Re: Re: [PATCH v2 5/5] sched/fair: cleanup for SIS_PROP

[Abel Wu <wuyun.abel@bytedance.com>]

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@bytedance.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