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

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Message-ID: <ff85b142-c833-4a79-970d-ff691f9b60b7@paulmck-laptop>
Date:   Wed, 18 Oct 2023 10:55:02 -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 10:31:46AM -0400, Steven Rostedt wrote:
> On Wed, 18 Oct 2023 15:16:12 +0200
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > > 14.	The kernel/trace/trace_osnoise.c file's run_osnoise() function
> > > 	might need to do something for non-preemptible RCU to make
> > > 	up for the lack of cond_resched() calls.  Maybe just drop the
> > > 	"IS_ENABLED()" and execute the body of the current "if" statement
> > > 	unconditionally.  
> 
> Right.
> 
> I'm guessing you are talking about this code:
> 
>                 /*
>                  * In some cases, notably when running on a nohz_full CPU with
>                  * a stopped tick PREEMPT_RCU has no way to account for QSs.
>                  * This will eventually cause unwarranted noise as PREEMPT_RCU
>                  * will force preemption as the means of ending the current
>                  * grace period. We avoid this problem by calling
>                  * rcu_momentary_dyntick_idle(), which performs a zero duration
>                  * EQS allowing PREEMPT_RCU to end the current grace period.
>                  * This call shouldn't be wrapped inside an RCU critical
>                  * section.
>                  *
>                  * Note that in non PREEMPT_RCU kernels QSs are handled through
>                  * cond_resched()
>                  */
>                 if (IS_ENABLED(CONFIG_PREEMPT_RCU)) {
>                         if (!disable_irq)
>                                 local_irq_disable();
> 
>                         rcu_momentary_dyntick_idle();
> 
>                         if (!disable_irq)
>                                 local_irq_enable();
>                 }

That is indeed the place!

>                 /*
>                  * For the non-preemptive kernel config: let threads runs, if
>                  * they so wish, unless set not do to so.
>                  */
>                 if (!disable_irq && !disable_preemption)
>                         cond_resched();
> 
> 
> 
> 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.

> > 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.  ;-)

							Thanx, Paul