linux-kernel - Re: [PATCH 11/20] sched: Handle CPU isolation on last resort fallback rq selection

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Message-ID: <Zx_BDLUDmL-3tLtd@localhost.localdomain>
Date: Mon, 28 Oct 2024 17:51:24 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Hocko <mhocko@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 11/20] sched: Handle CPU isolation on last resort
 fallback rq selection

Le Mon, Oct 28, 2024 at 04:25:15PM +0000, Will Deacon a écrit :
> > If nohz_full= isn't used then
> > it's cpu_possible_mask). If there is a housekeeping CPU supporting el0 32bits
> > then it will be disallowed to be ever offlined. But if the first mismatching
> > CPU supporting el0 that pops up is not housekeeping then we may end up
> > with that CPU disallowed to be offlined + later if a housekeeping CPU appears
> > that also supports 32bits el0 will also be disallowed to be offlined. Ideally
> > it should turn back the previous CPU to be offlinable but there may be
> > other things that have forbidden that CPU to be offline so...
> 
> I'd have thought the bigger problem would be if the set of nohz_full=
> CPUs was defined as the set of CPUs that support 32-bit. In that case,
> executing a 32-bit task will give the scheduler no choice but to run
> the task on a !housekeeping core.

Right.

> 
> So perhaps we could turn this on its head and explicitly mark the first
> 32-bit capable CPU as a housekeeping core when the mismatched mode is
> enabled? We're already preventing CPU hotplug for the thing, so it's
> "special" already. If that conflicts with the nohz_full_option, we can
> emit a warning message that we're overriding it. I think that's ok, as
> the user will have had to specify 'allow_mismatched_32bit_el0' as well.

It's very complicated to revert a CPU once it is set as nohz_full. But we can
retain a 32 bits capable nohz_full CPU from offlining until we finally find
a non-nohz_full 2bits capable CPU. I was about to repost the whole kthread
patchset but lemme post just the specific bits of interests here, it's "just"
two patches.

Thanks.