Re: [PATCH] mm: memcg: provide accurate stats for userspace reads

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Aug 14, 2023 at 5:48 PM Tejun Heo <tj@kernel.org> wrote:
>
> Hello,
>
> On Mon, Aug 14, 2023 at 05:39:15PM -0700, Yosry Ahmed wrote:
> > I believe dropping unified flushing, if possible of course, may fix
> > both problems.
>
> Yeah, flushing the whole tree for every stat read will push up the big O
> complexity of the operation. It shouldn't be too bad because only what's
> updated since the last read will need flushing but if you have a really big
> machine with a lot of constantly active cgroups, you're still gonna feel it.
> So, yeah, drop that and switch the global lock to mutex and we should all be
> good?

I hope so, but I am not sure.

The unified flushing was added initially to mitigate a thundering herd
problem from concurrent in-kernel flushers (e.g. concurrent reclaims),
but back then flushing was atomic so we had to keep the spinlock held
for a long time. I think it should be better now, but I am hoping
Shakeel will chime in since he added the unified flushing originally.

We also need to agree on what to do about stats_flushing_threshold and
flush_next_time since they're both global now (since all flushing is
global).

>
> Thanks.
>
> --
> tejun