From patchwork Wed Oct 9 03:21:09 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Dave Chinner X-Patchwork-Id: 11180273 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id C95B317D4 for ; Wed, 9 Oct 2019 03:21:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ABDD4218DE for ; Wed, 9 Oct 2019 03:21:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730495AbfJIDVd (ORCPT ); Tue, 8 Oct 2019 23:21:33 -0400 Received: from mail104.syd.optusnet.com.au ([211.29.132.246]:46267 "EHLO mail104.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730267AbfJIDVc (ORCPT ); Tue, 8 Oct 2019 23:21:32 -0400 Received: from dread.disaster.area (pa49-181-226-196.pa.nsw.optusnet.com.au [49.181.226.196]) by mail104.syd.optusnet.com.au (Postfix) with ESMTPS id 3FE9E43EC2F; Wed, 9 Oct 2019 14:21:27 +1100 (AEDT) Received: from discord.disaster.area ([192.168.253.110]) by dread.disaster.area with esmtp (Exim 4.92.2) (envelope-from ) id 1iI2XX-0006BY-0P; Wed, 09 Oct 2019 14:21:27 +1100 Received: from dave by discord.disaster.area with local (Exim 4.92) (envelope-from ) id 1iI2XW-00039Q-UQ; Wed, 09 Oct 2019 14:21:26 +1100 From: Dave Chinner To: linux-xfs@vger.kernel.org Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org Subject: [PATCH 11/26] shrinker: defer work only to kswapd Date: Wed, 9 Oct 2019 14:21:09 +1100 Message-Id: <20191009032124.10541-12-david@fromorbit.com> X-Mailer: git-send-email 2.23.0.rc1 In-Reply-To: <20191009032124.10541-1-david@fromorbit.com> References: <20191009032124.10541-1-david@fromorbit.com> MIME-Version: 1.0 X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.2 cv=FNpr/6gs c=1 sm=1 tr=0 a=dRuLqZ1tmBNts2YiI0zFQg==:117 a=dRuLqZ1tmBNts2YiI0zFQg==:17 a=jpOVt7BSZ2e4Z31A5e1TngXxSK0=:19 a=XobE76Q3jBoA:10 a=20KFwNOVAAAA:8 a=DY0x1sV2QIg9a-hhTtsA:9 a=MCR_tvXtDtI_MWXX:21 a=Jfgs850-GcERAMIO:21 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org From: Dave Chinner 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. 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. 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. 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. 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. Signed-off-by: Dave Chinner --- include/linux/shrinker.h | 2 +- mm/vmscan.c | 98 +++++++++++++++++++++------------------- 2 files changed, 52 insertions(+), 48 deletions(-) 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 { int id; #endif /* objs pending delete, per node */ - atomic_long_t *nr_deferred; + atomic64_t *nr_deferred; }; #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */ diff --git a/mm/vmscan.c b/mm/vmscan.c index de6b09ad97ed..d05f64bd26ff 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -516,16 +516,16 @@ static int64_t shrink_scan_count(struct shrink_control *shrinkctl, static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, struct shrinker *shrinker, int priority) { - unsigned long freed = 0; - long total_scan; + uint64_t freed = 0; int64_t freeable_objects = 0; int64_t scan_count; - long nr; - long new_nr; + int64_t scanned_objects = 0; + int64_t next_deferred = 0; + int64_t deferred_count = 0; + int64_t new_nr; int nid = shrinkctl->nid; long batch_size = shrinker->batch ? shrinker->batch : SHRINK_BATCH; - long scanned = 0, next_deferred; if (!(shrinker->flags & SHRINKER_NUMA_AWARE)) nid = 0; @@ -536,47 +536,51 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, return scan_count; /* - * copy the current shrinker scan count into a local variable - * and zero it so that other concurrent shrinker invocations - * don't also do this scanning work. + * If kswapd, we take all the deferred work and do it here. We don't let + * direct reclaim do this, because then it means some poor sod is going + * to have to do somebody else's GFP_NOFS reclaim, and it hides the real + * amount of reclaim work from concurrent kswapd operations. Hence we do + * the work in the wrong place, at the wrong time, and it's largely + * unpredictable. + * + * By doing the deferred work only in kswapd, we can schedule the