Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	linux-kernel@vger.kernel.org, tglx@linutronix.de,
	peterz@infradead.org, akpm@linux-foundation.org, luto@kernel.org,
	bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com,
	mingo@redhat.com, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, willy@infradead.org, mgorman@suse.de,
	jpoimboe@kernel.org, mark.rutland@arm.com, jgross@suse.com,
	andrew.cooper3@citrix.com, bristot@kernel.org,
	mathieu.desnoyers@efficios.com, geert@linux-m68k.org,
	glaubitz@physik.fu-berlin.de, anton.ivanov@cambridgegreys.com,
	mattst88@gmail.com, krypton@ulrich-teichert.org,
	rostedt@goodmis.org, David.Laight@aculab.com, richard@nod.at,
	mjguzik@gmail.com, jon.grimm@amd.com, bharata@amd.com,
	raghavendra.kt@amd.com, boris.ostrovsky@oracle.com,
	konrad.wilk@oracle.com
Subject: Re: [PATCH 26/30] sched: handle preempt=voluntary under PREEMPT_AUTO
Date: Mon, 11 Mar 2024 17:03:57 -0700	[thread overview]
Message-ID: <85cba8bd-5d2f-4da2-b4f9-93ae7c6a2a45@paulmck-laptop> (raw)
In-Reply-To: <CAHk-=whgp+Az+rR4L3oAnmbpSq7QqUEjJUNRpeeqsJ3O+Z8-QQ@mail.gmail.com>

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@oracle.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