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From: Sven Schnelle <svens@linux.ibm.com>
To: Valentin Schneider <vschneid@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>, 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>,
        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: <xhsmhzft86wap.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
	(Valentin Schneider's message of "Thu, 02 May 2024 17:57:50 +0200")
References: <20240429055415.3278672-1-svens@linux.ibm.com>
	<xhsmhzft86wap.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 08 May 2024 09:31:01 +0200
Message-ID: <yt9dcypwrc96.fsf@linux.ibm.com>
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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?