linux-kernel - Re: [patch 10/12] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers on RT

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Date:   Fri, 26 Jul 2019 17:30:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Julia Cartwright <julia@...com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [patch 10/12] hrtimer: Determine hard/soft expiry mode for
 hrtimer sleepers on RT

On Fri, 26 Jul 2019 21:16:24 +0000
Julia Cartwright <julia@...com> wrote:

> > +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +		if (task_is_realtime(current) && !(mode & HRTIMER_MODE_SOFT))
> > +			mode |= HRTIMER_MODE_HARD;  
> 
> Because this ends up sampling the tasks' scheduling parameters only at
> the time of enqueue, it doesn't take into consideration whether or not
> the task maybe holding a PI lock and later be boosted if contended by an
> RT thread.
> 
> Am I correct in assuming there is an induced inversion here in this
> case, because the deferred wakeup mechanism isn't part of the PI chain?
> 
> If so, is this just to be an accepted limitation at this point?  Is the
> intent to argue this away as bad RT application design? :)
> 

Well, it shouldn't be holding any kernel PI locks (aka spin_lock) when
it sleeps, but may be holding a PI futex. In which case, I would say is
a bad RT application, to have a thread sleep on a non RT timer while
holding a lock that an RT Task might take.

-- Steve