linux-kernel - Re: [PATCH] sched/rt: Don't try push tasks if there are none.

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Message-ID: <xhsmhv8dob13r.mognet@vschneid.remote.csb>
Date:   Wed, 09 Aug 2023 18:02:32 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Cc:     Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/rt: Don't try push tasks if there are none.

On 01/08/23 17:26, Sebastian Andrzej Siewior wrote:
> I have a RT task X at a high priority and cyclictest on each CPU with
> lower priority than X's. If X is active and each CPU wakes their own
> cylictest thread then it ends in a longer rto_push storm.
> A random CPU determines via balance_rt() that the CPU on which X is
> running needs to push tasks. X has the highest priority, cyclictest is
> next in line so there is nothing that can be done since the task with
> the higher priority is not touched.
>
> tell_cpu_to_push() increments rto_loop_next and schedules
> rto_push_irq_work_func() on X's CPU. The other CPUs also increment the
> loop counter and do the same. Once rto_push_irq_work_func() is active it
> does nothing because it has _no_ pushable tasks on its runqueue. Then
> checks rto_next_cpu() and decides to queue irq_work on the local CPU
> because another CPU requested a push by incrementing the counter.
>

For a CPU to be in the rto_mask, it needs:

  rt_rq->rt_nr_migratory && rt_rq->rt_nr_total > 1

But if that CPU has no pushable tasks, then that means only the current
task has p->nr_cpus_allowed > 1.

Should we change it so a CPU is only in the rto_mask iff it has pushable
tasks? AFAICT that should not break the case where we push the current task
away due to migration_disabled, as that still relies on the
migration_disabled task to be in the pushable list.