Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 4 Sep 2018 18:14:31 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Rik van Riel <riel@surriel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com,
        Josef Bacik <jbacik@fb.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: slowly shrink slabs with a relatively small number
 of objects
Message-ID: <20180904161431.GP14951@dhcp22.suse.cz>
References: <20180831203450.2536-1-guro@fb.com>
 <3b05579f964cca1d44551913f1a9ee79d96f198e.camel@surriel.com>
 <20180831213138.GA9159@tower.DHCP.thefacebook.com>
 <20180903182956.GE15074@dhcp22.suse.cz>
 <20180903202803.GA6227@castle.DHCP.thefacebook.com>
 <20180904070005.GG14951@dhcp22.suse.cz>
 <20180904153445.GA22328@tower.DHCP.thefacebook.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180904153445.GA22328@tower.DHCP.thefacebook.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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