts whatever list the callback was already
queued on.  This commit therefore makes __call_rcu() check the new
return value from debug-objects and leak the callback upon error.
This commit also substitutes rcu_leak_callback() for whatever callback
function was previously in place in order to avoid freeing the callback
out from under any readers that might still be referencing it.

These changes increase the probability that the debug-objects error
messages will actually make it somewhere visible.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Tested-by: NSedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

CONFIG_RCU_FAST_NO_HZ can increase grace-period durations by up to
a factor of four, which can result in long suspend and resume times.
Thus, this commit temporarily switches to expedited grace periods when
suspending the box and return to normal settings when resuming.  Similar
logic is applied to hibernation.

Because expedited grace periods are of dubious benefit on very large
systems, so this commit restricts their automated use during suspend
and resume to systems of 256 or fewer CPUs.  (Some day a number of
Linux-kernel facilities, including RCU's expedited grace periods,
will be more scalable, but I need to see bug reports first.)

[ paulmck: This also papers over an audio/irq bug, but hopefully that will
  be fixed soon. ]
Signed-off-by: NBorislav Petkov <bp@suse.de>
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Currently, RCU tracepoints save only a pointer to strings in the
ring buffer. When displayed via the /sys/kernel/debug/tracing/trace file
they are referenced like the printf "%s" that looks at the address
in the ring buffer and prints out the string it points too. This requires
that the strings are constant and persistent in the kernel.

The problem with this is for tools like trace-cmd and perf that read the
binary data from the buffers but have no access to the kernel memory to
find out what string is represented by the address in the buffer.

By using the tracepoint_string infrastructure, the RCU tracepoint strings
can be exported such that userspace tools can map the addresses to
the strings.

 # cat /sys/kernel/debug/tracing/printk_formats
0xffffffff81a4a0e8 : "rcu_preempt"
0xffffffff81a4a0f4 : "rcu_bh"
0xffffffff81a4a100 : "rcu_sched"
0xffffffff818437a0 : "cpuqs"
0xffffffff818437a6 : "rcu_sched"
0xffffffff818437a0 : "cpuqs"
0xffffffff818437b0 : "rcu_bh"
0xffffffff818437b7 : "Start context switch"
0xffffffff818437cc : "End context switch"
0xffffffff818437a0 : "cpuqs"
[...]

Now userspaces tools can display:

 rcu_utilization:      Start context switch
 rcu_dyntick:          Start 1 0
 rcu_utilization:      End context switch
 rcu_batch_start:      rcu_preempt CBs=0/5 bl=10
 rcu_dyntick:          End 0 140000000000000
 rcu_invoke_callback:  rcu_preempt rhp=0xffff880071c0d600 func=proc_i_callback
 rcu_invoke_callback:  rcu_preempt rhp=0xffff880077b5b230 func=__d_free
 rcu_dyntick:          Start 140000000000000 0
 rcu_invoke_callback:  rcu_preempt rhp=0xffff880077563980 func=file_free_rcu
 rcu_batch_end:        rcu_preempt CBs-invoked=3 idle=>c<>c<>c<>c<
 rcu_utilization:      End RCU core
 rcu_grace_period:     rcu_preempt 9741 start
 rcu_dyntick:          Start 1 0
 rcu_dyntick:          End 0 140000000000000
 rcu_dyntick:          Start 140000000000000 0

Instead of:

 rcu_utilization:      ffffffff81843110
 rcu_future_grace_period: ffffffff81842f1d 9939 9939 9940 0 0 3 ffffffff81842f32
 rcu_batch_start:      ffffffff81842f1d CBs=0/4 bl=10
 rcu_future_grace_period: ffffffff81842f1d 9939 9939 9940 0 0 3 ffffffff81842f3c
 rcu_grace_period:     ffffffff81842f1d 9939 ffffffff81842f80
 rcu_invoke_callback:  ffffffff81842f1d rhp=0xffff88007888aac0 func=file_free_rcu
 rcu_grace_period:     ffffffff81842f1d 9939 ffffffff81842f95
 rcu_invoke_callback:  ffffffff81842f1d rhp=0xffff88006aeb4600 func=proc_i_callback
 rcu_future_grace_period: ffffffff81842f1d 9939 9939 9940 0 0 3 ffffffff81842f32
 rcu_future_grace_period: ffffffff81842f1d 9939 9939 9940 0 0 3 ffffffff81842f3c
 rcu_invoke_callback:  ffffffff81842f1d rhp=0xffff880071cb9fc0 func=__d_free
 rcu_grace_period:     ffffffff81842f1d 9939 ffffffff81842f80
 rcu_invoke_callback:  ffffffff81842f1d rhp=0xffff88007888ae80 func=file_free_rcu
 rcu_batch_end:        ffffffff81842f1d CBs-invoked=4 idle=>c<>c<>c<>c<
 rcu_utilization:      ffffffff8184311f
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The RCU_STATE_INITIALIZER() macro is used only in the rcutree.c file
as well as the rcutree_plugin.h file. It is passed as a rvalue to
a variable of a similar name. A per_cpu variable is also created
with a similar name as well.

