Re: [PATCH] cgroup: rstat: optimize flush through speculative test

[Tejun Heo]

Hello, Shakeel.

On Wed, Sep 29, 2021 at 04:59:36PM -0700, Shakeel Butt wrote:
> Currently cgroup_rstat_updated() has a speculative already-on-list test
> to check if the given cgroup is already part of the rstat update tree.
> This helps in reducing the contention on the rstat cpu lock. This patch
> adds the similar speculative not-on-list test on the rstat flush
> codepath.
> 
> Recently the commit aa48e47e3906 ("memcg: infrastructure to flush memcg
> stats") added periodic rstat flush. On a large system which is not much
> busy, most of the per-cpu rstat tree would be empty. So, the speculative
> not-on-list test helps in eliminating unnecessary work and potentially
> reducing contention on the rstat cpu lock. Please note this might
> introduce temporary inaccuracy but with the frequent and periodic flush
> this would not be an issue.
> 
> To evaluate the impact of this patch, an 8 GiB tmpfs file is created on
> a system with swap-on-zram and the file was pushed to swap through
> memory.force_empty interface. On reading the whole file, the memcg stat
> flush in the refault code path is triggered. With this patch, we
> observed 38% reduction in the read time of 8 GiB file.

The patch looks fine to me but that's a lot of reduction in read time. Can
you elaborate a bit on why this makes such a huge difference? Who's hitting
on that lock so hard?

Thanks.

-- 
tejun