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Date: Thu, 13 Jun 2024 09:49:06 -0700
From: Shakeel Butt <shakeel.butt@linux.dev>
To: Nhat Pham <nphamcs@gmail.com>
Cc: Yosry Ahmed <yosryahmed@google.com>, 
	Takero Funaki <flintglass@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, 
	Chengming Zhou <chengming.zhou@linux.dev>, Jonathan Corbet <corbet@lwn.net>, 
	Andrew Morton <akpm@linux-foundation.org>, Domenico Cerasuolo <cerasuolodomenico@gmail.com>, 
	linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 1/3] mm: zswap: fix global shrinker memcg iteration
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On Thu, Jun 13, 2024 at 08:04:39AM GMT, Nhat Pham wrote:
[...]
> > > >
> > > > Is the idea here to avoid moving the iterator to another offline memcg
> > > > that zswap_memcg_offline_cleanup() was already called for, to avoid
> > > > holding a ref on that memcg until the next run of zswap shrinking?
> > > >
> > > > If yes, I think it's probably worth doing. But why do we need to
> > > > release and reacquire the lock in the loop above?
> > >
> > > Yes, the existing cleaner might leave the offline, already-cleaned memcg.
> > >
> > > The reacquiring lock is to not loop inside the critical section.
> > > In shrink_worker of v0 patch, the loop was restarted on offline memcg
> > > without releasing the lock. Nhat pointed out that we should drop the
> > > lock after every mem_cgroup_iter() call. v1 was changed to reacquire
> > > once per iteration like the cleaner code above.
> >
> > I am not sure how often we'll run into a situation where we'll be
> > holding the lock for too long tbh. It should be unlikely to keep
> > encountering offline memcgs for a long time.
> >
> > Nhat, do you think this could cause a problem in practice?
> 
> I don't remember prescribing anything to be honest :) I think I was
> just asking why can't we just drop the lock, then "continue;". This is
> mostly for simplicity's sake.
> 
> https://lore.kernel.org/linux-mm/CAKEwX=MwrRc43iM2050v5u-TPUK4Yn+a4G7+h6ieKhpQ7WtQ=A@mail.gmail.com/
> 
> But I think as Takero pointed out, it would still skip over the memcg
> that was (concurrently) updated to zswap_next_shrink by the memcg
> offline callback.

What's the issue with keep traversing until an online memcg is found?
Something like the following:


	spin_lock(&zswap_shrink_lock);
	do {
		zswap_next_shrink = mem_cgroup_iter(NULL, zswap_next_shrink, NULL);
	} while (zswap_next_shrink && !mem_cgroup_online(zswap_next_shrink));

	if (!zswap_next_shrink)
		zswap_next_shrink = mem_cgroup_iter(NULL, NULL, NULL);
	....

Is the concern that there can a lot of offlined memcgs which may cause
need resched warnings?