The uses of RCU_STATE_INITIALIZER() can be simplified to remove some
of the duplicate code that is done. Currently the three users of this
macro has this format:

struct rcu_state rcu_sched_state =
	RCU_STATE_INITIALIZER(rcu_sched, call_rcu_sched);
DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);

Notice that "rcu_sched" is called three times. This is the same with
the other two users. This can be condensed to just:

RCU_STATE_INITIALIZER(rcu_sched, call_rcu_sched);

by moving the rest into the macro itself.

This also opens the door to allow the RCU tracepoint strings and
their addresses to be exported so that userspace tracing tools can
translate the contents of the pointers of the RCU tracepoints.
The change will allow for helper code to be placed in the
RCU_STATE_INITIALIZER() macro to export the name that is used.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

All the RCU tracepoints and functions that reference char pointers do
so with just 'char *' even though they do not modify the contents of
the string itself. This will cause warnings if a const char * is used
in one of these functions.

The RCU tracepoints store the pointer to the string to refer back to them
when the trace output is displayed. As this can be minutes, hours or
even days later, those strings had better be constant.

This change also opens the door to allow the RCU tracepoint strings and
their addresses to be exported so that userspace tracing tools can
translate the contents of the pointers of the RCU tracepoints.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The __cpuinit type of throwaway sections might have made sense
some time ago when RAM was more constrained, but now the savings
do not offset the cost and complications.  For example, the fix in
commit 5e427ec2 ("x86: Fix bit corruption at CPU resume time")
is a good example of the nasty type of bugs that can be created
with improper use of the various __init prefixes.

After a discussion on LKML[1] it was decided that cpuinit should go
the way of devinit and be phased out.  Once all the users are gone,
we can then finally remove the macros themselves from linux/init.h.

This removes all the drivers/rcu uses of the __cpuinit macros
from all C files.

[1] https://lkml.org/lkml/2013/5/20/589

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@freedesktop.org>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Calling kthread_run with a single name parameter causes it to be handled
as a format string. Many callers are passing potentially dynamic string
content, so use "%s" in those cases to avoid any potential accidents.
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Systems with HZ=100 can have slow bootup times due to the default
three-jiffy delays between quiescent-state forcing attempts.  This
commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based
on the value of HZ.  However, this would break very large systems that
require more time between quiescent-state forcing attempts.  This
commit therefore also ups the default delay by one jiffy for each
256 CPUs that might be on the system (based off of nr_cpu_ids at
runtime, -not- NR_CPUS at build time).

Updated to collapse #ifdefs for RCU_JIFFIES_TILL_FORCE_QS into a
step-function definition as suggested by Josh Triplett.
Reported-by: NPaul Mackerras <paulus@au1.ibm.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The __rcu_process_callbacks() invokes note_gp_changes() immediately
before invoking rcu_check_quiescent_state(), which conditionally
invokes that same function.  This commit therefore eliminates the
call to note_gp_changes() in __rcu_process_callbacks() in favor of
making unconditional to call from rcu_check_quiescent_state() to
note_gp_changes().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Given the changes that introduce note_gp_change(), rcu_start_gp_per_cpu()
is now a trivial wrapper function with only one caller.  This commit
therefore inlines it into its sole call site.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

One of the calls to check_for_new_grace_period() is now redundant due to
an immediately preceding call to note_gp_changes().  Eliminating this
redundant call leaves a single caller, which is simpler if inlined.
This commit therefore eliminates the redundant call and inlines the
body of check_for_new_grace_period() into the single remaining call site.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit eliminates some duplicated code by merging
__rcu_process_gp_end() into __note_gp_changes().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Because note_gp_changes() now incorporates rcu_process_gp_end() function,
this commit switches to the former and eliminates the latter.  In
addition, this commit changes external calls from __rcu_process_gp_end()
to __note_gp_changes().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Because note_new_gpnum() now also checks for the ends of old grace periods,
this commit changes its name to note_gp_changes().  Later commits will merge
rcu_process_gp_end() into note_gp_changes().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The current implementation can detect the beginning of a new grace period
before noting the end of a previous grace period.  Although the current
implementation correctly handles this sort of nonsense, it would be
good to reduce RCU's state space by making such nonsense unnecessary,
which is now possible thanks to the fact that RCU's callback groups are
now numbered.

