linux-kernel - Re: [PATCH sched_ext/for-6.11] sched_ext: Disallow loading BPF scheduler if isolcpus= domain isolation is in effect

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Message-ID: <Zow-XrnYpDmdyg9O@slm.duckdns.org>
Date: Mon, 8 Jul 2024 09:30:38 -1000
From: Tejun Heo <tj@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: torvalds@...ux-foundation.org, mingo@...hat.com, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	bristot@...hat.com, vschneid@...hat.com, ast@...nel.org,
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...nel.org,
	joshdon@...gle.com, brho@...gle.com, pjt@...gle.com,
	derkling@...gle.com, haoluo@...gle.com, dvernet@...a.com,
	dschatzberg@...a.com, dskarlat@...cmu.edu, riel@...riel.com,
	changwoo@...lia.com, himadrics@...ia.fr, memxor@...il.com,
	andrea.righi@...onical.com, joel@...lfernandes.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	kernel-team@...a.com, Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH sched_ext/for-6.11] sched_ext: Disallow loading BPF
 scheduler if isolcpus= domain isolation is in effect

On Wed, Jun 26, 2024 at 03:27:02PM -1000, Tejun Heo wrote:
> sched_domains regulate the load balancing for sched_classes. A machine can
> be partitioned into multiple sections that are not load-balanced across
> using either isolcpus= boot param or cpuset partitions. In such cases, tasks
> that are in one partition are expected to stay within that partition.
> 
> cpuset configured partitions are always reflected in each member task's
> cpumask. As SCX always honors the task cpumasks, the BPF scheduler is
> automatically in compliance with the configured partitions.
> 
> However, for isolcpus= domain isolation, the isolated CPUs are simply
> omitted from the top-level sched_domain[s] without further restrictions on
> tasks' cpumasks, so, for example, a task currently running in an isolated
> CPU may have more CPUs in its allowed cpumask while expected to remain on
> the same CPU.
> 
> There is no straightforward way to enforce this partitioning preemptively on
> BPF schedulers and erroring out after a violation can be surprising.
> isolcpus= domain isolation is being replaced with cpuset partitions anyway,
> so keep it simple and simply disallow loading a BPF scheduler if isolcpus=
> domain isolation is in effect.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Link: http://lkml.kernel.org/r/20240626082342.GY31592@noisy.programming.kicks-ass.net
> Cc: David Vernet <void@...ifault.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Frederic Weisbecker <frederic@...nel.org>

Applied to cgroup/for-6.11.

Thanks.

-- 
tejun