linux-kernel - Re: [ANNOUNCE] v4.11.5-rt1

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Date:   Mon, 19 Jun 2017 10:08:38 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mike Galbraith <efault@....de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [ANNOUNCE] v4.11.5-rt1

On Mon, 19 Jun 2017 12:14:51 +0200
Mike Galbraith <efault@....de> wrote:

> On Mon, 2017-06-19 at 10:52 +0200, Sebastian Andrzej Siewior wrote:
> > On 2017-06-17 10:14:37 [+0200], Mike Galbraith wrote:
> >   
> > > During that rebase, migrate_disable() was changed to no longer map to
> > > preempt_disable() for nonrt, but some patches still assume it does.  It
> > > now depends upon PREEMPT_COUNT, the RT workaround in futex.c induces
> > > grumbling in nonrt builds with PREEMPT_COUNT enabled.  
> > 
> > argh, right. It was planned to get it merged upstream but due to
> > $reasons we never got that far. For that reason I would simply revert
> > that change and let migrate_disable() map to preempt_disable() as it did
> > earlier.  
> 
> Ok, doesn't matter for RT testing.  What does matter, is that...
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 30b24f774198..10e832da70b6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2284,7 +2284,7 @@ EXPORT_SYMBOL(wake_up_process);
>   */
>  int wake_up_lock_sleeper(struct task_struct *p)
>  {
> -       return try_to_wake_up(p, TASK_ALL, WF_LOCK_SLEEPER);
> +       return try_to_wake_up(p, TASK_UNINTERRUPTIBLE, WF_LOCK_SLEEPER);
>  }
> 
> ...appears to be inducing lost futex wakeups.

Hmm, it shouldn't affect futexes, as it's only called by rtmutex when
waiter->savestate is true. And that should always be false for futex.

-- Steve

> 
> Scratch that "appears", changing it to TASK_NORMAL just fixed my DL980
> running otherwise absolutely pristine 4.9-rt21, after having double
> verified that rt20 works fine.  Now to go back to 4.11/master/tip-rt,
> make sure that the little bugger really really REALLY ain't fscking
> with me for the sheer fun of it, futexes being made of pure evil :)
> 
> My testcase is to run futex_wait -n 4 in a modest sized loop.  Odd
> thing is that it only reproduces on the DL980 if I let it use multiple
> sockets, pin it to one, and all is peachy, (rather seems to be given)
> whereas on desktop box, the hang is far more intermittent, but there.
> 
> 	-Mike