Re: [PATCH v8 11/19] sched: Allow task CPU affinity to be restricted on asymmetric systems

From: Valentin Schneider <valentin.schneider@arm•com>
To: Will Deacon <will@kernel•org>
Cc: linux-arm-kernel@lists•infradead.org, linux-arch@vger•kernel.org,
	linux-kernel@vger•kernel.org,
	Catalin Marinas <catalin.marinas@arm•com>,
	Marc Zyngier <maz@kernel•org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation•org>,
	Peter Zijlstra <peterz@infradead•org>,
	Morten Rasmussen <morten.rasmussen@arm•com>,
	Qais Yousef <qais.yousef@arm•com>,
	Suren Baghdasaryan <surenb@google•com>,
	Quentin Perret <qperret@google•com>, Tejun Heo <tj@kernel•org>,
	Johannes Weiner <hannes@cmpxchg•org>,
	Ingo Molnar <mingo@redhat•com>,
	Juri Lelli <juri.lelli@redhat•com>,
	Vincent Guittot <vincent.guittot@linaro•org>,
	"Rafael J. Wysocki" <rjw@rjwysocki•net>,
	Dietmar Eggemann <dietmar.eggemann@arm•com>,
	Daniel Bristot de Oliveira <bristot@redhat•com>,
	kernel-team@android•com
Subject: Re: [PATCH v8 11/19] sched: Allow task CPU affinity to be restricted on asymmetric systems
Date: Thu, 10 Jun 2021 11:20:33 +0100	[thread overview]
Message-ID: <87eedac972.mognet@arm.com> (raw)
In-Reply-To: <20210607225202.GB8215@willie-the-truck>

On 07/06/21 23:52, Will Deacon wrote:
> On Fri, Jun 04, 2021 at 06:12:32PM +0100, Valentin Schneider wrote:
>> On 02/06/21 17:47, Will Deacon wrote:
>> > +	/*
>> > +	 * Forcefully restricting the affinity of a deadline task is
>> > +	 * likely to cause problems, so fail and noisily override the
>> > +	 * mask entirely.
>> > +	 */
>> > +	if (task_has_dl_policy(p) && dl_bandwidth_enabled()) {
>> > +		err = -EPERM;
>> > +		goto err_unlock;
>> > +	}
>> > +
>> > +	if (!cpumask_and(new_mask, &p->cpus_mask, subset_mask)) {
>> > +		err = -EINVAL;
>> > +		goto err_unlock;
>> > +	}
>> > +
>> > +	/*
>> > +	 * We're about to butcher the task affinity, so keep track of what
>> > +	 * the user asked for in case we're able to restore it later on.
>> > +	 */
>> > +	if (user_mask) {
>> > +		cpumask_copy(user_mask, p->cpus_ptr);
>> > +		p->user_cpus_ptr = user_mask;
>> > +	}
>> > +
>>
>> Shouldn't that be done before any of the bailouts above, so we can
>> potentially restore the mask even if we end up forcefully expanding the
>> affinity?
>
> I don't think so. I deliberately only track the old mask if we've managed
> to take a subset for the 32-bit task. If we end up having to override the
> mask entirely, then I treat it the same way as an explicit affinity change
> (only with a warning printed) and don't then try to restore the old mask --
> it feels like we'd be overriding the affinity twice if we tried to do that.
>

Put in this way, it does make sense to me. Thanks!

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