rcu-tasks.2014.09.10a: Add RCU-tasks flavor of RCU.

Merge branches 'doc.2014.09.07a', 'fixes.2014.09.10a', 'nocb-nohz.2014.09.16b' and 'torture.2014.09.07a' into HEAD

doc.2014.09.07a: Documentation updates.
fixes.2014.09.10a: Miscellaneous fixes.
nocb-nohz.2014.09.16b: No-CBs CPUs and NO_HZ_FULL updates.
torture.2014.09.07a: Torture-test updates.

The NOCB follower wakeup ordering depends on the store to the tail
pointer happening before the wakeup.  However, because atomic_long_add()
does not return a value, it does not provide ordering guarantees, and
the locking in wake_up() only guarantees that the store will happen
before the unlock, which might be too late.  Even though this is only a
theoretical issue, this commit adds a smp_mb__after_atomic() after the
final atomic_long_add() to provide the needed ordering guarantee.
Reported-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

If an RCU callback is queued on a no-CBs CPU from idle code with irqs
disabled, and if that CPU stays idle forever after, the callback will
never be invoked.  This commit therefore adds a check for this situation
in ____call_rcu_nocb(), invoking the RCU core solely for the purpose
of the ensuing return-to-idle transition.  (If the CPU doesn't return
to idle, the next scheduling-clock interrupt will fix things up.)
Reported-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

The NOCB leader wakeup ordering depends on the store to the header
happening before the check for the leader already being awake.  However,
because atomic_long_add() does not return a value, it does not provide
ordering guarantees, the incorrect comment in wake_nocb_leader()
notwithstanding.  This commit therefore adds a smp_mb__after_atomic()
after the final atomic_long_add() to provide the needed ordering
guarantee.
Reported-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

If there are no nohz_full= CPUs, then there is currently no reason to
track sysidle state.  This commit therefore short-circuits this state
tracking if !tick_nohz_full_enabled().

Note that these checks will need to be revisited if nohz_full= state
can ever be changed at runtime.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Now that we have rcu_state_p, which references rcu_preempt_state for
TREE_PREEMPT_RCU and rcu_sched_state for TREE_RCU, we don't need a
separate rcu_sysidle_state variable.  This commit therefore eliminates
rcu_preempt_state in favor of rcu_state_p.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

If we configure a kernel with CONFIG_NOCB_CPU=y, CONFIG_RCU_NOCB_CPU_NONE=y and
CONFIG_CPUMASK_OFFSTACK=n and do not pass in a rcu_nocb= boot parameter, the
cpumask rcu_nocb_mask can be garbage instead of NULL.

Hence this commit replaces checks for rcu_nocb_mask == NULL with a check for
have_rcu_nocb_mask.
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

RCU currently uses for_each_possible_cpu() to spawn rcuo kthreads,
which can result in more rcuo kthreads than one would expect, for
example, derRichard reported 64 CPUs worth of rcuo kthreads on an
8-CPU image.  This commit therefore creates rcuo kthreads only for
those CPUs that actually come online.

This was reported by derRichard on the OFTC IRC network.
Reported-by: NRichard Weinberger <richard@nod.at>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Currently, RCU spawns kthreads from several different early_initcall()
functions.  Although this has served RCU well for quite some time,
as more kthreads are added a more deterministic approach is required.
This commit therefore causes all of RCU's early-boot kthreads to be
spawned from a single early_initcall() function.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Return false instead of 0 in rcu_nocb_adopt_orphan_cbs() as this has
bool as return type.
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Return false instead of 0 in __call_rcu_nocb() as this has bool as
return type.
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Return true/false in rcu_nocb_adopt_orphan_cbs() instead of 0/1 as
this function has return type of bool.
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Return true/false instead of 0/1 in __call_rcu_nocb() as this returns a
bool type.
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

This commit checks the return value of the zalloc_cpumask_var() used for
allocating cpumask for rcu_nocb_mask.
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Commit b58cc46c (rcu: Don't offload callbacks unless specifically
requested) failed to adjust the callback lists of the CPUs that are
known to be no-CBs CPUs only because they are also nohz_full= CPUs.
This failure can result in callbacks that are posted during early boot
getting stranded on nxtlist for CPUs whose no-CBs property becomes
apparent late, and there can also be spurious warnings about offline
CPUs posting callbacks.

This commit fixes these problems by adding an early-boot rcu_init_nohz()
that properly initializes the no-CBs CPUs.

