linux-kernel - Re: [PATCH v2 5/5] sched/fair: cleanup for SIS

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Date:   Thu, 1 Sep 2022 15:03:11 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Abel Wu <wuyun.abel@...edance.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Josh Don <joshdon@...gle.com>, Chen Yu <yu.c.chen@...el.com>,
        Yicong Yang <yangyicong@...wei.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/5] sched/fair: cleanup for SIS_PROP

On Thu, Sep 01, 2022 at 09:11:07PM +0800, Abel Wu wrote:
> The sched-domain of this cpu is only used when SIS_PROP is enabled,
> and it should be irrelevant whether the local sd_llc is valid or
> not, since all we care about is target sd_llc if !SIS_PROP.
> 
> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>

This could conceivably result in an uninitialised memory access if
SIS_PROP was enabled while select_idle_cpu is running. I'm not sure if
it can happen when jump labels are in use but I think it could happen
for !CONFIG_JUMP_LABEL updating the sysctl_sched_features bitmap updated
via sysctl.

The patch is still a good idea because it moves an unlikely rcu_deference
out of the default path for sched features but either this_sd needs to
be initialised to NULL and checked or the this_sd lookup needs to happen
twice at a slight additional cost to the default-disabled SIS_PROP path.

-- 
Mel Gorman
SUSE Labs