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

[Shakeel Butt]

On Mon, Oct 4, 2021 at 10:44 AM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> On Mon, Oct 04, 2021 at 10:25:12AM -0700, Shakeel Butt wrote:
> > > > 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?
> >
> > It was actually due to machine size. I ran a single threaded workload
> > without any interference on a 112 cpus machine. So, most of the time
> > the flush was acquiring and releasing the per-cpu rstat lock for empty
> > trees.
>
> Sorry for being so slow but can you point to the exact call path which gets
> slowed down so significantly?

This is the mem_cgroup_flush_stats() inside workingset_refault() in
mm/workingset.c.

> I'm mostly wondering whether we need some sort
> of time-batched flushes because even with lock avoidance the flush path
> really isn't great when called frequently. We can mitigate it further if
> necessary - e.g. by adding an "updated" bitmap so that the flusher doesn't
> have to go around touching the cachelines for all the cpus.

For the memcg stats, I already proposed a batched flush at [1].

I actually did perform the same experiment with the proposed patch
along with [1] and it improves around just 1%. More specifically for
memcg stats [1] is good enough but that is memcg specific and this
patch has merits on its own.

[1] https://lkml.kernel.org/r/20210930044711.2892660-1-shakeelb@xxxxxxxxxx

>
> Thanks.

>
> --
> tejun