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Subject: Re: [PATCH 0/9] sched: Migrate disable support
To:     Peter Zijlstra <peterz@infradead.org>, tglx@linutronix.de,
        mingo@kernel.org
Cc:     linux-kernel@vger.kernel.org, bigeasy@linutronix.de,
        qais.yousef@arm.com, swood@redhat.com, valentin.schneider@arm.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vincent.donnefort@arm.com
References: <20200921163557.234036895@infradead.org>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <6f55a303-0e5c-8e84-65d3-798b589a5d75@arm.com>
Date:   Fri, 25 Sep 2020 11:12:09 +0200
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On 21/09/2020 18:35, Peter Zijlstra wrote:
> Hi,
> 
> Here's my take on migrate_disable(). It avoids growing a second means of
> changing the affinity, documents how the things violates locking rules but
> still mostly works.
> 
> It also avoids blocking completely, so no more futex band-aids required.
> 
> Also, no more atomics/locks on the fast path.
> 
> I also put in a comment that explains how the whole concept is fundamentally
> flawed but a necessary evil to get PREEMPT_RT going -- for now.
> 
> Somewhat tested with PREEMPT_RT.
> 
> ---
>  include/linux/cpuhotplug.h    |    1 
>  include/linux/preempt.h       |   60 +++
>  include/linux/sched.h         |    4 
>  include/linux/sched/hotplug.h |    2 
>  include/linux/stop_machine.h  |    5 
>  kernel/cpu.c                  |    9 
>  kernel/sched/core.c           |  673 +++++++++++++++++++++++++++++++-----------
>  kernel/sched/deadline.c       |    5 
>  kernel/sched/sched.h          |   24 +
>  kernel/stop_machine.c         |   23 +
>  lib/dump_stack.c              |    2 
>  lib/smp_processor_id.c        |    5 
>  12 files changed, 635 insertions(+), 178 deletions(-)
> 

I get this when running 6 (periodic) RT50 tasks with CPU hp stress on my
6 CPU JUNO board (!CONFIG_PREEMPT_RT).

[   55.490263] ------------[ cut here ]------------
[   55.505261] Modules linked in:
[   55.508322] CPU: 3 PID: 24 Comm: migration/3 Not tainted
5.9.0-rc1-00132-gc096e6406c50-dirty #90
[   55.517119] Hardware name: ARM Juno development board (r0) (DT)
[   55.523058] Stopper: multi_cpu_stop+0x0/0x170 <- 0x0
[   55.528029] pstate: 20000085 (nzCv daIf -PAN -UAO BTYPE=--)
[   55.533612] pc : sched_cpu_dying+0x124/0x130
[   55.537887] lr : sched_cpu_dying+0xd8/0x130
[   55.542071] sp : ffff800011f0bca0
[   55.545385] x29: ffff800011f0bca0 x28: 0000000000000002
[   55.550703] x27: 0000000000000000 x26: 0000000000000060
[   55.556022] x25: 0000000000000000 x24: 0000000000000001
[   55.561340] x23: 0000000000000000 x22: 0000000000000003
[   55.566659] x21: 0000000000000080 x20: 0000000000000003
[   55.571977] x19: ffff00097ef9e1c0 x18: 0000000000000010
[   55.577295] x17: 0000000000000000 x16: 0000000000000000
[   55.582613] x15: 0000000000000000 x14: 000000000000015c
[   55.587932] x13: 0000000000000000 x12: 00000000000006f1
[   55.593250] x11: 0000000000000080 x10: 0000000000000000
[   55.598567] x9 : 0000000000000003 x8 : ffff0009743f5900
[   55.603886] x7 : 0000000000000003 x6 : 0000000000000000
[   55.609204] x5 : 0000000000000001 x4 : 0000000000000002
[   55.614521] x3 : 0000000000000000 x2 : 0000000000000013
[   55.619839] x1 : 0000000000000008 x0 : 0000000000000003
[   55.625158] Call trace:
[   55.627607]  sched_cpu_dying+0x124/0x130
[   55.631535]  cpuhp_invoke_callback+0x88/0x210
[   55.635897]  take_cpu_down+0x7c/0xd8
[   55.639475]  multi_cpu_stop+0xac/0x170
[   55.643227]  cpu_stopper_thread+0x98/0x130
[   55.647327]  smpboot_thread_fn+0x1c4/0x280
[   55.651427]  kthread+0x140/0x160
[   55.654658]  ret_from_fork+0x10/0x34
[   55.658239] Code: f000e1c1 913fc021 1400034a 17ffffde (d4210000)
[   55.664342] ---[ end trace c5b8988b7b701e56 ]---
[   55.668963] note: migration/3[24] exited with preempt_count 3

7309 int sched_cpu_dying(unsigned int cpu)
    ...
    BUG_ON(rq->nr_running != 1 || rq_has_pinned_tasks(rq));
    ...

rq->nr_running is always 2 here in this cases.

balance_hotplug_wait and sched_cpu_wait_empty run in cpuhp/X (CFS)
whereas sched_cpu_dying in migration/X ?