This commit therefore makes __note_new_gpnum() invoke
__rcu_process_gp_end() in order to note the ends of prior grace
periods before noting the beginnings of new grace periods.
Of course, this now means that note_new_gpnum() notes both the
beginnings and ends of grace periods, and could therefore be
used in place of rcu_process_gp_end().  But that is a job for
later commits.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The addition of callback numbering allows combining the detection of the
ends of old grace periods and the beginnings of new grace periods.  This
commit moves code to set the stage for this combining.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit converts printk() calls to the corresponding pr_*() calls.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

In Steven Rostedt's words:

> I've been debugging the last couple of days why my tests have been
> locking up. One of my tracing tests, runs all available tracers. The
> lockup always happened with the mmiotrace, which is used to trace
> interactions between priority drivers and the kernel. But to do this
> easily, when the tracer gets registered, it disables all but the boot
> CPUs. The lockup always happened after it got done disabling the CPUs.
>
> Then I decided to try this:
>
> while :; do
> 	for i in 1 2 3; do
> 		echo 0 > /sys/devices/system/cpu/cpu$i/online
> 	done
> 	for i in 1 2 3; do
> 		echo 1 > /sys/devices/system/cpu/cpu$i/online
> 	done
> done
>
> Well, sure enough, that locked up too, with the same users. Doing a
> sysrq-w (showing all blocked tasks):
>
> [ 2991.344562]   task                        PC stack   pid father
> [ 2991.344562] rcu_preempt     D ffff88007986fdf8     0    10      2 0x00000000
> [ 2991.344562]  ffff88007986fc98 0000000000000002 ffff88007986fc48 0000000000000908
> [ 2991.344562]  ffff88007986c280 ffff88007986ffd8 ffff88007986ffd8 00000000001d3c80
> [ 2991.344562]  ffff880079248a40 ffff88007986c280 0000000000000000 00000000fffd4295
> [ 2991.344562] Call Trace:
> [ 2991.344562]  [<ffffffff815437ba>] schedule+0x64/0x66
> [ 2991.344562]  [<ffffffff81541750>] schedule_timeout+0xbc/0xf9
> [ 2991.344562]  [<ffffffff8154bec0>] ? ftrace_call+0x5/0x2f
> [ 2991.344562]  [<ffffffff81049513>] ? cascade+0xa8/0xa8
> [ 2991.344562]  [<ffffffff815417ab>] schedule_timeout_uninterruptible+0x1e/0x20
> [ 2991.344562]  [<ffffffff810c980c>] rcu_gp_kthread+0x502/0x94b
> [ 2991.344562]  [<ffffffff81062791>] ? __init_waitqueue_head+0x50/0x50
> [ 2991.344562]  [<ffffffff810c930a>] ? rcu_gp_fqs+0x64/0x64
> [ 2991.344562]  [<ffffffff81061cdb>] kthread+0xb1/0xb9
> [ 2991.344562]  [<ffffffff81091e31>] ? lock_release_holdtime.part.23+0x4e/0x55
> [ 2991.344562]  [<ffffffff81061c2a>] ? __init_kthread_worker+0x58/0x58
> [ 2991.344562]  [<ffffffff8154c1dc>] ret_from_fork+0x7c/0xb0
> [ 2991.344562]  [<ffffffff81061c2a>] ? __init_kthread_worker+0x58/0x58
> [ 2991.344562] kworker/0:1     D ffffffff81a30680     0    47      2 0x00000000
> [ 2991.344562] Workqueue: events cpuset_hotplug_workfn
> [ 2991.344562]  ffff880078dbbb58 0000000000000002 0000000000000006 00000000000000d8
> [ 2991.344562]  ffff880078db8100 ffff880078dbbfd8 ffff880078dbbfd8 00000000001d3c80
> [ 2991.344562]  ffff8800779ca5c0 ffff880078db8100 ffffffff81541fcf 0000000000000000
> [ 2991.344562] Call Trace:
> [ 2991.344562]  [<ffffffff81541fcf>] ? __mutex_lock_common+0x3d4/0x609
> [ 2991.344562]  [<ffffffff815437ba>] schedule+0x64/0x66
> [ 2991.344562]  [<ffffffff81543a39>] schedule_preempt_disabled+0x18/0x24
> [ 2991.344562]  [<ffffffff81541fcf>] __mutex_lock_common+0x3d4/0x609
> [ 2991.344562]  [<ffffffff8103d11b>] ? get_online_cpus+0x3c/0x50
> [ 2991.344562]  [<ffffffff8103d11b>] ? get_online_cpus+0x3c/0x50
> [ 2991.344562]  [<ffffffff815422ff>] mutex_lock_nested+0x3b/0x40
> [ 2991.344562]  [<ffffffff8103d11b>] get_online_cpus+0x3c/0x50
> [ 2991.344562]  [<ffffffff810af7e6>] rebuild_sched_domains_locked+0x6e/0x3a8
