linux-kernel - Re: [PATCH RT] Defer migrate_enable migration while task state != TASK

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Date:   Mon, 26 Mar 2018 11:35:15 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     joe.korty@...current-rt.com
Cc:     Julia Cartwright <julia@...com>, <bigeasy@...utronix.de>,
        <mingo@...hat.com>, <tglx@...utronix.de>,
        <linux-rt-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RT] Defer migrate_enable migration while task state !=
 TASK_RUNNING

On Fri, 23 Mar 2018 13:21:31 -0400
joe.korty@...current-rt.com wrote:

> My understanding is, in standard Linux and in rt, setting
> task state to anything other than TASK_RUNNING in of itself
> blocks preemption.

That is clearly false. The only thing that blocks preemption with a
CONFIG_PREEMPT kernel is preempt_disable() and local_irq*() disabling.

(Note spin_locks call preempt_disable in non RT).

Otherwise, nothing will stop preemption.

>  A preemption is not really needed here
> as it is expected that there is a schedule() written in that
> will shortly be executed.  And if a 'involuntary schedule'
> (ie, preemption) were allowed to occur between the task
> state set and the schedule(), that would change the task
> state back to TASK_RUNNING, which would cause the schedule
> to NOP.  Thus we risk not having paused long enough here
> for the condition we were waiting for to become true.

That is also incorrect. As Julia mentioned, a preemption keeps the
state of the task.

-- Steve