Building rcutorture as a module requires cpu_up() as well as cpu_down()
exported, so apply EXPORT_SYMBOL_GPL().
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Both TINY_RCU's and TREE_RCU's implementations of rcu_boost() access
the ->boost_tasks and ->exp_tasks fields without preventing concurrent
changes to these fields.  This commit therefore applies ACCESS_ONCE in
order to prevent compiler mischief.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This reverts commit 5342e269.

The approach taken in this patch was deemed too abusive to mutexes,
and thus too likely to result in maintenance problems in the future.
Instead, we will disallow RCU read-side critical sections that partially
overlap with interrupt-disbled code segments.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Tyler Hicks pointed me at an additional article on RCU and I figured
it should probably be mentioned with the others.
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The current rcu_batch_end event trace records only the name of the RCU
flavor and the total number of callbacks that remain queued on the
current CPU.  This is insufficient for testing and tuning the new
dyntick-idle RCU_FAST_NO_HZ code, so this commit adds idle state along
with whether or not any of the callbacks that were ready to invoke
at the beginning of rcu_do_batch() are still queued.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds simple rcutorture tests for srcu_read_lock_raw() and
srcu_read_unlock_raw().  It does not test doing srcu_read_lock_raw()
in an exception handler and releasing it in the corresponding process
context.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The rcutorture test now can automatically exercise CPU hotplug and
collect success statistics, which can be correlated with other rcutorture
activity.  This permits rcutorture to completely exercise RCU regardless
of what sort of userspace and filesystem layout is in use.  Unfortunately,
rcutorture is happy to attempt to offline CPUs that cannot be offlined,
for example, CPU 0 in both the x86 and ARM architectures.  Although this
allows rcutorture testing to proceed normally, it confounds attempts at
error analysis due to the resulting flood of spurious CPU-hotplug errors.

Therefore, this commit uses the new cpu_is_hotpluggable() function to
avoid attempting to offline CPUs that are not hotpluggable, which in
turn avoids spurious CPU-hotplug errors.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

When architectures register CPUs, they indicate whether the CPU allows
hotplugging; notably, x86 and ARM don't allow hotplugging CPU 0.
Userspace can easily query the hotpluggability of a CPU via sysfs;
however, the kernel has no convenient way of accessing that property in
an architecture-independent way.  While the kernel can simply try it and
see, some code needs to distinguish between "hotplug failed" and
"hotplug has no hope of working on this CPU"; for example, rcutorture's
CPU hotplug tests want to avoid drowning out real hotplug failures with
expected failures.

Expose this property via a new cpu_is_hotpluggable function, so that the
rest of the kernel can access it in an architecture-independent way.
Signed-off-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

No point in having two identical rcu_cpu_stall_suppress declarations,
so remove the more obscure of the two.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

If there are other CPUs active at a given point in time, then there is a
limit to what a given CPU can do to advance the current RCU grace period.
Beyond this limit, attempting to force the RCU grace period forward will
do nothing but consume energy burning CPU cycles.

Therefore, this commit takes an adaptive approach to RCU_FAST_NO_HZ
preparations for idle.  It pushes the RCU core state machine for
two cycles unconditionally, and then it will push from zero to three
additional cycles, but only as long as the RCU core has work for this
CPU to do immediately.  The rcu_pending() function is used to check
whether the RCU core has such work.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The rcu_do_batch() function that invokes callbacks for TREE_RCU and
TREE_PREEMPT_RCU normally throttles callback invocation to avoid degrading
scheduling latency.  However, as long as the CPU would otherwise be idle,
there is no downside to continuing to invoke any callbacks that have passed
through their grace periods.  In fact, processing such callbacks in a
timely manner has the benefit of increasing the probability that the
CPU can enter the power-saving dyntick-idle mode.

Therefore, this commit allows callback invocation to continue beyond the
preset limit as long as the scheduler does not have some other task to
run and as long as context is that of the idle task or the relevant
RCU kthread.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Because tasks don't nest, the ->dyntick_nesting must always be zero upon
entry to rcu_idle_enter_common().  Therefore, pass "0" rather than the
counter itself.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Because tasks do not nest, rcu_idle_enter() and rcu_idle_exit() do
not need to check for nesting.  This commit therefore moves nesting
checks from rcu_idle_enter_common() to rcu_irq_exit() and from
rcu_idle_exit_common() to rcu_irq_enter().
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The current implementation of RCU_FAST_NO_HZ prevents CPUs from entering
dyntick-idle state if they have RCU callbacks pending.  Unfortunately,
this has the side-effect of often preventing them from entering this
state, especially if at least one other CPU is not in dyntick-idle state.
However, the resulting per-tick wakeup is wasteful in many cases: if the
CPU has already fully responded to the current RCU grace period, there
will be nothing for it to do until this grace period ends, which will
frequently take several jiffies.