> [ 2991.344562]  [<ffffffff810b0ec6>] rebuild_sched_domains+0x1c/0x2a
> [ 2991.344562]  [<ffffffff810b109b>] cpuset_hotplug_workfn+0x1c7/0x1d3
> [ 2991.344562]  [<ffffffff810b0ed9>] ? cpuset_hotplug_workfn+0x5/0x1d3
> [ 2991.344562]  [<ffffffff81058e07>] process_one_work+0x2d4/0x4d1
> [ 2991.344562]  [<ffffffff81058d3a>] ? process_one_work+0x207/0x4d1
> [ 2991.344562]  [<ffffffff8105964c>] worker_thread+0x2e7/0x3b5
> [ 2991.344562]  [<ffffffff81059365>] ? rescuer_thread+0x332/0x332
> [ 2991.344562]  [<ffffffff81061cdb>] kthread+0xb1/0xb9
> [ 2991.344562]  [<ffffffff81061c2a>] ? __init_kthread_worker+0x58/0x58
> [ 2991.344562]  [<ffffffff8154c1dc>] ret_from_fork+0x7c/0xb0
> [ 2991.344562]  [<ffffffff81061c2a>] ? __init_kthread_worker+0x58/0x58
> [ 2991.344562] bash            D ffffffff81a4aa80     0  2618   2612 0x10000000
> [ 2991.344562]  ffff8800379abb58 0000000000000002 0000000000000006 0000000000000c2c
> [ 2991.344562]  ffff880077fea140 ffff8800379abfd8 ffff8800379abfd8 00000000001d3c80
> [ 2991.344562]  ffff8800779ca5c0 ffff880077fea140 ffffffff81541fcf 0000000000000000
> [ 2991.344562] Call Trace:
> [ 2991.344562]  [<ffffffff81541fcf>] ? __mutex_lock_common+0x3d4/0x609
> [ 2991.344562]  [<ffffffff815437ba>] schedule+0x64/0x66
> [ 2991.344562]  [<ffffffff81543a39>] schedule_preempt_disabled+0x18/0x24
> [ 2991.344562]  [<ffffffff81541fcf>] __mutex_lock_common+0x3d4/0x609
> [ 2991.344562]  [<ffffffff81530078>] ? rcu_cpu_notify+0x2f5/0x86e
> [ 2991.344562]  [<ffffffff81530078>] ? rcu_cpu_notify+0x2f5/0x86e
> [ 2991.344562]  [<ffffffff815422ff>] mutex_lock_nested+0x3b/0x40
> [ 2991.344562]  [<ffffffff81530078>] rcu_cpu_notify+0x2f5/0x86e
> [ 2991.344562]  [<ffffffff81091c99>] ? __lock_is_held+0x32/0x53
> [ 2991.344562]  [<ffffffff81548912>] notifier_call_chain+0x6b/0x98
> [ 2991.344562]  [<ffffffff810671fd>] __raw_notifier_call_chain+0xe/0x10
> [ 2991.344562]  [<ffffffff8103cf64>] __cpu_notify+0x20/0x32
> [ 2991.344562]  [<ffffffff8103cf8d>] cpu_notify_nofail+0x17/0x36
> [ 2991.344562]  [<ffffffff815225de>] _cpu_down+0x154/0x259
> [ 2991.344562]  [<ffffffff81522710>] cpu_down+0x2d/0x3a
> [ 2991.344562]  [<ffffffff81526351>] store_online+0x4e/0xe7
> [ 2991.344562]  [<ffffffff8134d764>] dev_attr_store+0x20/0x22
> [ 2991.344562]  [<ffffffff811b3c5f>] sysfs_write_file+0x108/0x144
> [ 2991.344562]  [<ffffffff8114c5ef>] vfs_write+0xfd/0x158
> [ 2991.344562]  [<ffffffff8114c928>] SyS_write+0x5c/0x83
> [ 2991.344562]  [<ffffffff8154c494>] tracesys+0xdd/0xe2
>
> As well as held locks:
>
> [ 3034.728033] Showing all locks held in the system:
> [ 3034.728033] 1 lock held by rcu_preempt/10:
> [ 3034.728033]  #0:  (rcu_preempt_state.onoff_mutex){+.+...}, at: [<ffffffff810c9471>] rcu_gp_kthread+0x167/0x94b
> [ 3034.728033] 4 locks held by kworker/0:1/47:
> [ 3034.728033]  #0:  (events){.+.+.+}, at: [<ffffffff81058d3a>] process_one_work+0x207/0x4d1
> [ 3034.728033]  #1:  (cpuset_hotplug_work){+.+.+.}, at: [<ffffffff81058d3a>] process_one_work+0x207/0x4d1
> [ 3034.728033]  #2:  (cpuset_mutex){+.+.+.}, at: [<ffffffff810b0ec1>] rebuild_sched_domains+0x17/0x2a
> [ 3034.728033]  #3:  (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff8103d11b>] get_online_cpus+0x3c/0x50
> [ 3034.728033] 1 lock held by mingetty/2563:
> [ 3034.728033]  #0:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8131e28a>] n_tty_read+0x252/0x7e8
> [ 3034.728033] 1 lock held by mingetty/2565:
> [ 3034.728033]  #0:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8131e28a>] n_tty_read+0x252/0x7e8
> [ 3034.728033] 1 lock held by mingetty/2569:
> [ 3034.728033]  #0:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8131e28a>] n_tty_read+0x252/0x7e8
> [ 3034.728033] 1 lock held by mingetty/2572:
> [ 3034.728033]  #0:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8131e28a>] n_tty_read+0x252/0x7e8
> [ 3034.728033] 1 lock held by mingetty/2575:
> [ 3034.728033]  #0:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8131e28a>] n_tty_read+0x252/0x7e8
