linux-kernel - Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT

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Date: Mon, 11 Mar 2024 17:03:57 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ankur Arora <ankur.a.arora@...cle.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	peterz@...radead.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, jpoimboe@...nel.org,
	mark.rutland@....com, jgross@...e.com, andrew.cooper3@...rix.com,
	bristot@...nel.org, mathieu.desnoyers@...icios.com,
	geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
	anton.ivanov@...bridgegreys.com, mattst88@...il.com,
	krypton@...ich-teichert.org, rostedt@...dmis.org,
	David.Laight@...lab.com, richard@....at, mjguzik@...il.com,
	jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
	boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO

On Mon, Mar 11, 2024 at 01:23:09PM -0700, Linus Torvalds wrote:
> On Mon, 11 Mar 2024 at 13:10, Ankur Arora <ankur.a.arora@...cle.com> wrote:
> >
> > Ah, I see your point. Basically, keep the lazy semantics but -- in
> > addition -- also provide the ability to dynamically toggle
> > cond_resched(), might_reshed() as a feature to help move this along
> > further.
> 
> Please, let's not make up any random hypotheticals.
> 
> Honestly, if we ever hit the hypothetical scenario that Paul outlined, let's
> 
>  (a) deal with it THEN, when we actually know what the situation is
> 
>  (b) learn and document what it is that actually causes the odd behavior
> 
> IOW, instead of assuming that some "cond_resched()" case would even be
> the right thing to do, maybe there are other issues going on?  Let's
> not paper over them by keeping some hack around - and *if* some
> cond_resched() model is actually the right model in some individual
> place, let's make it the rule that *when* we hit that case, we
> document it.
> 
> And we should absolutely not have some hypothetical case keep us from
> just doing the right thing and getting rid of the existing
> cond_resched().
> 
> Because any potential future case is *not* going to be the same
> cond_resched() that the current case is anyway. It is going to have
> some very different cause.

Fair enough, and that approach just means that we will be reaching out
to Ankur and Thomas sooner rather than later if something goes sideways
latency-wise.  ;-)

							Thanx, Paul