Note that kernels built with CONFIG_RCU_NOCB_CPU_ALL=y or with
CONFIG_RCU_NOCB_CPU=n do not exhibit this bug.  Neither do kernels
booted without the nohz_full= boot parameter.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>
Tested-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Although the test cases have been added, they must be specified explicitly
via the kvm.sh --configs argument in order to run them.  This commit
therefore adds the RCU-tasks tests to the CFLIST so that they will be
run automatically by default.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The rcu_bh_qs(), rcu_preempt_qs(), and rcu_sched_qs() functions use
old-style per-CPU variable access and write to ->passed_quiesce even
if it is already set.  This commit therefore updates to use the new-style
per-CPU variable access functions and avoids the spurious writes.
This commit also eliminates the "cpu" argument to these functions because
they are always invoked on the indicated CPU.
Reported-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The rcu_preempt_note_context_switch() function is on a scheduling fast
path, so it would be good to avoid disabling irqs.  The reason that irqs
are disabled is to synchronize process-level and irq-handler access to
the task_struct ->rcu_read_unlock_special bitmask.  This commit therefore
makes ->rcu_read_unlock_special instead be a union of bools with a short
allowing single-access checks in RCU's __rcu_read_unlock().  This results
in the process-level and irq-handler accesses being simple loads and
stores, so that irqs need no longer be disabled.  This commit therefore
removes the irq disabling from rcu_preempt_note_context_switch().
Reported-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

In theory, synchronize_sched() requires a read-side critical section
to order against.  In practice, preemption can be thought of as
being disabled across every machine instruction, at least for those
machine instructions that are not in the idle loop and not on offline
CPUs.  So this commit removes the redundant preempt_disable() from
rcu_note_voluntary_context_switch().

Please note that the single instruction in question is the store of
zero to ->rcu_tasks_holdout.  The "if" is simply a performance optimization
that avoids unnecessary stores.  To see this, keep in mind that both
the "if" condition and the store are in a quiescent state.  Therefore,
even if the task is preempted for a full grace period (presumably due
to its having done a context switch beforehand), the store will be
recording a legitimate quiescent state.
Reported-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Conflicts:
	include/linux/rcupdate.h

The grace-period-wait loop in rcu_tasks_kthread() is under (unnecessary)
RCU protection, and therefore has no preemption points in a PREEMPT=n
kernel.  This commit therefore removes the RCU protection and inserts
cond_resched().
Reported-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently TASKS_RCU would ignore a CPU running a task in nohz_full=
usermode execution.  There would be neither a context switch nor a
scheduling-clock interrupt to tell TASKS_RCU that the task in question
had passed through a quiescent state.  The grace period would therefore
extend indefinitely.  This commit therefore makes RCU's dyntick-idle
subsystem record the task_struct structure of the task that is running
in dyntick-idle mode on each CPU.  The TASKS_RCU grace period can
then access this information and record a quiescent state on
behalf of any CPU running in dyntick-idle usermode.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

It is expected that many sites will have CONFIG_TASKS_RCU=y, but
will never actually invoke call_rcu_tasks().  For such sites, creating
rcu_tasks_kthread() at boot is wasteful.  This commit therefore defers
creation of this kthread until the time of the first call_rcu_tasks().

This of course means that the first call_rcu_tasks() must be invoked
from process context after the scheduler is fully operational.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit documents RCU-tasks stall warning messages and also describes
when to use the new cond_resched_rcu_qs() API.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The current RCU-tasks implementation uses strict polling to detect
callback arrivals.  This works quite well, but is not so good for
energy efficiency.  This commit therefore replaces the strict polling
with a wait queue.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds a ten-minute RCU-tasks stall warning.  The actual
time is controlled by the boot/sysfs parameter rcu_task_stall_timeout,
with values less than or equal to zero disabling the stall warnings.
The default value is ten minutes, which means that the tasks that have
not yet responded will get their stacks dumped every ten minutes, until
they pass through a voluntary context switch.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds the TASKS01 and TASKS02 Kconfig fragments, along with
the corresponding TASKS01.boot and TASKS02.boot boot-parameter files
specifying that rcutorture test RCU-tasks instead of the default flavor.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds torture tests for RCU-tasks.  It also fixes a bug that
would segfault for an RCU flavor lacking a callback-barrier function.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit exports the RCU-tasks synchronous APIs,
synchronize_rcu_tasks() and rcu_barrier_tasks(), to
GPL-licensed kernel modules.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Once a task has passed exit_notify() in the do_exit() code path, it
is no longer on the task lists, and is therefore no longer visible
to rcu_tasks_kthread().  This means that an almost-exited task might
be preempted while within a trampoline, and this task won't be waited
on by rcu_tasks_kthread().  This commit fixes this bug by adding an
srcu_struct.  An exiting task does srcu_read_lock() just before calling
exit_notify(), and does the corresponding srcu_read_unlock() after
doing the final preempt_disable().  This means that rcu_tasks_kthread()
can do synchronize_srcu() to wait for all mostly-exited tasks to reach
their final preempt_disable() region, and then use synchronize_sched()
to wait for those tasks to finish exiting.
Reported-by: NOleg Nesterov <oleg@redhat.com>
Suggested-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