> [ 3034.728033] 7 locks held by bash/2618:
> [ 3034.728033]  #0:  (sb_writers#5){.+.+.+}, at: [<ffffffff8114bc3f>] file_start_write+0x2a/0x2c
> [ 3034.728033]  #1:  (&buffer->mutex#2){+.+.+.}, at: [<ffffffff811b3b93>] sysfs_write_file+0x3c/0x144
> [ 3034.728033]  #2:  (s_active#54){.+.+.+}, at: [<ffffffff811b3c3e>] sysfs_write_file+0xe7/0x144
> [ 3034.728033]  #3:  (x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff810217c2>] cpu_hotplug_driver_lock+0x17/0x19
> [ 3034.728033]  #4:  (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff8103d196>] cpu_maps_update_begin+0x17/0x19
> [ 3034.728033]  #5:  (cpu_hotplug.lock){+.+.+.}, at: [<ffffffff8103cfd8>] cpu_hotplug_begin+0x2c/0x6d
> [ 3034.728033]  #6:  (rcu_preempt_state.onoff_mutex){+.+...}, at: [<ffffffff81530078>] rcu_cpu_notify+0x2f5/0x86e
> [ 3034.728033] 1 lock held by bash/2980:
> [ 3034.728033]  #0:  (&ldata->atomic_read_lock){+.+...}, at: [<ffffffff8131e28a>] n_tty_read+0x252/0x7e8
>
> Things looked a little weird. Also, this is a deadlock that lockdep did
> not catch. But what we have here does not look like a circular lock
> issue:
>
> Bash is blocked in rcu_cpu_notify():
>
> 1961		/* Exclude any attempts to start a new grace period. */
> 1962		mutex_lock(&rsp->onoff_mutex);
>
>
> kworker is blocked in get_online_cpus(), which makes sense as we are
> currently taking down a CPU.
>
> But rcu_preempt is not blocked on anything. It is simply sleeping in
> rcu_gp_kthread (really rcu_gp_init) here:
>
> 1453	#ifdef CONFIG_PROVE_RCU_DELAY
> 1454			if ((prandom_u32() % (rcu_num_nodes * 8)) == 0 &&
> 1455			    system_state == SYSTEM_RUNNING)
> 1456				schedule_timeout_uninterruptible(2);
> 1457	#endif /* #ifdef CONFIG_PROVE_RCU_DELAY */
>
> And it does this while holding the onoff_mutex that bash is waiting for.
>
> Doing a function trace, it showed me where it happened:
>
> [  125.940066] rcu_pree-10      3.... 28384115273: schedule_timeout_uninterruptible <-rcu_gp_kthread
> [...]
> [  125.940066] rcu_pree-10      3d..3 28384202439: sched_switch: prev_comm=rcu_preempt prev_pid=10 prev_prio=120 prev_state=D ==> next_comm=watchdog/3 next_pid=38 next_prio=120
>
> The watchdog ran, and then:
>
> [  125.940066] watchdog-38      3d..3 28384692863: sched_switch: prev_comm=watchdog/3 prev_pid=38 prev_prio=120 prev_state=P ==> next_comm=modprobe next_pid=2848 next_prio=118
>
> Not sure what modprobe was doing, but shortly after that:
>
> [  125.940066] modprobe-2848    3d..3 28385041749: sched_switch: prev_comm=modprobe prev_pid=2848 prev_prio=118 prev_state=R+ ==> next_comm=migration/3 next_pid=40 next_prio=0
>
> Where the migration thread took down the CPU:
>
> [  125.940066] migratio-40      3d..3 28389148276: sched_switch: prev_comm=migration/3 prev_pid=40 prev_prio=0 prev_state=P ==> next_comm=swapper/3 next_pid=0 next_prio=120
>
> which finally did:
>
> [  125.940066]   <idle>-0       3...1 28389282142: arch_cpu_idle_dead <-cpu_startup_entry
> [  125.940066]   <idle>-0       3...1 28389282548: native_play_dead <-arch_cpu_idle_dead
> [  125.940066]   <idle>-0       3...1 28389282924: play_dead_common <-native_play_dead
> [  125.940066]   <idle>-0       3...1 28389283468: idle_task_exit <-play_dead_common
> [  125.940066]   <idle>-0       3...1 28389284644: amd_e400_remove_cpu <-play_dead_common
>
>
> CPU 3 is now offline, the rcu_preempt thread that ran on CPU 3 is still
> doing a schedule_timeout_uninterruptible() and it registered it's
> timeout to the timer base for CPU 3. You would think that it would get
> migrated right? The issue here is that the timer migration happens at
> the CPU notifier for CPU_DEAD. The problem is that the rcu notifier for
> CPU_DOWN is blocked waiting for the onoff_mutex to be released, which is
> held by the thread that just put itself into a uninterruptible sleep,
> that wont wake up until the CPU_DEAD notifier of the timer
> infrastructure is called, which wont happen until the rcu notifier
> finishes. Here's our deadlock!

