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

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Message-ID: <20230911191623.3408d66d@gandalf.local.home>
Date:   Mon, 11 Sep 2023 19:16:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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, tglx@...utronix.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On Mon, 11 Sep 2023 16:10:31 -0700
Ankur Arora <ankur.a.arora@...cle.com> wrote:

> An even crazier version of that idea would be to have
> preempt_disable/enable() demarcate regions, and the compiler putting all
> of the preemption disabled region out-of-line to a special section.
> Seems to me, that then we could do away to preempt_enable/disable()?
> (Ignoring the preempt_count used in hardirq etc.)
> 

I thought about this too, but wasn't sure if it would be easier or harder
to implement. This would still require the duplicate functions (which I
guess would be the most difficult part).

> This would allow preemption always, unless executing in the
> preemption-disabled section.
> 
> Though I don't have any intuition for how much extra call overhead this
> would add.

I don't think this version would have as high of an overhead. You would get
a direct jump (which isn't bad as all speculation knows exactly where to
look), and it would improve the look up. No table, just a simple range
check.

-- Steve