From: Peter Zijlstra <peterz@infradead.org>
To: "Kohli, Gaurav" <gkohli@codeaurora.org>
Cc: tglx@linutronix.de, mpe@ellerman.id.au, mingo@kernel.org,
bigeasy@linutronix.de, linux-kernel@vger.kernel.org,
linux-arm-msm@vger.kernel.org,
Neeraj Upadhyay <neeraju@codeaurora.org>,
Will Deacon <will.deacon@arm.com>,
Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup
Date: Tue, 1 May 2018 13:31:32 +0200 [thread overview]
Message-ID: <20180501113132.GF12217@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <f134c8d8-e56f-bf29-0177-e6702ae79f2b@codeaurora.org>
On Tue, May 01, 2018 at 04:10:53PM +0530, Kohli, Gaurav wrote:
> Yes with loop, it will reset TASK_PARKED but that is not happening in the
> dumps we have seen.
But was that with or without the fixed wait-loop? I don't care about
stuff you might have seen with the current code, that is clearly broken.
> > takedown_cpu() can proceed beyond smpboot_park_threads() and kill the
> > CPU before any of the threads are parked -- per having the complete()
> > before hitting schedule().
> >
> > And, afaict, that is harmless. When we go offline, sched_cpu_dying() ->
> > migrate_tasks() will migrate any still runnable threads off the cpu.
> > But because at this point the thread must be in the PARKED wait-loop, it
> > will hit schedule() and go to sleep eventually.
> >
> > Also note that kthread_unpark() does __kthread_bind() to rebind the
> > threads.
> >
> > Aaaah... I think I've spotted a problem there. We clear SHOULD_PARK
> > before we rebind, so if the thread lost the first PARKED store,
> > does the completion, gets migrated, cycles through the loop and now
> > observes !SHOULD_PARK and bails the wait-loop, then __kthread_bind()
> > will forever wait.
> >
>
> So during next unpark
> __kthread_unpark -> __kthread_bind -> wait_task_inactive (this got failed,
> as current state is running so failed on below call:
Aah, yes, I seem to have mis-remembered how wait_task_inactive() works.
And it is indeed still a problem..
Let me ponder what the best solution is, it's a bit of a mess.
next prev parent reply other threads:[~2018-05-01 11:31 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-25 8:33 [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup Gaurav Kohli
2018-04-25 20:09 ` Peter Zijlstra
2018-04-26 4:04 ` Kohli, Gaurav
2018-04-26 9:14 ` Peter Zijlstra
2018-04-26 8:41 ` Peter Zijlstra
2018-04-26 8:57 ` Peter Zijlstra
2018-04-26 15:53 ` Kohli, Gaurav
2018-04-30 11:17 ` Peter Zijlstra
2018-05-01 7:50 ` Kohli, Gaurav
2018-05-01 10:18 ` Peter Zijlstra
2018-05-01 10:40 ` Peter Zijlstra
2018-05-01 10:40 ` Kohli, Gaurav
2018-05-01 11:31 ` Peter Zijlstra [this message]
2018-05-01 11:46 ` Kohli, Gaurav
2018-05-01 13:19 ` Peter Zijlstra
2018-05-02 5:15 ` Kohli, Gaurav
2018-05-02 8:20 ` Peter Zijlstra
2018-05-02 10:13 ` Kohli, Gaurav
2018-05-07 11:09 ` Kohli, Gaurav
2018-05-07 11:23 ` Kohli, Gaurav
2018-06-05 11:13 ` Kohli, Gaurav
2018-06-05 15:08 ` Oleg Nesterov
2018-06-05 15:22 ` Peter Zijlstra
2018-06-05 15:40 ` Peter Zijlstra
2018-06-05 16:35 ` Oleg Nesterov
2018-06-05 18:21 ` Kohli, Gaurav
2018-06-05 20:13 ` Peter Zijlstra
2018-06-06 13:51 ` Oleg Nesterov
2018-06-06 15:03 ` Peter Zijlstra
2018-06-06 15:04 ` Peter Zijlstra
2018-06-06 15:22 ` Peter Zijlstra
2018-06-06 18:59 ` Peter Zijlstra
2018-06-07 8:30 ` Kohli, Gaurav
2018-05-01 10:44 ` Peter Zijlstra
2018-04-26 16:02 ` Andrea Parri
2018-04-26 16:18 ` Oleg Nesterov
2018-04-30 11:20 ` Peter Zijlstra
2018-04-30 11:56 ` Peter Zijlstra
2018-04-28 6:43 ` [lkp-robot] [kthread/smpboot] cad8e99675: inconsistent{IN-HARDIRQ-W}->{HARDIRQ-ON-W}usage kernel test robot
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