This commit breaks this deadlock cycle by substituting a shorter udelay()
for the previous schedule_timeout_uninterruptible(), while at the same
time increasing the probability of the delay.  This maintains the intensity
of the testing.
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NSteven Rostedt <rostedt@goodmis.org>

This commit fixes a lockdep-detected deadlock by moving a wake_up()
call out from a rnp->lock critical section.  Please see below for
the long version of this story.

On Tue, 2013-05-28 at 16:13 -0400, Dave Jones wrote:

> [12572.705832] ======================================================
> [12572.750317] [ INFO: possible circular locking dependency detected ]
> [12572.796978] 3.10.0-rc3+ #39 Not tainted
> [12572.833381] -------------------------------------------------------
> [12572.862233] trinity-child17/31341 is trying to acquire lock:
> [12572.870390]  (rcu_node_0){..-.-.}, at: [<ffffffff811054ff>] rcu_read_unlock_special+0x9f/0x4c0
> [12572.878859]
> but task is already holding lock:
> [12572.894894]  (&ctx->lock){-.-...}, at: [<ffffffff811390ed>] perf_lock_task_context+0x7d/0x2d0
> [12572.903381]
> which lock already depends on the new lock.
>
> [12572.927541]
> the existing dependency chain (in reverse order) is:
> [12572.943736]
> -> #4 (&ctx->lock){-.-...}:
> [12572.960032]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
> [12572.968337]        [<ffffffff816ebc90>] _raw_spin_lock+0x40/0x80
> [12572.976633]        [<ffffffff8113c987>] __perf_event_task_sched_out+0x2e7/0x5e0
> [12572.984969]        [<ffffffff81088953>] perf_event_task_sched_out+0x93/0xa0
> [12572.993326]        [<ffffffff816ea0bf>] __schedule+0x2cf/0x9c0
> [12573.001652]        [<ffffffff816eacfe>] schedule_user+0x2e/0x70
> [12573.009998]        [<ffffffff816ecd64>] retint_careful+0x12/0x2e
> [12573.018321]
> -> #3 (&rq->lock){-.-.-.}:
> [12573.034628]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
> [12573.042930]        [<ffffffff816ebc90>] _raw_spin_lock+0x40/0x80
> [12573.051248]        [<ffffffff8108e6a7>] wake_up_new_task+0xb7/0x260
> [12573.059579]        [<ffffffff810492f5>] do_fork+0x105/0x470
> [12573.067880]        [<ffffffff81049686>] kernel_thread+0x26/0x30
> [12573.076202]        [<ffffffff816cee63>] rest_init+0x23/0x140
> [12573.084508]        [<ffffffff81ed8e1f>] start_kernel+0x3f1/0x3fe
> [12573.092852]        [<ffffffff81ed856f>] x86_64_start_reservations+0x2a/0x2c
> [12573.101233]        [<ffffffff81ed863d>] x86_64_start_kernel+0xcc/0xcf
> [12573.109528]
> -> #2 (&p->pi_lock){-.-.-.}:
> [12573.125675]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
> [12573.133829]        [<ffffffff816ebe9b>] _raw_spin_lock_irqsave+0x4b/0x90
> [12573.141964]        [<ffffffff8108e881>] try_to_wake_up+0x31/0x320
> [12573.150065]        [<ffffffff8108ebe2>] default_wake_function+0x12/0x20
> [12573.158151]        [<ffffffff8107bbf8>] autoremove_wake_function+0x18/0x40
> [12573.166195]        [<ffffffff81085398>] __wake_up_common+0x58/0x90
> [12573.174215]        [<ffffffff81086909>] __wake_up+0x39/0x50
> [12573.182146]        [<ffffffff810fc3da>] rcu_start_gp_advanced.isra.11+0x4a/0x50
> [12573.190119]        [<ffffffff810fdb09>] rcu_start_future_gp+0x1c9/0x1f0
> [12573.198023]        [<ffffffff810fe2c4>] rcu_nocb_kthread+0x114/0x930
> [12573.205860]        [<ffffffff8107a91d>] kthread+0xed/0x100
> [12573.213656]        [<ffffffff816f4b1c>] ret_from_fork+0x7c/0xb0
> [12573.221379]
> -> #1 (&rsp->gp_wq){..-.-.}:
> [12573.236329]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
> [12573.243783]        [<ffffffff816ebe9b>] _raw_spin_lock_irqsave+0x4b/0x90
> [12573.251178]        [<ffffffff810868f3>] __wake_up+0x23/0x50
> [12573.258505]        [<ffffffff810fc3da>] rcu_start_gp_advanced.isra.11+0x4a/0x50
> [12573.265891]        [<ffffffff810fdb09>] rcu_start_future_gp+0x1c9/0x1f0
> [12573.273248]        [<ffffffff810fe2c4>] rcu_nocb_kthread+0x114/0x930
> [12573.280564]        [<ffffffff8107a91d>] kthread+0xed/0x100
> [12573.287807]        [<ffffffff816f4b1c>] ret_from_fork+0x7c/0xb0

Notice the above call chain.

rcu_start_future_gp() is called with the rnp->lock held. Then it calls
rcu_start_gp_advance, which does a wakeup.

You can't do wakeups while holding the rnp->lock, as that would mean
that you could not do a rcu_read_unlock() while holding the rq lock, or
any lock that was taken while holding the rq lock. This is because...
(See below).