work + * according the the reclaim priority - low priority reclaim will do + * less deferred work, hence we'll do more of the deferred work the more + * desperate we become for free memory. This avoids the need for needing + * to specifically avoid deferred work windup as low amount os memory + * pressure won't excessive trim caches anymore. */ - nr = atomic_long_xchg(&shrinker->nr_deferred[nid], 0); + if (current_is_kswapd()) { + int64_t deferred_scan; - total_scan = nr + scan_count; - if (total_scan < 0) { - pr_err("shrink_slab: %pS negative objects to delete nr=%ld\n", - shrinker->scan_objects, total_scan); - total_scan = scan_count; - next_deferred = nr; - } else - next_deferred = total_scan; + deferred_count = atomic64_xchg(&shrinker->nr_deferred[nid], 0); - /* - * We need to avoid excessive windup on filesystem shrinkers - * due to large numbers of GFP_NOFS allocations causing the - * shrinkers to return -1 all the time. This results in a large - * nr being built up so when a shrink that can do some work - * comes along it empties the entire cache due to nr >>> - * freeable. This is bad for sustaining a working set in - * memory. - * - * Hence only allow the shrinker to scan the entire cache when - * a large delta change is calculated directly. - */ - if (scan_count < freeable_objects / 4) - total_scan = min_t(long, total_scan, freeable_objects / 2); + /* we want to scan 5-10% of the deferred work here at minimum */ + deferred_scan = deferred_count; + if (priority) + do_div(deferred_scan, priority); + scan_count += deferred_scan; + + /* + * If there is more deferred work than the number of freeable + * items in the cache, limit the amount of work we will carry + * over to the next kswapd run on this cache. This prevents + * deferred work windup. + */ + deferred_count = min(deferred_count, freeable_objects * 2); + + } /* * Avoid risking looping forever due to too large nr value: * never try to free more than twice the estimate number of * freeable entries. */ - if (total_scan > freeable_objects * 2) - total_scan = freeable_objects * 2; + scan_count = min(scan_count, freeable_objects * 2); - trace_mm_shrink_slab_start(shrinker, shrinkctl, nr, + trace_mm_shrink_slab_start(shrinker, shrinkctl, deferred_count, freeable_objects, scan_count, - total_scan, priority); + scan_count, priority); /* * If the shrinker can't run (e.g. due to gfp_mask constraints), then @@ -600,10 +604,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, * scanning at high prio and therefore should try to reclaim as much as * possible. */ - while (total_scan >= batch_size || - total_scan >= freeable_objects) { + while (scan_count >= batch_size || + scan_count >= freeable_objects) { unsigned long ret; - unsigned long nr_to_scan = min(batch_size, total_scan); + unsigned long nr_to_scan = min_t(long, batch_size, scan_count); shrinkctl->nr_to_scan = nr_to_scan; shrinkctl->nr_scanned = nr_to_scan; @@ -613,29 +617,29 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl, freed += ret; count_vm_events(SLABS_SCANNED, shrinkctl->nr_scanned); - total_scan -= shrinkctl->nr_scanned; - scanned += shrinkctl->nr_scanned; + scan_count -= shrinkctl->nr_scanned; + scanned_objects += shrinkctl->nr_scanned; cond_resched(); } - done: - if (next_deferred >= scanned) - next_deferred -= scanned; + if (deferred_count) + next_deferred = deferred_count - scanned_objects; else - next_deferred = 0; + next_deferred = scan_count; /* * move the unused scan count back into the shrinker in a * manner that handles concurrent updates. If we exhausted the * scan, there is no need to do an update. */ if (next_deferred > 0) - new_nr = atomic_long_add_return(next_deferred, + new_nr = atomic64_add_return(next_deferred, &shrinker->nr_deferred[nid]); else - new_nr = atomic_long_read(&shrinker->nr_deferred[nid]); + new_nr = atomic64_read(&shrinker->nr_deferred[nid]); - trace_mm_shrink_slab_end(shrinker, nid, freed, nr, new_nr, total_scan); + trace_mm_shrink_slab_end(shrinker, nid, freed, deferred_count, new_nr, + scan_count); return freed; }