linux-kernel - Re: [PATCH tip/core/rcu 02/26] mm/mmap.c: Add cond_resched() for exit

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Date:   Tue, 23 Jun 2020 13:55:08 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, linux-mm@...ck.org
Subject: Re: [PATCH tip/core/rcu 02/26] mm/mmap.c: Add cond_resched() for
 exit_mmap() CPU stalls

On Tue, Jun 23, 2020 at 03:34:31PM -0400, Joel Fernandes wrote:
> On Mon, Jun 22, 2020 at 05:21:23PM -0700, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.org>
> > 
> > A large process running on a heavily loaded system can encounter the
> > following RCU CPU stall warning:
> > 
> >   rcu: INFO: rcu_sched self-detected stall on CPU
> >   rcu: \x093-....: (20998 ticks this GP) idle=4ea/1/0x4000000000000002 softirq=556558/556558 fqs=5190
> >   \x09(t=21013 jiffies g=1005461 q=132576)
> >   NMI backtrace for cpu 3
> >   CPU: 3 PID: 501900 Comm: aio-free-ring-w Kdump: loaded Not tainted 5.2.9-108_fbk12_rc3_3858_gb83b75af7909 #1
> >   Hardware name: Wiwynn   HoneyBadger/PantherPlus, BIOS HBM6.71 02/03/2016
> >   Call Trace:
> >    <IRQ>
> >    dump_stack+0x46/0x60
> >    nmi_cpu_backtrace.cold.3+0x13/0x50
> >    ? lapic_can_unplug_cpu.cold.27+0x34/0x34
> >    nmi_trigger_cpumask_backtrace+0xba/0xca
> >    rcu_dump_cpu_stacks+0x99/0xc7
> >    rcu_sched_clock_irq.cold.87+0x1aa/0x397
> >    ? tick_sched_do_timer+0x60/0x60
> >    update_process_times+0x28/0x60
> >    tick_sched_timer+0x37/0x70
> >    __hrtimer_run_queues+0xfe/0x270
> >    hrtimer_interrupt+0xf4/0x210
> >    smp_apic_timer_interrupt+0x5e/0x120
> >    apic_timer_interrupt+0xf/0x20
> >    </IRQ>
> >   RIP: 0010:kmem_cache_free+0x223/0x300
> >   Code: 88 00 00 00 0f 85 ca 00 00 00 41 8b 55 18 31 f6 f7 da 41 f6 45 0a 02 40 0f 94 c6 83 c6 05 9c 41 5e fa e8 a0 a7 01 00 41 56 9d <49> 8b 47 08 a8 03 0f 85 87 00 00 00 65 48 ff 08 e9 3d fe ff ff 65
> >   RSP: 0018:ffffc9000e8e3da8 EFLAGS: 00000206 ORIG_RAX: ffffffffffffff13
> >   RAX: 0000000000020000 RBX: ffff88861b9de960 RCX: 0000000000000030
> >   RDX: fffffffffffe41e8 RSI: 000060777fe3a100 RDI: 000000000001be18
> >   RBP: ffffea00186e7780 R08: ffffffffffffffff R09: ffffffffffffffff
> >   R10: ffff88861b9dea28 R11: ffff88887ffde000 R12: ffffffff81230a1f
> >   R13: ffff888854684dc0 R14: 0000000000000206 R15: ffff8888547dbc00
> >    ? remove_vma+0x4f/0x60
> >    remove_vma+0x4f/0x60
> >    exit_mmap+0xd6/0x160
> >    mmput+0x4a/0x110
> >    do_exit+0x278/0xae0
> >    ? syscall_trace_enter+0x1d3/0x2b0
> >    ? handle_mm_fault+0xaa/0x1c0
> >    do_group_exit+0x3a/0xa0
> >    __x64_sys_exit_group+0x14/0x20
> >    do_syscall_64+0x42/0x100
> >    entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > 
> > And on a PREEMPT=n kernel, the "while (vma)" loop in exit_mmap() can run
> > for a very long time given a large process.  This commit therefore adds
> > a cond_resched() to this loop, providing RCU any needed quiescent states.
> > 
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: <linux-mm@...ck.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >  mm/mmap.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 59a4682..972f839 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -3159,6 +3159,7 @@ void exit_mmap(struct mm_struct *mm)
> >  		if (vma->vm_flags & VM_ACCOUNT)
> >  			nr_accounted += vma_pages(vma);
> >  		vma = remove_vma(vma);
> > +		cond_resched();
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Thank you!  I will apply this on my next rebase.

> Just for my understanding, cond_resched_tasks_rcu_qs() may not help here
> because preemption is not disabled right? Still I see no harm in using it
> here either as it may give a slight speed up for tasks-RCU.

The RCU-tasks stall-warning interval is ten minutes, and I have not yet
seen evidence that we are getting close to that.  If we do, then yes,
a cond_resched_tasks_rcu_qs() might be in this code's future.  But it
does add overhead, so we need to see the evidence first.

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> >  	}
> >  	vm_unacct_memory(nr_accounted);
> >  }
> > -- 
> > 2.9.5
> >