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

[Michal Hocko]

On Wed 01-03-17 12:36:28, Johannes Weiner wrote:
> 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.

Yeah, this is not really easy to spot because there are usually other
memcgs which can be reclaimed.

> 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.

well the memory pressure spills over to others in the same hierarchy.
But I agree this shouldn't a disaster.

> > 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.

As I've already mentioned, I really love this patch I just think this is
a subtle side effect. The above reasoning should be good enough I
believe.

Anyway I forgot to add, I will leave the decision whether to have this
in a separate patch or just added to the changelog to you.
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
-- 
Michal Hocko
SUSE Labs