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Date:   Wed, 13 Jan 2021 14:02:44 -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 Wed, Jan 13, 2021 at 06:31:41PM +0000, Valentin Schneider wrote:
> Since commit
> 
>   1cf12e08bc4d ("sched/hotplug: Consolidate task migration on CPU unplug")
> 
> tasks are expected to move themselves out of a out-going CPU. For most
> tasks this will be done automagically via BALANCE_PUSH, but percpu kthreads
> will have to cooperate and move themselves away one way or another.
> 
> Currently, some percpu kthreads (workqueues being a notable exemple) do not
> cooperate nicely and can end up on an out-going CPU at the time
> sched_cpu_dying() is invoked.
> 
> Print the dying rq's tasks to shed some light on the stragglers.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
> As Peter pointed out, this should really be caught much earlier than
> sched_cpu_dying().
> 
> If we go down the route of preventing kthreads from being affined to
> !active CPUs in __set_cpus_allowed_ptr() (genuine percpu kthreads sidestep
> it via kthread_bind_mask()), then I *think* we could catch this in wakeups,
> i.e. select_task_rq(). I've been playing around there, but it's not as
> straightforward as I'd have hoped.
> ---

Given that I am not seeing much sched_cpu_dying(), this patch didn't
produce any output.  (I will try other configurations.)

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
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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