> [12573.295067]
> -> #0 (rcu_node_0){..-.-.}:
> [12573.309293]        [<ffffffff810b8d36>] __lock_acquire+0x1786/0x1af0
> [12573.316568]        [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
> [12573.323825]        [<ffffffff816ebc90>] _raw_spin_lock+0x40/0x80
> [12573.331081]        [<ffffffff811054ff>] rcu_read_unlock_special+0x9f/0x4c0
> [12573.338377]        [<ffffffff810760a6>] __rcu_read_unlock+0x96/0xa0
> [12573.345648]        [<ffffffff811391b3>] perf_lock_task_context+0x143/0x2d0
> [12573.352942]        [<ffffffff8113938e>] find_get_context+0x4e/0x1f0
> [12573.360211]        [<ffffffff811403f4>] SYSC_perf_event_open+0x514/0xbd0
> [12573.367514]        [<ffffffff81140e49>] SyS_perf_event_open+0x9/0x10
> [12573.374816]        [<ffffffff816f4dd4>] tracesys+0xdd/0xe2

Notice the above trace.

perf took its own ctx->lock, which can be taken while holding the rq
lock. While holding this lock, it did a rcu_read_unlock(). The
perf_lock_task_context() basically looks like:

rcu_read_lock();
raw_spin_lock(ctx->lock);
rcu_read_unlock();

Now, what looks to have happened, is that we scheduled after taking that
first rcu_read_lock() but before taking the spin lock. When we scheduled
back in and took the ctx->lock, the following rcu_read_unlock()
triggered the "special" code.

The rcu_read_unlock_special() takes the rnp->lock, which gives us a
possible deadlock scenario.

	CPU0		CPU1		CPU2
	----		----		----

				     rcu_nocb_kthread()
    lock(rq->lock);
		    lock(ctx->lock);
				     lock(rnp->lock);

				     wake_up();

				     lock(rq->lock);

		    rcu_read_unlock();

		    rcu_read_unlock_special();

		    lock(rnp->lock);
    lock(ctx->lock);

**** DEADLOCK ****

> [12573.382068]
> other info that might help us debug this:
>
> [12573.403229] Chain exists of:
>   rcu_node_0 --> &rq->lock --> &ctx->lock
>
> [12573.424471]  Possible unsafe locking scenario:
>
> [12573.438499]        CPU0                    CPU1
> [12573.445599]        ----                    ----
> [12573.452691]   lock(&ctx->lock);
> [12573.459799]                                lock(&rq->lock);
> [12573.467010]                                lock(&ctx->lock);
> [12573.474192]   lock(rcu_node_0);
> [12573.481262]
>  *** DEADLOCK ***
>
> [12573.501931] 1 lock held by trinity-child17/31341:
> [12573.508990]  #0:  (&ctx->lock){-.-...}, at: [<ffffffff811390ed>] perf_lock_task_context+0x7d/0x2d0
> [12573.516475]
> stack backtrace:
> [12573.530395] CPU: 1 PID: 31341 Comm: trinity-child17 Not tainted 3.10.0-rc3+ #39
> [12573.545357]  ffffffff825b4f90 ffff880219f1dbc0 ffffffff816e375b ffff880219f1dc00
> [12573.552868]  ffffffff816dfa5d ffff880219f1dc50 ffff88023ce4d1f8 ffff88023ce4ca40
> [12573.560353]  0000000000000001 0000000000000001 ffff88023ce4d1f8 ffff880219f1dcc0
> [12573.567856] Call Trace:
> [12573.575011]  [<ffffffff816e375b>] dump_stack+0x19/0x1b
> [12573.582284]  [<ffffffff816dfa5d>] print_circular_bug+0x200/0x20f
> [12573.589637]  [<ffffffff810b8d36>] __lock_acquire+0x1786/0x1af0
> [12573.596982]  [<ffffffff810918f5>] ? sched_clock_cpu+0xb5/0x100
> [12573.604344]  [<ffffffff810b9851>] lock_acquire+0x91/0x1f0
> [12573.611652]  [<ffffffff811054ff>] ? rcu_read_unlock_special+0x9f/0x4c0
> [12573.619030]  [<ffffffff816ebc90>] _raw_spin_lock+0x40/0x80
> [12573.626331]  [<ffffffff811054ff>] ? rcu_read_unlock_special+0x9f/0x4c0
> [12573.633671]  [<ffffffff811054ff>] rcu_read_unlock_special+0x9f/0x4c0
> [12573.640992]  [<ffffffff811390ed>] ? perf_lock_task_context+0x7d/0x2d0
> [12573.648330]  [<ffffffff810b429e>] ? put_lock_stats.isra.29+0xe/0x40
> [12573.655662]  [<ffffffff813095a0>] ? delay_tsc+0x90/0xe0
> [12573.662964]  [<ffffffff810760a6>] __rcu_read_unlock+0x96/0xa0
> [12573.670276]  [<ffffffff811391b3>] perf_lock_task_context+0x143/0x2d0
> [12573.677622]  [<ffffffff81139070>] ? __perf_event_enable+0x370/0x370
> [12573.684981]  [<ffffffff8113938e>] find_get_context+0x4e/0x1f0
> [12573.692358]  [<ffffffff811403f4>] SYSC_perf_event_open+0x514/0xbd0
> [12573.699753]  [<ffffffff8108cd9d>] ? get_parent_ip+0xd/0x50
> [12573.707135]  [<ffffffff810b71fd>] ? trace_hardirqs_on_caller+0xfd/0x1c0
> [12573.714599]  [<ffffffff81140e49>] SyS_perf_event_open+0x9/0x10
> [12573.721996]  [<ffffffff816f4dd4>] tracesys+0xdd/0xe2

