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

[Tejun Heo]

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? 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.

Thanks.

-- 
tejun