This commit therefore permits a CPU that has done everything that the
current grace period has asked of it (rcu_pending() == 0) even if it
still as RCU callbacks pending.  However, such a CPU posts a timer to
wake it up several jiffies later (6 jiffies, based on experience with
grace-period lengths).  This wakeup is required to handle situations
that can result in all CPUs being in dyntick-idle mode, thus failing
to ever complete the current grace period.  If a CPU wakes up before
the timer goes off, then it cancels that timer, thus avoiding spurious
wakeups.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The intent is that a given RCU read-side critical section be confined
to a single context.  For example, it is illegal to invoke rcu_read_lock()
in an exception handler and then invoke rcu_read_unlock() from the
context of the task that received the exception.
Suggested-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Fixes and workarounds for a number of issues (for example, that in
df4012edc) make it safe to once again detect dyntick-idle CPUs on the
first pass of force_quiescent_state(), so this commit makes that change.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Assertions in rcu_init_percpu_data() unknowingly relied on outgoing
CPUs being turned off before reaching the idle loop.  Unfortunately,
when running under kvm/qemu on x86, CPUs really can get to idle before
begin shut off.  These CPUs are then born in dyntick-idle mode from an
RCU perspective, which results in splats in rcu_init_percpu_data() and
in RCU wrongly ignoring those CPUs despite them being active.  This in
turn can cause RCU to end grace periods prematurely, potentially freeing
up memory that the newly onlined CPUs were still using.  This is most
decidedly not what we need to see in an RCU implementation.

This commit therefore replaces the assertions in rcu_init_percpu_data()
with code that forces RCU's dyntick-idle view of newly onlined CPUs to
match reality.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Re-enable interrupts across calls to quiescent-state functions and
also across force_quiescent_state() to reduce latency.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

With the new implementation of RCU_FAST_NO_HZ, it was possible to hang
RCU grace periods as follows:

o	CPU 0 attempts to go idle, cycles several times through the
	rcu_prepare_for_idle() loop, then goes dyntick-idle when
	RCU needs nothing more from it, while still having at least
	on RCU callback pending.

o	CPU 1 goes idle with no callbacks.

Both CPUs can then stay in dyntick-idle mode indefinitely, preventing
the RCU grace period from ever completing, possibly hanging the system.

This commit therefore prevents CPUs that have RCU callbacks from entering
dyntick-idle mode.  This approach also eliminates the need for the
end-of-grace-period IPIs used previously.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

If a CPU enters dyntick-idle mode with callbacks pending, it will need
an IPI at the end of the grace period.  However, if it exits dyntick-idle
mode before the grace period ends, it will be needlessly IPIed at the
end of the grace period.

Therefore, this commit clears the per-CPU rcu_awake_at_gp_end flag
when a CPU determines that it does not need it.  This in turn requires
disabling interrupts across much of rcu_prepare_for_idle() in order to
avoid having nested interrupts clearing this state out from under us.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The earlier version would attempt to push callbacks through five times
before going into dyntick-idle mode if callbacks remained, but the CPU
had done all that it needed to do for the current RCU grace periods.
This is wasteful:  In most cases, once the CPU has done all that it
needs to for the current RCU grace periods, it will make no further
progress on the callbacks no matter how many times it loops through
the RCU core processing and the idle-entry code.

This commit therefore goes to dyntick-idle mode whenever the current
CPU has done all it can for the current grace period.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds trace_rcu_prep_idle(), which is invoked from
rcu_prepare_for_idle() and rcu_wake_cpu() to trace attempts on
the part of RCU to force CPUs into dyntick-idle mode.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit updates the trace_rcu_dyntick() header comment to reflect
events added by commit 4b4f421.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

An IRC discussion uncovered many conflicting opinions on what types
of data may be atomically loaded and stored.  This commit therefore
calls this out the official set: pointers, longs, ints, and chars (but
not shorts).  This commit also gives some examples of compiler mischief
that can thwart atomicity.

Please note that this discussion is relevant to !SMP kernels if
CONFIG_PREEMPT=y: preemption can cause almost as much trouble as can SMP.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Haavard Skinnemoen <hskinnemoen@gmail.com>
Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Mikael Starvik <starvik@axis.com>
Cc: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Richard Kuo <rkuo@codeaurora.org>
Cc: Jes Sorensen <jes@sgi.com>
Cc: Hirokazu Takata <takata@linux-m32r.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com>
Cc: Jonas Bonn <jonas@southpole.se>
Cc: Kyle McMartin <kyle@mcmartin.ca>
Cc: Helge Deller <deller@gmx.de>
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Chen Liqin <liqin.chen@sunplusct.com>
Cc: Lennox Wu <lennox.wu@gmail.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Chris Zankel <chris@zankel.net>

Those two APIs were provided to optimize the calls of
tick_nohz_idle_enter() and rcu_idle_enter() into a single
irq disabled section. This way no interrupt happening in-between would
needlessly process any RCU job.

