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

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Message-ID: <98f9ff35-5d71-4706-afe1-7e039afbcaaf@paulmck-laptop>
Date:   Wed, 18 Oct 2023 11:13:17 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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>,
        Daniel Bristot de Oliveira <bristot@...nel.org>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

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.

> > > > 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.  ;-)

						Thanx, Paul