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From: Valentin Schneider <vschneid@redhat.com>
To: Sven Schnelle <svens@linux.ibm.com>, 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>
Cc: 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: <20240429055415.3278672-1-svens@linux.ibm.com>
References: <20240429055415.3278672-1-svens@linux.ibm.com>
Date: Thu, 02 May 2024 17:57:50 +0200
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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>