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

From: Shakeel Butt <shakeelb@google.com>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Tejun Heo <tj@kernel.org>, Michal Hocko <mhocko@suse.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Muchun Song <muchun.song@linux.dev>,
	cgroups@vger.kernel.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, Ivan Babrou <ivan@cloudflare.com>
Subject: Re: [PATCH] mm: memcg: provide accurate stats for userspace reads
Date: Tue, 15 Aug 2023 17:23:36 -0700	[thread overview]
Message-ID: <CALvZod6KRxiDzrppCgx+=SHg2+96nFE5crwXCKwe9PZbWM_6cQ@mail.gmail.com> (raw)
In-Reply-To: <CAJD7tkYgCySTX28zK9GZiWwsabR4nv7M2hQ57y12si-fqtv7zg@mail.gmail.com>

+Ivan

On Mon, Aug 14, 2023 at 5:51 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> 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).
>

I thought we already reached the decision on how to proceed here. Let
me summarize what I think we should do:

1. Completely remove the sync flush from stat files read from userspace.
2. Provide a separate way/interface to explicitly flush stats for
users who want more accurate stats and can pay the cost. This is
similar to the stat_refresh interface.
3. Keep the 2 sec periodic stats flusher.