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

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Date:   Fri, 2 Sep 2022 11:28:21 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Mel Gorman <mgorman@...e.de>
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: Re: [PATCH v2 5/5] sched/fair: cleanup for SIS_PROP

On 9/1/22 10:03 PM, Mel Gorman Wrote:
> 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.

Nice catch!

> 
> 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.
> 

I'd prefer the former.

Thanks & BR,
Abel