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

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Message-ID: <05066f9d-b8c9-4a22-b573-928df4263e49@kernel.org>
Date:   Thu, 19 Oct 2023 14:37:23 +0200
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     paulmck@...nel.org, 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>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

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.

>>>>> 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 :-).

Thanks!
-- Daniel
> 						Thanx, Paul