linux-kernel - Re: [PATCH] mm: slowly shrink slabs with a relatively small number of objects

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Date:   Tue, 4 Sep 2018 18:14:31 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Rik van Riel <riel@...riel.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Josef Bacik <jbacik@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] mm: slowly shrink slabs with a relatively small number
 of objects

On Tue 04-09-18 08:34:49, Roman Gushchin wrote:
> On Tue, Sep 04, 2018 at 09:00:05AM +0200, Michal Hocko wrote:
> > On Mon 03-09-18 13:28:06, Roman Gushchin wrote:
> > > On Mon, Sep 03, 2018 at 08:29:56PM +0200, Michal Hocko wrote:
> > > > On Fri 31-08-18 14:31:41, Roman Gushchin wrote:
> > > > > On Fri, Aug 31, 2018 at 05:15:39PM -0400, Rik van Riel wrote:
> > > > > > On Fri, 2018-08-31 at 13:34 -0700, Roman Gushchin wrote:
> > > > > > 
> > > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > > > > index fa2c150ab7b9..c910cf6bf606 100644
> > > > > > > --- a/mm/vmscan.c
> > > > > > > +++ b/mm/vmscan.c
> > > > > > > @@ -476,6 +476,10 @@ static unsigned long do_shrink_slab(struct
> > > > > > > shrink_control *shrinkctl,
> > > > > > >  	delta = freeable >> priority;
> > > > > > >  	delta *= 4;
> > > > > > >  	do_div(delta, shrinker->seeks);
> > > > > > > +
> > > > > > > +	if (delta == 0 && freeable > 0)
> > > > > > > +		delta = min(freeable, batch_size);
> > > > > > > +
> > > > > > >  	total_scan += delta;
> > > > > > >  	if (total_scan < 0) {
> > > > > > >  		pr_err("shrink_slab: %pF negative objects to delete
> > > > > > > nr=%ld\n",
> > > > > > 
> > > > > > I agree that we need to shrink slabs with fewer than
> > > > > > 4096 objects, but do we want to put more pressure on
> > > > > > a slab the moment it drops below 4096 than we applied
> > > > > > when it had just over 4096 objects on it?
> > > > > > 
> > > > > > With this patch, a slab with 5000 objects on it will
> > > > > > get 1 item scanned, while a slab with 4000 objects on
> > > > > > it will see shrinker->batch or SHRINK_BATCH objects
> > > > > > scanned every time.
> > > > > > 
> > > > > > I don't know if this would cause any issues, just
> > > > > > something to ponder.
> > > > > 
> > > > > Hm, fair enough. So, basically we can always do
> > > > > 
> > > > >     delta = max(delta, min(freeable, batch_size));
> > > > > 
> > > > > Does it look better?
> > > > 
> > > > Why don't you use the same heuristic we use for the normal LRU raclaim?
> > > 
> > > Because we do reparent kmem lru lists on offlining.
> > > Take a look at memcg_offline_kmem().
> > 
> > Then I must be missing something. Why are we growing the number of dead
> > cgroups then?
> 
> We do reparent LRU lists, but not objects. Objects (or, more precisely, pages)
> are still holding a reference to the memcg.

OK, this is what I missed. I thought that the reparenting includes all
the pages as well. Is there any strong reason that we cannot do that?
Performance/Locking/etc.?

Or maybe do not reparent at all and rely on the same reclaim heuristic
we do for normal pages?

I am not opposing your patch but I am trying to figure out whether that
is the best approach.
-- 
Michal Hocko
SUSE Labs