Re: [tip:sched/core] sched/preempt: Optimize preemption operations on __schedule() callers

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Jul 20, 2015 at 06:14:27PM -0400, Sasha Levin wrote:
> On 05/19/2015 03:17 AM, tip-bot for Frederic Weisbecker wrote:
> > Commit-ID:  b30f0e3ffedfa52b1d67a302ae5860c49998e5e2
> > Gitweb:     http://git.kernel.org/tip/b30f0e3ffedfa52b1d67a302ae5860c49998e5e2
> > Author:     Frederic Weisbecker <fweisbec@gmail.com>
> > AuthorDate: Tue, 12 May 2015 16:41:49 +0200
> > Committer:  Ingo Molnar <mingo@kernel.org>
> > CommitDate: Tue, 19 May 2015 08:39:12 +0200
> > 
> > sched/preempt: Optimize preemption operations on __schedule() callers
> > 
> > __schedule() disables preemption and some of its callers
> > (the preempt_schedule*() family) also set PREEMPT_ACTIVE.
> > 
> > So we have two preempt_count() modifications that could be performed
> > at once.
> > 
> > Lets remove the preemption disablement from __schedule() and pull
> > this responsibility to its callers in order to optimize preempt_count()
> > operations in a single place.
> > 
> > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Link: http://lkml.kernel.org/r/1431441711-29753-5-git-send-email-fweisbec@gmail.com
> > Signed-off-by: Ingo Molnar <mingo@kernel.org>
> 
> Hi all,
> 
> I was seeing RCU stall warnings that appeared to have different backtrace
> each time, but would reliably trigger when fuzzing and looked something like this:
> 
> [4394886.414687] INFO: rcu_preempt detected stalls on CPUs/tasks:
> [4394886.416717]        Tasks blocked on level-0 rcu_node (CPUs 0-23): P13356
> [4394886.418699]        (detected by 2, t=20502 jiffies, g=3548, c=3547, q=120)
> [4394886.420712] trinity-c42     R  running task    26936 13356   9574 0x10000000
> [4394886.422466]  ffff8807d1c7fbe8 ffff8807d1c7fb88 0000000000000022 ffff8801081e1bb0
> [4394886.423711]  ffff8801081e1b88 ffff8807d1eb8780 ffff8801081e11d8 ffff8807e2564000
> [4394886.424956]  ffff8807d1eb8000 ffff8807d1c7fbd8 ffff8807d1c78000 0000000000000000
> [4394886.426196] Call Trace:
> [4394886.426627] preempt_schedule_irq (./arch/x86/include/asm/paravirt.h:807 kernel/sched/core.c:3218)
> [4394886.427582] ? lockdep_reset_lock (kernel/locking/lockdep.c:3105)
> [4394886.428548] ? kill_pid_info (include/linux/rcupdate.h:857 kernel/signal.c:1340)
> [4394886.429438] retint_kernel (arch/x86/entry/entry_64.S:578)
> [4394886.430286] ? kill_pid_info (include/linux/rcupdate.h:857 kernel/signal.c:1340)
> [4394886.431176] ? native_restore_fl (./arch/x86/include/asm/irqflags.h:35)
> [4394886.432376] lock_is_held (kernel/locking/lockdep.c:3661)
> [4394886.433241] ? kill_pid_info (include/linux/rcupdate.h:914 kernel/signal.c:1344)
> [4394886.434135] rcu_read_lock_held (kernel/rcu/update.c:275)
> [4394886.435012] pid_task (kernel/pid.c:440 (discriminator 5))
> [4394886.435656] kill_pid_info (kernel/signal.c:1341)
> [4394886.436351] ? kill_pid_info (include/linux/rcupdate.h:857 kernel/signal.c:1340)
> [4394886.437065] SYSC_kill (kernel/signal.c:1426 kernel/signal.c:2903)
> [4394886.437726] ? SYSC_kill (include/linux/rcupdate.h:857 kernel/signal.c:1425 kernel/signal.c:2903)
> [4394886.438419] ? find_get_pid (include/linux/rcupdate.h:914 kernel/pid.c:494)
> [4394886.439134] ? kill_pid (kernel/signal.c:2894)
> [4394886.439782] ? find_get_pid (kernel/pid.c:497)
> [4394886.440492] ? find_get_pid (kernel/pid.c:489)
> [4394886.441183] ? lock_is_held (kernel/locking/lockdep.c:3661)
> [4394886.441929] ? rcu_read_lock_sched_held (kernel/rcu/update.c:109)
> [4394886.442772] ? syscall_trace_enter_phase2 (arch/x86/entry/common.c:196)
> [4394886.443632] SyS_kill (kernel/signal.c:2893)
> [4394886.444255] tracesys_phase2 (arch/x86/entry/entry_64.S:270)
> 
> I worked with Paul to rule out RCU as the cause.
> 
> I've noticed that all traces had one thing in common: being stuck in preempt_schedule_irq(),
> so I've looked at recent changes there and noticed this commit.
> 
> I've tried testing the commit before that, and the problem went away. Checking out this
> commit the problem reappeared.

Do you have any idea how to reproduce that?

Thanks.