This commit delays the wakeup via irq_work(), which is what
perf and ftrace use to perform wakeups in critical sections.
Reported-by: NDave Jones <davej@redhat.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Use preferable function name which implies using a pseudo-random
number generator.
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

We need full dynticks CPU to also be RCU nocb so
that we don't have to keep the tick to handle RCU
callbacks.

Make sure the range passed to nohz_full= boot
parameter is a subset of rcu_nocbs=

The CPUs that fail to meet this requirement will be
excluded from the nohz_full range. This is checked
early in boot time, before any CPU has the opportunity
to stop its tick.
Suggested-by: NSteven Rostedt <rostedt@goodmis.org>
Reviewed-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

Adaptive-ticks CPUs inform RCU when they enter kernel mode, but they do
not necessarily turn the scheduler-clock tick back on.  This state of
affairs could result in RCU waiting on an adaptive-ticks CPU running
for an extended period in kernel mode.  Such a CPU will never run the
RCU state machine, and could therefore indefinitely extend the RCU state
machine, sooner or later resulting in an OOM condition.

This patch, inspired by an earlier patch by Frederic Weisbecker, therefore
causes RCU's force-quiescent-state processing to check for this condition
and to send an IPI to CPUs that remain in that state for too long.
"Too long" currently means about three jiffies by default, which is
quite some time for a CPU to remain in the kernel without blocking.
The rcu_tree.jiffies_till_first_fqs and rcutree.jiffies_till_next_fqs
sysfs variables may be used to tune "too long" if needed.
Reported-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

Now that rcu_start_future_gp() has been abstracted from
rcu_nocb_wait_gp(), rcu_accelerate_cbs() can invoke rcu_start_future_gp()
so as to register the need for any future grace periods needed by a
CPU about to enter dyntick-idle mode.  This commit makes this change.
Note that some refactoring of rcu_start_gp() is carried out to avoid
recursion and subsequent self-deadlocks.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

CPUs going idle will need to record the need for a future grace
period, but won't actually need to block waiting on it.  This commit
therefore splits rcu_start_future_gp(), which does the recording, from
rcu_nocb_wait_gp(), which now invokes rcu_start_future_gp() to do the
recording, after which rcu_nocb_wait_gp() does the waiting.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

If CPUs are to give prior notice of needed grace periods, it will be
necessary to invoke rcu_start_gp() without dropping the root rcu_node
structure's ->lock.  This commit takes a second step in this direction
by moving the release of this lock to rcu_start_gp()'s callers.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

If CPUs are to give prior notice of needed grace periods, it will be
necessary to invoke rcu_start_gp() without dropping the root rcu_node
structure's ->lock.  This commit takes a first step in this direction
by moving the release of this lock to the end of rcu_start_gp().
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Because RCU callbacks are now associated with the number of the grace
period that they must wait for, CPUs can now take advance callbacks
corresponding to grace periods that ended while a given CPU was in
dyntick-idle mode.  This eliminates the need to try forcing the RCU
state machine while entering idle, thus reducing the CPU intensiveness
of RCU_FAST_NO_HZ, which should increase its energy efficiency.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Now that callback acceleration is idempotent, it is safe to accelerate
callbacks during grace-period cleanup on any CPUs that the kthread happens
to be running on.  This commit therefore propagates the completion
of the grace period to the per-CPU data structures, and also adds an
rcu_advance_cbs() just before the cpu_needs_another_gp() check in order
to reduce false-positive grace periods.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently, the per-no-CBs-CPU kthreads are named "rcuo" followed by
the CPU number, for example, "rcuo".  This is problematic given that
there are either two or three RCU flavors, each of which gets a per-CPU
kthread with exactly the same name.  This commit therefore introduces
a one-letter abbreviation for each RCU flavor, namely 'b' for RCU-bh,
'p' for RCU-preempt, and 's' for RCU-sched.  This abbreviation is used
to distinguish the "rcuo" kthreads, for example, for CPU 0 we would have
"rcuob/0", "rcuop/0", and "rcuos/0".
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NDietmar Eggemann <dietmar.eggemann@arm.com>

Currently, the no-CBs kthreads do repeated timed waits for grace periods
to elapse.  This is crude and energy inefficient, so this commit allows
no-CBs kthreads to specify exactly which grace period they are waiting
for and also allows them to block for the entire duration until the
desired grace period completes.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>