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

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Message-ID: <ZRAPbXk4N2dntgyA@casper.infradead.org>
Date:   Sun, 24 Sep 2023 11:29:01 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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, mgorman@...e.de,
        rostedt@...dmis.org, jon.grimm@....com, bharata@....com,
        raghavendra.kt@....com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com, jgross@...e.com, andrew.cooper3@...rix.com
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On Sun, Sep 24, 2023 at 09:55:52AM +0200, Thomas Gleixner wrote:
> On Sun, Sep 24 2023 at 08:19, Matthew Wilcox wrote:
> > On Sun, Sep 24, 2023 at 12:50:43AM +0200, Thomas Gleixner wrote:
> >> cond_resched() cannot nest and is obviously scope-less.
> >> 
> >> The TIF_ALLOW_RESCHED mechanism, which sparked this discussion only
> >> pretends to be scoped.
> >> 
> >> As Peter pointed out it does not properly nest with other mechanisms and
> >> it cannot even nest in itself because it is boolean.
> >
> > We can nest a single bit without turning it into a counter -- we
> > do this for memalloc_nofs_save() for example.  Simply return the
> > current value of the bit, and pass it to _restore().
> 
> Right.
> 
> That works, but the reverse logic still does not make sense:
> 
>         allow_resched();
>         ....
>         spin_lock();
> 
> while
>         resched_now_is_suboptimal();
>         ...
>         spin_lock();
> 
> works.

Oh, indeed.  I had in mind

	state = resched_now_is_suboptimal();
	spin_lock();
	...
	spin_unlock();
	resched_might_be_optimal_again(state);

... or we could bundle it up ...

	state = spin_lock_resched_disable();
	...
	spin_unlock_resched_restore(state);