Re: [PATCH 6/9] mm: don't avoid high-priority reclaim on memcg limit reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Mar 01, 2017 at 04:40:27PM +0100, Michal Hocko wrote:
> On Tue 28-02-17 16:40:04, Johannes Weiner wrote:
> > 246e87a93934 ("memcg: fix get_scan_count() for small targets") sought
> > to avoid high reclaim priorities for memcg by forcing it to scan a
> > minimum amount of pages when lru_pages >> priority yielded nothing.
> > This was done at a time when reclaim decisions like dirty throttling
> > were tied to the priority level.
> > 
> > Nowadays, the only meaningful thing still tied to priority dropping
> > below DEF_PRIORITY - 2 is gating whether laptop_mode=1 is generally
> > allowed to write. But that is from an era where direct reclaim was
> > still allowed to call ->writepage, and kswapd nowadays avoids writes
> > until it's scanned every clean page in the system. Potential changes
> > to how quick sc->may_writepage could trigger are of little concern.
> > 
> > Remove the force_scan stuff, as well as the ugly multi-pass target
> > calculation that it necessitated.
> 
> I _really_ like this, I hated the multi-pass part. One thig that I am
> worried about and changelog doesn't mention it is what we are going to
> do about small (<16MB) memcgs. On one hand they were already ignored in
> the global reclaim so this is nothing really new but maybe we want to
> preserve the behavior for the memcg reclaim at least which would reduce
> side effect of this patch which is a great cleanup otherwise. Or at
> least be explicit about this in the changelog.

<16MB groups are a legitimate concern during global reclaim, but we
have done it this way for a long time and it never seemed to have
mattered in practice.

And for limit reclaim, this should be much less of a concern. It just
means we no longer scan these groups at DEF_PRIORITY and will have to
increase the scan window. I don't see a problem with that. And that
consequence of higher priorities is right in the patch subject.

> Btw. why cannot we simply force scan at least SWAP_CLUSTER_MAX
> unconditionally?
> 
> > +		/*
> > +		 * If the cgroup's already been deleted, make sure to
> > +		 * scrape out the remaining cache.
> 		   Also make sure that small memcgs will not get
> 		   unnoticed during the memcg reclaim
> 
> > +		 */
> > +		if (!scan && !mem_cgroup_online(memcg))
> 
> 		if (!scan && (!mem_cgroup_online(memcg) || !global_reclaim(sc)))

With this I'd be worried about regressing the setups pointed out in
6f04f48dc9c0 ("mm: only force scan in reclaim when none of the LRUs
are big enough.").

Granted, that patch is a little dubious. IMO, we should be steering
the LRU balance through references and, in that case in particular,
with swappiness. Using the default 60 for zswap is too low.

Plus, I would expect the refault detection code that was introduced
around the same time as this patch to counter-act the hot file
thrashing that is mentioned in that patch's changelog.

Nevertheless, it seems a bit gratuitous to go against that change so
directly when global reclaim hasn't historically been a problem with
groups <16MB. Limit reclaim should be fine too.