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

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Date:   Fri, 23 Mar 2018 12:42:02 -0500
From:   Julia Cartwright <julia@...com>
To:     Joe Korty <joe.korty@...current-rt.com>
CC:     <bigeasy@...utronix.de>, <mingo@...hat.com>, <tglx@...utronix.de>,
        <rostedt@...dmis.org>, <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, Mar 23, 2018 at 01:21:31PM -0400, joe.korty@...current-rt.com wrote:
> On Fri, Mar 23, 2018 at 11:59:21AM -0500, Julia Cartwright wrote:
> > On Fri, Mar 23, 2018 at 11:09:59AM -0400, joe.korty@...current-rt.com wrote:
> > > I see the below kernel splat in 4.9-rt when I run a test program that
> > > continually changes the affinity of some set of running pids:
> > >
> > >    do not call blocking ops when !TASK_RUNNING; state=2 set at ...
> > >       ...
> > >       stop_one_cpu+0x60/0x80
> > >       migrate_enable+0x21f/0x3e0
> > >       rt_spin_unlock+0x2f/0x40
> > >       prepare_to_wait+0x5c/0x80
> > >       ...
> >
> > This is clearly a problem.
> >
> > > The reason is that spin_unlock, write_unlock, and read_unlock call
> > > migrate_enable, and since 4.4-rt, migrate_enable will sleep if it discovers
> > > that a migration is in order.  But sleeping in the unlock services is not
> > > expected by most kernel developers,
> >
> > I don't buy this, see below:
> >
> > > and where that counts most is in code sequences like the following:
> > >
> > >   set_current_state(TASK_UNINTERRUPIBLE);
> > >   spin_unlock(&s);
> > >   schedule();
> >
> > The analog in mainline is CONFIG_PREEMPT and the implicit
> > preempt_enable() in spin_unlock().  In this configuration, a kernel
> > developer should _absolutely_ expect their task to be suspended (and
> > potentially migrated), _regardless of the task state_ if there is a
> > preemption event on the CPU on which this task is executing.
> >
> > Similarly, on RT, there is nothing _conceptually_ wrong on RT with
> > migrating on migrate_enable(), regardless of task state, if there is a
> > pending migration event.
>
> My understanding is, in standard Linux and in rt, setting
> task state to anything other than TASK_RUNNING in of itself
> blocks preemption.

I'm assuming you're referring to the window of time between a task
setting its state to !TASK_RUNNING and schedule()?  The task remains
preemptible in this window.

> 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;

This isn't the case.  A preempted task preserves its state.

   Julia