linux-kernel - Re: [PATCH v2 7/9] sched: define TIF_ALLOW

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Message-ID: <cdec2fb1-f5f4-48ab-a941-8cc963daa102@paulmck-laptop>
Date:   Thu, 19 Oct 2023 10:08:12 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jon.grimm@....com,
        bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On Thu, Oct 19, 2023 at 02:37:23PM +0200, Daniel Bristot de Oliveira wrote:
> On 10/18/23 20:13, Paul E. McKenney wrote:
> > On Wed, Oct 18, 2023 at 02:00:35PM -0400, Steven Rostedt wrote:
> >> On Wed, 18 Oct 2023 10:55:02 -0700
> >> "Paul E. McKenney" <paulmck@...nel.org> wrote:
> >>
> >>>> If everything becomes PREEMPT_RCU, then the above should be able to be
> >>>> turned into just:
> >>>>
> >>>>                 if (!disable_irq)
> >>>>                         local_irq_disable();
> >>>>
> >>>>                 rcu_momentary_dyntick_idle();
> >>>>
> >>>>                 if (!disable_irq)
> >>>>                         local_irq_enable();
> >>>>
> >>>> And no cond_resched() is needed.  
> >>>
> >>> Even given that CONFIG_PREEMPT_RCU=n still exists, the fact that
> >>> run_osnoise() is running in kthread context with preemption and everything
> >>> else enabled (am I right?), then the change you suggest should work fine.
> >>
> >> There's a user space option that lets you run that loop with preemption and/or
> >> interrupts disabled.
> > 
> > Ah, thank you.  Then as long as this function is not expecting an RCU
> > reader to span that call to rcu_momentary_dyntick_idle(), all is well.
> > This is a kthread, so there cannot be something else expecting an RCU
> > reader to span that call.
> 
> Sorry for the delay, this thread is quite long (and I admit I should be paying
> attention to it).
> 
> It seems that you both figure it out without me anyways. This piece of
> code is preemptive unless a config is set to disable irq or preemption (as
> steven mentioned). That call is just a ping to RCU to say that things
> are fine.
> 
> So Steven's suggestion should work.

Very good!

> >>>>> Again. There is no non-preemtible RCU with this model, unless I'm
> >>>>> missing something important here.  
> >>>>
> >>>> Daniel?  
> >>>
> >>> But very happy to defer to Daniel.  ;-)
> >>
> >> But Daniel could also correct me ;-)
> > 
> > If he figures out a way that it is broken, he gets to fix it.  ;-)
> 
> It works for me, keep in the loop for the patches and I can test and
> adjust osnoise accordingly. osnoise should not be a reason to block more
> important things like this patch set, and we can find a way out in
> the osnoise tracer side. (I might need an assistance from rcu
> people, but I know I can count on them :-).

For good or for bad, we will be here.  ;-)

							Thanx, Paul