linux-kernel - Re: [PATCH 1/6] sched: Fix ttwu_queue

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Date:   Mon, 22 Jun 2020 11:41:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     mingo@...nel.org, tglx@...utronix.de, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        paulmck@...nel.org, frederic@...nel.org
Subject: Re: [PATCH 1/6] sched: Fix ttwu_queue_cond()

On Mon, Jun 22, 2020 at 10:11:22AM +0100, Mel Gorman wrote:
> On Mon, Jun 15, 2020 at 02:56:55PM +0200, Peter Zijlstra wrote:
> > Where the condition:
> > 
> >   !cpus_share_cache(smp_processor_id(), cpu)
> > 
> > already implies 'cpu != smp_processor_id()', because a CPU always
> > shares cache with itself, the secondary condition added in commit:
> > 
> >   2ebb17717550 ("sched/core: Offload wakee task activation if it the wakee is descheduling")
> > 
> > voids that implication, resulting in attempting to do local wake-ups
> > through the queue mechanism.
> > 
> > Fixes: 2ebb17717550 ("sched/core: Offload wakee task activation if it the wakee is descheduling")
> > Reported-by: Paul E. McKenney <paulmck@...nel.org>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Tested-by: Paul E. McKenney <paulmck@...nel.org>
> 
> Yep, I mistakenly though this would be covered by the self-wakeup check
> early in try_to_wake_up() but it is not

It is, one should not be able to observe 'p->on_cpu && task_cpu(cpu) ==
smp_processor_id()); I've since found the actual problem, find here:

  https://lkml.kernel.org/r/20200620184622.GA19696@paulmck-ThinkPad-P72

I'm currently polishing the changelog a little, and will shortly post a
new version of that.