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From: Sven Schnelle <svens@linux.ibm.com>
To: Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>
Cc: Valentin Schneider <vschneid@redhat.com>,
        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>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Heiko Carstens <hca@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/core: Test online status in available_idle_cpu()
In-Reply-To: <yt9dcypwrc96.fsf@linux.ibm.com> (Sven Schnelle's message of
	"Wed, 08 May 2024 09:31:01 +0200")
References: <20240429055415.3278672-1-svens@linux.ibm.com>
	<xhsmhzft86wap.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
	<yt9dcypwrc96.fsf@linux.ibm.com>
Date: Thu, 16 May 2024 18:38:06 +0200
Message-ID: <yt9dcyplg1ap.fsf@linux.ibm.com>
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Sven Schnelle <svens@linux.ibm.com> writes:

> Valentin Schneider <vschneid@redhat.com> writes:
>
>> On 29/04/24 07:54, Sven Schnelle wrote:
>>> The current implementation of available_idle_cpu() doesn't test
>>> whether a possible cpu is offline. On s390 this dereferences a
>>> NULL pointer in arch_vcpu_is_preempted() because lowcore is not
>>> allocated for offline cpus. On x86, tracing also shows calls to
>>> available_idle_cpu() after a cpu is disabled, but it looks like
>>> this isn't causing any (obvious) issue. Nevertheless, add a check
>>> and return early if the cpu isn't online.
>>>
>>> Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
>>
>>
>> So most of the uses of that function is in wakeup task placement.
>> o find_idlest_cpu() works on the sched_domain spans, so shouldn't
> deal with
>>   offline CPUs.
>> o select_idle_sibling() may issue an available_idle_cpu(prev) with
> an
>>   offline previous, which would trigger your issue.
>>
>> Currently, even if select_idle_sibling() picks an offline CPU, this
> will
>> get corrected by select_fallback_rq() at the end of
>> select_task_rq(). However, it would make sense to realize @prev
> isn't a
>> suitable pick before making it to the fallback machinery, in which
> case
>> your patch makes sense beyond just fixing s390.
>>
>> Reviewed-by: Valentin Schneider <vschneid@redhat.com>
>
> Thanks for the review! Ingo/Peter, gentle ping, are you planning to
> take
> this patch?

Ping?

Thanks,

Sven