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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Qian Cai <cai@lca.pw>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>, paulmck@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: suspicious RCU due to "Prefer using an idle CPU as a migration target instead of comparing tasks"
In-reply-to: <1582814862.7365.135.camel@lca.pw>
Date:   Thu, 27 Feb 2020 15:26:41 +0000
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On Thu, Feb 27 2020, Qian Cai wrote:

> On Thu, 2020-02-27 at 09:09 -0500, Qian Cai wrote:
>> The linux-next commit ff7db0bf24db ("sched/numa: Prefer using an idle CPU as a
>> migration target instead of comparing tasks") introduced a boot warning,
>
> This?
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a61d83ea2930..ca780cd1eae2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1607,7 +1607,9 @@ static void update_numa_stats(struct task_numa_env *env,
> if (ns->idle_cpu == -1)
> ns->idle_cpu = cpu;
>
> +rcu_read_lock();
> idle_core = numa_idle_core(idle_core, cpu);
> +rcu_read_unlock();
> }
> }
>


Hmph right, we have
numa_idle_core()->test_idle_cores()->rcu_dereference().

Dunno if it's preferable to wrap the entirety of update_numa_stats() or
if that fine-grained read-side section is ok.