linux-kernel - Re: [PATCH RT 6/8] sched: migrate_enable: Set state to TASK

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Date:   Tue, 17 Sep 2019 17:31:10 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Scott Wood <swood@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Clark Williams <williams@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: [PATCH RT 6/8] sched: migrate_enable: Set state to TASK_RUNNING

On 2019-07-27 00:56:36 [-0500], Scott Wood wrote:
> If migrate_enable() is called while a task is preparing to sleep
> (state != TASK_RUNNING), that triggers a debug check in stop_one_cpu().
> Explicitly reset state to acknowledge that we're accepting the spurious
> wakeup.
> 
> Signed-off-by: Scott Wood <swood@...hat.com>
> ---
>  kernel/sched/core.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 38a9a9df5638..eb27a9bf70d7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7396,6 +7396,14 @@ void migrate_enable(void)
>  			unpin_current_cpu();
>  			preempt_lazy_enable();
>  			preempt_enable();
> +
> +			/*
> +			 * Avoid sleeping with an existing non-running
> +			 * state.  This will result in a spurious wakeup
> +			 * for the calling context.
> +			 */
> +			__set_current_state(TASK_RUNNING);

Do you have an example for this? I'm not too sure if we are not using a
state by doing this. Actually we were losing it and get yelled at.  We do:
|rt_spin_unlock()
|{
|         rt_spin_lock_fastunlock();
|         migrate_enable();
|}

So save the state as part of the locking process and lose it as part of
migrate_enable() if the CPU mask was changed. I *think* we need to
preserve that state until after the migration to the other CPU.

>  			sleeping_lock_inc();
>  			stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
>  			sleeping_lock_dec();

Sebastian