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Fri, 15 Nov 2019 17:23:03 +0000 (UTC) Received: from bfoster (dhcp-41-2.bos.redhat.com [10.18.41.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0DC0828DDE; Fri, 15 Nov 2019 17:23:02 +0000 (UTC) Date: Fri, 15 Nov 2019 12:23:01 -0500 From: Brian Foster To: Dave Chinner Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 12/28] shrinker: defer work only to kswapd Message-ID: <20191115172301.GB55854@bfoster> References: <20191031234618.15403-1-david@fromorbit.com> <20191031234618.15403-13-david@fromorbit.com> <20191104152954.GC10665@bfoster> <20191114211150.GE4614@dread.disaster.area> MIME-Version: 1.0 In-Reply-To: <20191114211150.GE4614@dread.disaster.area> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-MC-Unique: R4y48ZzWOkGEjZwVb4saJQ-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Fri, Nov 15, 2019 at 08:11:50AM +1100, Dave Chinner wrote: > On Mon, Nov 04, 2019 at 10:29:54AM -0500, Brian Foster wrote: > > On Fri, Nov 01, 2019 at 10:46:02AM +1100, Dave Chinner wrote: > > > @@ -601,10 +605,10 @@ static unsigned long do_shrink_slab(struct shri= nk_control *shrinkctl, > > > =09 * scanning at high prio and therefore should try to reclaim as m= uch as > > > =09 * possible. > > > =09 */ > > > -=09while (total_scan >=3D batch_size || > > > -=09 total_scan >=3D freeable_objects) { > > > +=09while (scan_count >=3D batch_size || > > > +=09 scan_count >=3D freeable_objects) { > > > =09=09unsigned long ret; > > > -=09=09unsigned long nr_to_scan =3D min(batch_size, total_scan); > > > +=09=09unsigned long nr_to_scan =3D min_t(long, batch_size, scan_coun= t); > > > =20 > > > =09=09shrinkctl->nr_to_scan =3D nr_to_scan; > > > =09=09shrinkctl->nr_scanned =3D nr_to_scan; > > > @@ -614,29 +618,29 @@ static unsigned long do_shrink_slab(struct shri= nk_control *shrinkctl, > > > =09=09freed +=3D ret; > > > =20 > > > =09=09count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned); > > > -=09=09total_scan -=3D shrinkctl->nr_scanned; > > > -=09=09scanned +=3D shrinkctl->nr_scanned; > > > +=09=09scan_count -=3D shrinkctl->nr_scanned; > > > +=09=09scanned_objects +=3D shrinkctl->nr_scanned; > > > =20 > > > =09=09cond_resched(); > > > =09} > > > - > > > done: > > > -=09if (next_deferred >=3D scanned) > > > -=09=09next_deferred -=3D scanned; > > > +=09if (deferred_count) > > > +=09=09next_deferred =3D deferred_count - scanned_objects; > > > =09else > > > -=09=09next_deferred =3D 0; > > > +=09=09next_deferred =3D scan_count; > >=20 > > Hmm.. so if there was no deferred count on this cycle, we set > > next_deferred to whatever is left from scan_count and add that back int= o > > the shrinker struct below. If there was a pending deferred count on thi= s > > cycle, we subtract what we scanned from that and add that value back. > > But what happens to the remaining scan_count in the latter case? Is it > > lost, or am I missing something? >=20 > if deferred_count is not zero, then it is kswapd that is running. It > does the deferred work, and if it doesn't make progress then adding > it's scan count to the deferred work doesn't matter. That's because > it will come back with an increased priority in a short while and > try to scan more of the deferred count plus it's larger scan count. >=20 Ok, so perhaps there is no functional reason to defer remaining scan count from a context (i.e. kswapd) that attempts to process deferred work... > IOWs, if we defer kswapd unused scan count, we effectively increase > the pressure as the priority goes up, potentially making the > deferred count increase out of control. i.e. kswapd can make > progress and free items, but the result is that it increased the > deferred scan count rather than reducing it. This leads to excessive > reclaim of the slab caches and kswapd can trash the caches long > after the memory pressure has gone away... >=20 ... yet if kswapd runs without pre-existing deferred work, that's precisely what it does. next_deferred is set to remaining scan_count and that is added back to the shrinker struct. So should kswapd generally defer work or not? If the answer is sometimes, then please add a comment to the next_deferred assignment to explain when/why. > > For example, suppose we start this cycle with a large scan_count and > > ->scan_objects() returned SHRINK_STOP before doing much work. In that > > scenario, it looks like whether ->nr_deferred is 0 or not is the only > > thing that determines whether we defer the entire remaining scan_count > > or just what is left from the previous ->nr_deferred. The existing code > > appears to consistently factor in what is left from the current scan > > with the previous deferred count. Hm? >=20 > If kswapd doesn't have any deferred work, then it's largely no > different in behaviour to direct reclaim. If it has no deferred > work, then the shrinker is not getting stopped early in direct > reclaim, so it's unlikely that kswapd is going to get stopped early, > either.... >=20 Then perhaps the logic could be simplified to explicitly not defer from kswapd..? Brian > Cheers, >=20 > Dave. > --=20 > Dave Chinner > david@fromorbit.com >=20