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Date:   Wed, 13 Jan 2021 16:36:13 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...nel.org, tglx@...utronix.de, jiangshanlai@...il.com,
        cai@...hat.com, vincent.donnefort@....com, decui@...rosoft.com,
        vincent.guittot@...aro.org, rostedt@...dmis.org, tj@...nel.org
Subject: Re: [PATCH] sched/core: Print out straggler tasks in
 sched_cpu_dying()

On Thu, Jan 14, 2021 at 12:15:24AM +0000, Valentin Schneider wrote:
> On 13/01/21 14:02, Paul E. McKenney wrote:
> >
> > Given that I am not seeing much sched_cpu_dying(), this patch didn't
> > produce any output.  (I will try other configurations.)
> 
> Thanks for giving it a spin! I think with the current series (either
> Lai's or Peter's) sched_cpu_dying() should go smoothly, but you never
> know.

I was running the patch set having one of Lai's and three of Peter's,
which sounds like Peter's.  If I understand which series is which,
Peter's has the advantage of not requiring rcutorture changes.  ;-)

> > However, it did produce the following new-to-me splat, which will
> > hopefully be of some help.
> >
> >                                                       Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > WARNING: CPU: 2 PID: 23 at kernel/kthread.c:508 kthread_set_per_cpu+0x3b/0x50
> 
> Aha, so that's that warning I was expecting to see [1].
> Did you also get the process_one_work() one?

Yes.  Of 112 one-hour runs, there were five process_one_work() splats
and two kthread_set_per_cpu() splats.  Each splat-ridden run had exactly
one splat.

> FWIW I think Peter's suggested approach of killing+respawning the pcpu
> kworkers should prevent at least this one from showing up - all of the
> affinity / flag changes would happen before the task gets enqueued
> anywhere.

Here is hoping!  ;-)

							Thanx, Paul

> [1]: http://lore.kernel.org/r/jhjturkzzv9.mognet@arm.com
> 
> > Modules linked in:
> > CPU: 2 PID: 23 Comm: cpuhp/2 Not tainted 5.11.0-rc3+ #1180
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.11.0-2.el7 04/01/2014
> > RIP: 0010:kthread_set_per_cpu+0x3b/0x50
> > Code: 00 48 85 c0 74 1f 40 84 f6 74 16 81 e2 00 00 00 04 74 1b 83 bf a0 03 00 00
> > +01 75 0e f0 80 08 01 c3 f0 80 20 fe c3 0f 0b eb d0 <0f> 0b eb ee 0f 0b eb e1 0f
> > +1f 00 66 2e 0f 1f 84 00 00 00 00 00 f6
> > RSP: 0000:ffffb25c80103df8 EFLAGS: 00010202
> > RAX: ffff94ac8188ec00 RBX: ffff94ac81390240 RCX: 0000000000000000
> > RDX: 0000000004000000 RSI: 0000000000000001 RDI: ffff94ac818fde00
> > RBP: ffff94ac9f4aadc0 R08: 0000000000000001 R09: 0000000000000001
> > R10: 0000000000000004 R11: 0000000000000000 R12: ffff94ac9f4ab0e8
> > R13: 0000000000000002 R14: ffffffffb9868c40 R15: 00000000ffffffff
> > FS:  0000000000000000(0000) GS:ffff94ac9f480000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000000000000000 CR3: 000000001b022000 CR4: 00000000000006e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> >  workqueue_online_cpu+0x19c/0x360
> >  ? workqueue_prepare_cpu+0x70/0x70
> >  cpuhp_invoke_callback+0x9e/0x890
> >  cpuhp_thread_fun+0x199/0x230
> >  ? _raw_spin_unlock_irqrestore+0x2f/0x50
> >  ? sort_range+0x20/0x20
> >  smpboot_thread_fn+0x193/0x230
> >  kthread+0x13b/0x160
> >  ? kthread_insert_work_sanity_check+0x50/0x50
> >  ret_from_fork+0x22/0x30
> > irq event stamp: 38113
> > hardirqs last  enabled at (38121): [<ffffffffb80dafaa>]
> > +console_unlock+0x46a/0x550
> > hardirqs last disabled at (38130): [<ffffffffb80daf16>]
> > +console_unlock+0x3d6/0x550
> > softirqs last  enabled at (37574): [<ffffffffb9000342>] __do_softirq+0x342/0x48e
> > softirqs last disabled at (37567): [<ffffffffb8e00f92>]
> > +asm_call_irq_on_stack+0x12/0x20
> > ---[ end trace 0b77ae0f211adc14 ]---

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