linux-kernel - Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

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Date:   Wed, 6 Jun 2018 20:59:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     "Kohli, Gaurav" <gkohli@...eaurora.org>, tglx@...utronix.de,
        mpe@...erman.id.au, mingo@...nel.org, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against
 wakeup

On Wed, Jun 06, 2018 at 03:51:16PM +0200, Oleg Nesterov wrote:
> IIUC, this will only affect smpboot_update_cpumask_percpu_thread() which can hit
> an already parked thread, but it doesn't need to wait.
> 
> And it seems that smpboot_update_cpumask_percpu_thread() in turn needs some cleanups.
> Hmm. and its single user: kernel/watchdog.c.
> 
> And speaking of watchdog.c, can't we simply kill the "watchdog/%u" threads? This is
> off-topic, but can't watchdog_timer_fn() use stop_one_cpu_nowait(watchdog) ?
> 
> And I really think we should unexport kthread_park/unpark(), only smpboot_thread_fn()
> should use them. kthread() should not play with __kthread_parkme(). And even
> KTHREAD_SHOULD_PARK must die, I mean it should live in struct smp_hotplug_thread,
> not in struct kthread.
> 
> OK, this is off-topic too.

> And, let me repeat, can't we avoid complete_all() ?

Yes, or at least if that watchdog crap is the only user.

I have most of the patch reworking watchdog.c to use stop_one_cpu*(),
and that cleans up lots -- of course, I've not tested it yet, so it
could also be breaking lots :-)