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

From: Sasha Levin <sasha.levin@oracle.com>
To: linux-kernel@vger.kernel.org, mingo@kernel.org,
	fweisbec@gmail.com, tglx@linutronix.de, hpa@zytor.com,
	torvalds@linux-foundation.org, peterz@infradead.org,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [tip:sched/core] sched/preempt: Optimize preemption operations on __schedule() callers
Date: Mon, 20 Jul 2015 18:14:27 -0400	[thread overview]
Message-ID: <55AD72C3.4000300@oracle.com> (raw)
In-Reply-To: <tip-b30f0e3ffedfa52b1d67a302ae5860c49998e5e2@git.kernel.org>

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.

Thanks,
Sasha