It turns out to be easier to add the synchronous grace-period waiting
functions to RCU-tasks than to work around their absense in rcutorture,
so this commit adds them.  The key point is that the existence of
call_rcu_tasks() means that rcutorture needs an rcu_barrier_tasks().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

RCU-tasks requires the occasional voluntary context switch
from CPU-bound in-kernel tasks.  In some cases, this requires
instrumenting cond_resched().  However, there is some reluctance
to countenance unconditionally instrumenting cond_resched() (see
http://lwn.net/Articles/603252/), so this commit creates a separate
cond_resched_rcu_qs() that may be used in place of cond_resched() in
locations prone to long-duration in-kernel looping.

This commit currently instruments only RCU-tasks.  Future possibilities
include also instrumenting RCU, RCU-bh, and RCU-sched in order to reduce
IPI usage.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds a new RCU-tasks flavor of RCU, which provides
call_rcu_tasks().  This RCU flavor's quiescent states are voluntary
context switch (not preemption!) and userspace execution (not the idle
loop -- use some sort of schedule_on_each_cpu() if you need to handle the
idle tasks.  Note that unlike other RCU flavors, these quiescent states
occur in tasks, not necessarily CPUs.  Includes fixes from Steven Rostedt.

This RCU flavor is assumed to have very infrequent latency-tolerant
updaters.  This assumption permits significant simplifications, including
a single global callback list protected by a single global lock, along
with a single task-private linked list containing all tasks that have not
yet passed through a quiescent state.  If experience shows this assumption
to be incorrect, the required additional complexity will be added.
Suggested-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

When starting a new torture run while an old one is still running, both
qemu processes can be outputting to the same console.out file.  This can
cause quite a bit of confusion, so this commit checks for this situation,
which is normally indicated by nul bytes in the console output.  Yes,
if your new run uses up an exact number of blocks of the file, this
check will be ineffective, but the odds are not bad.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>

Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>

Although RCU is designed to handle arbitrary floods of callbacks, this
capability is not routinely tested.   This commit therefore adds a
cbflood capability in which kthreads repeatedly registers large numbers
of callbacks.  One such kthread is created for each four CPUs (rounding
up), and the test may be controlled by several cbflood_* kernel boot
parameters, which control the number of bursts per flood, the number
of callbacks per burst, the time between bursts, and the time between
floods.  The default values are large enough to exercise RCU's emergency
responses to callback flooding.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David Miller <davem@davemloft.net>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>

User pr_alert/pr_cont for printing the logs from rcutorture module directly
instead of writing it to a buffer and then printing it. This allows us from not
having to allocate such buffers. Also remove a resulting empty function.

I tested this using the parse-torture.sh script as follows:

$ dmesg | grep torture > log.txt
$ bash parse-torture.sh log.txt test
$

There were no warnings which means that parsing went fine.
Signed-off-by: NJoe Perches <joe@perches.com>
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit specifies offstack cpumasks in TREE07 in order to catch
references to unallocated cpumask_var_t variables.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>

Setting CONFIG_MAXSMP=y causes cpumasks to be moved offstack, which
introduces the possibility of NULL cpumask_var_t pointers.  This commit
therefore enables CONFIG_MAXSMP=y in TREE01 to increase test coverage.
However, because CONFIG_MAXSMP=y implies 8192 CPUs, we need to use
the maxcpus= boot parameter to limit the number of CPUs to something
reasonable, which in turn requires updating the scripts to handle this.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NPranith Kumar <bobby.prani@gmail.com>