From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ed1-f69.google.com (mail-ed1-f69.google.com [209.85.208.69]) by kanga.kvack.org (Postfix) with ESMTP id A87CB6B6E59 for ; Tue, 4 Sep 2018 12:14:35 -0400 (EDT) Received: by mail-ed1-f69.google.com with SMTP id g15-v6so1593741edm.11 for ; Tue, 04 Sep 2018 09:14:35 -0700 (PDT) Received: from mx1.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id 40-v6si626763edz.192.2018.09.04.09.14.34 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 04 Sep 2018 09:14:34 -0700 (PDT) Date: Tue, 4 Sep 2018 18:14:31 +0200 From: Michal Hocko 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> Sender: owner-linux-mm@kvack.org List-ID: To: Roman Gushchin Cc: Rik van Riel , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, Josef Bacik , Johannes Weiner , Andrew Morton 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