Now we are talking about an optimization for which benefits
have yet to be measured. Let's start simple and completely decouple
idle rcu and dyntick idle logics to simplify.
Signed-off-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Running CPU-hotplug operations concurrently with rcutorture has
historically been a good way to find bugs in both RCU and CPU hotplug.
This commit therefore adds an rcutorture module parameter called
"onoff_interval" that causes a randomly selected CPU-hotplug operation to
be executed at the specified interval, in seconds.  The default value of
"onoff_interval" is zero, which disables rcutorture-instigated CPU-hotplug
operations.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Change from direct comparison of ->pid with zero to is_idle_task().
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Acked-by: NChris Metcalf <cmetcalf@tilera.com>

Change from direct comparison of ->pid with zero to is_idle_task().
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Change from direct comparison of ->pid with zero to is_idle_task().
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jason Wessel <jason.wessel@windriver.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Change from direct comparison of ->pid with zero to is_idle_task().
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Change from direct comparison of ->pid with zero to is_idle_task().
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Commit 908a3283 (Fix idle_cpu()) invalidated some uses of idle_cpu(),
which used to say whether or not the CPU was running the idle task,
but now instead says whether or not the CPU is running the idle task
in the absence of pending wakeups.  Although this new implementation
gives a better answer to the question "is this CPU idle?", it also
invalidates other uses that were made of idle_cpu().

This commit therefore introduces a new is_idle_task() API member
that determines whether or not the specified task is one of the
idle tasks, allowing open-coded "->pid == 0" sequences to be replaced
by something more meaningful.
Suggested-by: NJosh Triplett <josh@joshtriplett.org>
Suggested-by: NPeter Zijlstra <peterz@infradead.org>
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently, if rcutorture is built into the kernel, it must be manually
started or started from an init script.  This is inconvenient for
automated KVM testing, where it is good to be able to fully control
rcutorture execution from the kernel parameters.  This patch therefore
adds a module parameter named "rcutorture_runnable" that defaults
to zero ("don't start automatically"), but which can be set to one
to cause rcutorture to start up immediately during boot.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Although it is easy to run rcutorture tests under KVM, there is currently
no nice way to run such a test for a fixed time period, collect all of
the rcutorture data, and then shut the system down cleanly.  This commit
therefore adds an rcutorture module parameter named "shutdown_secs" that
specified the run duration in seconds, after which rcutorture terminates
the test and powers the system down.  The default value for "shutdown_secs"
is zero, which disables shutdown.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The new implementation of RCU_FAST_NO_HZ is compatible with preemptible
RCU, so this commit removes the Kconfig restriction that previously
prohibited this.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

RCU has traditionally relied on idle_cpu() to determine whether a given
CPU is running in the context of an idle task, but commit 908a3283
(Fix idle_cpu()) has invalidated this approach.  After commit 908a3283,
idle_cpu() will return true if the current CPU is currently running the
idle task, and will be doing so for the foreseeable future.  RCU instead
needs to know whether or not the current CPU is currently running the
idle task, regardless of what the near future might bring.

This commit therefore switches from idle_cpu() to "current->pid != 0".
Reported-by: NWu Fengguang <fengguang.wu@intel.com>
Suggested-by: NCarsten Emde <C.Emde@osadl.org>
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Acked-by: NSteven Rostedt <rostedt@goodmis.org>
Tested-by: NWu Fengguang <fengguang.wu@intel.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Currently, RCU does not permit a CPU to enter dyntick-idle mode if that
CPU has any RCU callbacks queued.  This means that workloads for which
each CPU wakes up and does some RCU updates every few ticks will never
enter dyntick-idle mode.  This can result in significant unnecessary power
consumption, so this patch permits a given to enter dyntick-idle mode if
it has callbacks, but only if that same CPU has completed all current
work for the RCU core.  We determine use rcu_pending() to determine
whether a given CPU has completed all current work for the RCU core.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The current code just complains if the current task is not the idle task.
This commit therefore adds printing of the identity of the idle task.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The trace_rcu_dyntick() trace event did not print both the old and
the new value of the nesting level, and furthermore printed only
the low-order 32 bits of it.  This could result in some confusion
when interpreting trace-event dumps, so this commit prints both
the old and the new value, prints the full 64 bits, and also selects
the process-entry/exit increment to print nicely in hexadecimal.
Signed-off-by: NPaul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Update various files in Documentation/RCU to reflect srcu_read_lock_raw()
and srcu_read_unlock_raw().  Credit to Peter Zijlstra for suggesting
use of the existing _raw suffix instead of the earlier bulkref names.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>