Re: [PATCH] sched: __set_cpus_allowed_ptr(): Check cpus_mask, not cpus_ptr

From: Valentin Schneider <valentin.schneider@arm.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Scott Wood <swood@redhat.com>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] sched: __set_cpus_allowed_ptr(): Check cpus_mask, not cpus_ptr
Date: Thu, 18 Jun 2020 09:51:18 +0100	[thread overview]
Message-ID: <jhj366seozd.mognet@arm.com> (raw)
In-Reply-To: <20200618080700.cig4x4y7n3thmneu@linutronix.de>

On 18/06/20 09:07, Sebastian Andrzej Siewior wrote:
> On 2020-06-17 17:49:48 [-0500], Scott Wood wrote:
>> > Makes sense, but what about the rest of the checks? Further down there is
>> >
>> >         /* Can the task run on the task's current CPU? If so, we're done
>> > */
>> >         if (cpumask_test_cpu(task_cpu(p), new_mask))
>> >                 goto out;
>> >
>> > If the task is currently migrate disabled and for some stupid reason it
>> > gets affined elsewhere, we could try to move it out - which AFAICT we
>> > don't
>> > want to do because migrate disabled. So I suppose you'd want an extra
>> > bailout condition here when the task is migrate disabled.
>> >
>> > ISTR in RT you do re-check the affinity and potentially move the task away
>> > when re-enabling migration, so that should work out all fine.
>>
>> On RT the above test is:
>>
>>         /* Can the task run on the task's current CPU? If so, we're done */
>>         if (cpumask_test_cpu(task_cpu(p), new_mask) ||
>>             p->cpus_ptr != &p->cpus_mask)
>>                 goto out;
>>
>> ...so we do bail out if we're migrate disabled.
>
> correct. There is a complete migrate_disable() patch in the RT queue
> which has to wait. This patch however looked to be independent of that
> and could "fix" the pointer part which is already here so I sent it.
>

Okay, thanks. I don't see any harm in including that extra check with the
patch, but either way:

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

>> -Scott
>
> Sebastian