From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.3 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 95AD7CA9ECF for ; Mon, 4 Nov 2019 15:30:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 62A472080F for ; Mon, 4 Nov 2019 15:30:04 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="B09HhwSx" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729217AbfKDPaD (ORCPT ); Mon, 4 Nov 2019 10:30:03 -0500 Received: from us-smtp-delivery-1.mimecast.com ([207.211.31.120]:22056 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728957AbfKDPaD (ORCPT ); Mon, 4 Nov 2019 10:30:03 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1572881402; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=EcgDDpXNskbpfNmglvt+lRA/X0pT5r2sa1muBARm9Tc=; b=B09HhwSx32vE7e8F78uwLLrlvDLgQuusbxe7UOiXa9fQ0w8Riy8SnKUeXh1PEIG5SyhAU+ c2ieRsxYgLOGtmk0fS+pkv/R5SsXG7PHSyx/u449RYpfRJnZfQRYqRNT+eTwN7anPz4Y0o AyyVG10csMifjpZn2DldhDKSbCyM7Sw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-17-Oze8pseKP-emimqWNAKh_Q-1; Mon, 04 Nov 2019 10:29:58 -0500 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 53ECD800C73; Mon, 4 Nov 2019 15:29:57 +0000 (UTC) Received: from bfoster (dhcp-41-2.bos.redhat.com [10.18.41.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A46C85D9CD; Mon, 4 Nov 2019 15:29:56 +0000 (UTC) Date: Mon, 4 Nov 2019 10:29:54 -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: <20191104152954.GC10665@bfoster> References: <20191031234618.15403-1-david@fromorbit.com> <20191031234618.15403-13-david@fromorbit.com> MIME-Version: 1.0 In-Reply-To: <20191031234618.15403-13-david@fromorbit.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-MC-Unique: Oze8pseKP-emimqWNAKh_Q-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 01, 2019 at 10:46:02AM +1100, Dave Chinner wrote: > From: Dave Chinner >=20 > Right now deferred work is picked up by whatever GFP_KERNEL context > reclaimer that wins the race to empty the node's deferred work > counter. However, if there are lots of direct reclaimers, that > work might be continually picked up by contexts taht can't do any > work and so the opportunities to do the work are missed by contexts > that could do them. >=20 > A further problem with the current code is that the deferred work > can be picked up by a random direct reclaimer, resulting in that > specific process having to do all the deferred reclaim work and > hence can take extremely long latencies if the reclaim work blocks > regularly. This is not good for direct reclaim fairness or for > minimising long tail latency events. >=20 > To avoid these problems, simply limit deferred work to kswapd > contexts. We know kswapd is a context that can always do reclaim > work, and hence deferring work to kswapd allows the deferred work to > be done in the background and not adversely affect any specific > process context doing direct reclaim. >=20 > The advantage of this is that amount of work to be done in direct > reclaim is now bound and predictable - it is entirely based on > the cache's freeable objects and the reclaim priority. hence all > direct reclaimers running at the same time should be doing > relatively equal amounts of work, thereby reducing the incidence of > long tail latencies due to uneven reclaim workloads. >=20 > Note that we use signed integers for everything except the freed > count as the returns from the shrinker callouts cannot be guaranteed > untainted. Indeed, the shrinkers can return scan counts larger that > were fed in, so we need scan counts to underflow in a detectable > manner to terminate loops. This is necessary to avoid a misbehaving > shrinker from triggering endless scanning loops. >=20 > Signed-off-by: Dave Chinner > --- > include/linux/shrinker.h | 2 +- > mm/vmscan.c | 100 ++++++++++++++++++++------------------- > 2 files changed, 53 insertions(+), 49 deletions(-) >=20 > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h > index 3405c39ab92c..30c10f42109f 100644 > --- a/include/linux/shrinker.h > +++ b/include/linux/shrinker.h > @@ -81,7 +81,7 @@ struct shrinker { > =09int id; > #endif > =09/* objs pending delete, per node */ > -=09atomic_long_t *nr_deferred; > +=09atomic64_t *nr_deferred; > }; > #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */ > =20 > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 2d39ec37c04d..c0e2bf656e3f 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -517,16 +517,16 @@ static int64_t shrink_scan_count(struct shrink_cont= rol *shrinkctl, > static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, > =09=09=09=09 struct shrinker *shrinker, int priority) > { > -=09unsigned long freed =3D 0; > -=09long total_scan; > +=09uint64_t freed =3D 0; > =09int64_t freeable_objects =3D 0; > =09int64_t scan_count; > -=09long nr; > -=09long new_nr; > +=09int64_t scanned_objects =3D 0; > +=09int64_t next_deferred =3D 0; > +=09int64_t deferred_count =3D 0; > +=09int64_t new_nr; > =09int nid =3D shrinkctl->nid; > =09long batch_size =3D shrinker->batch ? shrinker->batch > =09=09=09=09=09 : SHRINK_BATCH; > -=09long scanned =3D 0, next_deferred; > =20 > =09if (!