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Message-ID: <f18c42e4-ead7-fbcd-b7da-6677e8485be9@codeaurora.org> Date: Thu, 26 Apr 2018 09:34:36 +0530 From: "Kohli, Gaurav" <gkohli@...eaurora.org> To: Peter Zijlstra <peterz@...radead.org> Cc: 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 4/26/2018 1:39 AM, Peter Zijlstra wrote: > On Wed, Apr 25, 2018 at 02:03:19PM +0530, Gaurav Kohli wrote: >> diff --git a/kernel/smpboot.c b/kernel/smpboot.c >> index 5043e74..c5c5184 100644 >> --- a/kernel/smpboot.c >> +++ b/kernel/smpboot.c >> @@ -122,7 +122,45 @@ static int smpboot_thread_fn(void *data) >> } >> >> if (kthread_should_park()) { >> + /* >> + * Serialize against wakeup. > * > * Prior wakeups must complete and later wakeups > * will observe TASK_RUNNING. > * > * This avoids the case where the TASK_RUNNING > * store from ttwu() competes with the > * TASK_PARKED store from kthread_parkme(). > * > * If the TASK_PARKED store looses that > * competition, kthread_unpark() will go wobbly. >> + */ >> + raw_spin_lock(¤t->pi_lock); >> __set_current_state(TASK_RUNNING); >> + raw_spin_unlock(¤t->pi_lock); >> preempt_enable(); >> if (ht->park && td->status == HP_THREAD_ACTIVE) { >> BUG_ON(td->cpu != smp_processor_id()); > Does that work for you? We have given patch for testing, usually it takes around 2-3 days for reproduction(we will update for the same). > > But looking at this a bit more; don't we have the exact same problem > with the TASK_RUNNING store in the !ht->thread_should_run() case? > Suppose a ttwu() happens concurrently there, it can end up competing > against the TASK_INTERRUPTIBLE store, no? > > Of course, that race is not fatal, we'll just end up going around the > loop once again I suppose. Maybe a comment there too? > > /* > * A similar race is possible here, but loosing > * the TASK_INTERRUPTIBLE store is harmless and > * will make us go around the loop once more. > */ Actually instead of race, i am seeing wakeup miss problem which is very rare, if we take case of hotplug thread Controller Hotplug Loop start set_current_state(TASK_INTERRUPTIBLE); if (kthread_should_park()) { -> fails Set Should_park then wake_up if (!ht->thread_should_run(td->cpu)) { preempt_enable_no_resched(); schedule(); Again went to schedule(which is very rare to occur,not sure whether it hits) > > And of course, I suspect we actually want to use TASK_IDLE, smpboot > threads don't want signals do they? But that probably ought to be a > separate patch. Yes I agree, we can control race from here as well, Please suggest would below change be any help here: } else { __set_current_state(TASK_RUNNING); preempt_enable(); ht->thread_fn(td->cpu); + set_current_state(TASK_INTERRUPTIBLE); + schedule(); } > -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
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