linux-kernel - Re: [RFC PATCH 54/86] sched: add cond_resched

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Message-ID: <87cywjfa2h.ffs@tglx>
Date:   Thu, 09 Nov 2023 12:19:34 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org
Cc:     peterz@...radead.org, torvalds@...ux-foundation.org,
        paulmck@...nel.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, mingo@...nel.org,
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        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com,
        Ankur Arora <ankur.a.arora@...cle.com>
Subject: Re: [RFC PATCH 54/86] sched: add cond_resched_stall()

On Tue, Nov 07 2023 at 13:57, Ankur Arora wrote:
> The kernel has a lot of intances of cond_resched() where it is used
> as an alternative to spinning in a tight-loop while waiting to
> retry an operation, or while waiting for a device state to change.
>
> Unfortunately, because the scheduler is unlikely to have an
> interminable supply of runnable tasks on the runqueue, this just
> amounts to spinning in a tight-loop with a cond_resched().
> (When running in a fully preemptible kernel, cond_resched()
> calls are stubbed out so it amounts to even less.)
>
> In sum, cond_resched() in error handling/retry contexts might
> be useful in avoiding softlockup splats, but not very good at
> error handling. Ideally, these should be replaced with some kind
> of timed or event wait.
>
> For now add cond_resched_stall(), which tries to schedule if
> possible, and failing that executes a cpu_relax().

What's the point of this new variant of cond_resched()? We really do not
want it at all. 

> +int __cond_resched_stall(void)
> +{
> +	if (tif_need_resched(RESCHED_eager)) {
> +		__preempt_schedule();

Under the new model TIF_NEED_RESCHED is going to reschedule if the
preemption counter goes to zero.

So the typical

   while (readl(mmio) & BUSY)
   	cpu_relax();

will just be preempted like any other loop, no?

Confused.