(shrinker->flags & SHRINKER_NUMA_AWARE)) > =09=09nid =3D 0; > @@ -537,47 +537,51 @@ static unsigned long do_shrink_slab(struct shrink_c= ontrol *shrinkctl, > =09=09return scan_count; > =20 > =09/* > -=09 * copy the current shrinker scan count into a local variable > -=09 * and zero it so that other concurrent shrinker invocations > -=09 * don't also do this scanning work. > -=09 */ > -=09nr =3D atomic_long_xchg(&shrinker->nr_deferred[nid], 0); > - > -=09total_scan =3D nr + scan_count; > -=09if (total_scan < 0) { > -=09=09pr_err("shrink_slab: %pS negative objects to delete nr=3D%ld\n", > -=09=09 shrinker->scan_objects, total_scan); > -=09=09total_scan =3D scan_count; > -=09=09next_deferred =3D nr; > -=09} else > -=09=09next_deferred =3D total_scan; > - > -=09/* > -=09 * We need to avoid excessive windup on filesystem shrinkers > -=09 * due to large numbers of GFP_NOFS allocations causing the > -=09 * shrinkers to return -1 all the time. This results in a large > -=09 * nr being built up so when a shrink that can do some work > -=09 * comes along it empties the entire cache due to nr >>> > -=09 * freeable. This is bad for sustaining a working set in > -=09 * memory. > +=09 * If kswapd, we take all the deferred work and do it here. We don't = let > +=09 * direct reclaim do this, because then it means some poor sod is goi= ng > +=09 * to have to do somebody else's GFP_NOFS reclaim, and it hides the r= eal > +=09 * amount of reclaim work from concurrent kswapd operations. Hence we= do > +=09 * the work in the wrong place, at the wrong time, and it's largely > +=09 * unpredictable. > =09 * > -=09 * Hence only allow the shrinker to scan the entire cache when > -=09 * a large delta change is calculated directly. > +=09 * By doing the deferred work only in kswapd, we can schedule the wor= k > +=09 * according the the reclaim priority - low priority reclaim will do > +=09 * less deferred work, hence we'll do more of the deferred work the m= ore > +=09 * desperate we become for free memory. This avoids the need for need= ing > +=09 * to specifically avoid deferred work windup as low amount os memory > +=09 * pressure won't excessive trim caches anymore. That last sentence is hard to read. ;) > =09 */ > -=09if (scan_count < freeable_objects / 4) > -=09=09total_scan =3D min_t(long, total_scan, freeable_objects / 2); > +=09if (current_is_kswapd()) { > +=09=09int64_t=09deferred_scan; > + > +=09=09deferred_count =3D atomic64_xchg(&shrinker->nr_deferred[nid], 0); > + > +=09=09/* we want to scan 5-10% of the deferred work here at minimum */ > +=09=09deferred_scan =3D deferred_count; > +=09=09if (priority) > +=09=09=09do_div(deferred_scan, priority); > +=09=09scan_count +=3D deferred_scan; > + > +=09=09/* > +=09=09 * If there is more deferred work than the number of freeable > +=09=09 * items in the cache, limit the amount of work we will carry > +=09=09 * over to the next kswapd run on this cache. This prevents > +=09=09 * deferred work windup. > +=09=09 */ > +=09=09deferred_count =3D min(deferred_count, freeable_objects * 2); > + Extra whitespace above. > +=09} > =20 > =09/* > =09 * Avoid risking looping forever due to too large nr value: > =09 * never try to free more than twice the estimate number of > =09 * freeable entries. > =09 */ The comment refers to a variable that no longer exists. I also wonder if it's a little cleaner to move the deferred_count =3D min(...); statement above down here and condense the two comments. > -=09if (total_scan > freeable_objects * 2) > -=09=09total_scan =3D freeable_objects * 2; > +=09scan_count =3D min(scan_count, freeable_objects * 2); > =20 > -=09trace_mm_shrink_slab_start(shrinker, shrinkctl, nr, > +=09trace_mm_shrink_slab_start(shrinker, shrinkctl, deferred_count, > =09=09=09=09 freeable_objects, scan_count, > -=09=09=09=09 total_scan, priority); > +=09=09=09=09 scan_count, priority); > =20 > =09/* > =09 * If the shrinker can't run (e.g. due to gfp_mask constraints), then > @@ -601,10 +605,10 @@ static unsigned long do_shrink_slab(struct shrink_c= ontrol *shrinkctl, > =09 * scanning at high prio and therefore should try to reclaim as much = 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_count); > =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 shrink_c= ontrol *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; 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 into the shrinker struct below. If there was a pending deferred count on this 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? 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? > =09/* > =09 * move the unused scan count back into the shrinker in a > =09 * manner that handles concurrent updates. If we exhausted the > =09 * scan, there is no need to do an update. > =09 */ > =09if (next_deferred > 0) > -=09=09new_nr =3D atomic_long_add_return(next_deferred, > +=09=09new_nr =3D atomic64_add_return(next_deferred, > =09=09=09=09=09=09&shrinker->nr_deferred[nid]); > =09else > -=09=09new_nr =3D atomic_long_read(&shrinker->nr_deferred[nid]); > +=09=09new_nr =3D atomic64_read(&shrinker->nr_deferred[nid]); It looks like we could kill new_nr and just reuse next_deferred here too. Brian > =20 > -=09trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan= ); > +=09trace_mm_shrink_slab_end(shrinker, nid, freed, deferred_count, new_nr= , > +=09=09=09=09=09scan_count); > =09return freed; > } > =20 > --=20 > 2.24.0.rc0 >=20