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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9DEF1C433F5 for ; Fri, 13 May 2022 18:23:06 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 27E5F6B0073; Fri, 13 May 2022 14:23:06 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 22D136B0075; Fri, 13 May 2022 14:23:06 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0F6086B0078; Fri, 13 May 2022 14:23:06 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id EF1EA6B0073 for ; Fri, 13 May 2022 14:23:05 -0400 (EDT) Received: from smtpin21.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay12.hostedemail.com (Postfix) with ESMTP id CA239120282 for ; Fri, 13 May 2022 18:23:05 +0000 (UTC) X-FDA: 79461541530.21.15977D0 Received: from outbound-smtp58.blacknight.com (outbound-smtp58.blacknight.com [46.22.136.242]) by imf01.hostedemail.com (Postfix) with ESMTP id 1AF40400BB for ; Fri, 13 May 2022 18:22:51 +0000 (UTC) Received: from mail.blacknight.com (pemlinmail04.blacknight.ie [81.17.254.17]) by outbound-smtp58.blacknight.com (Postfix) with ESMTPS id 58DB3FB488 for ; Fri, 13 May 2022 19:23:03 +0100 (IST) Received: (qmail 1396 invoked from network); 13 May 2022 18:23:03 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.198.246]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 13 May 2022 18:23:03 -0000 Date: Fri, 13 May 2022 19:23:01 +0100 From: Mel Gorman To: Nicolas Saenz Julienne Cc: Andrew Morton , Marcelo Tosatti , Vlastimil Babka , Michal Hocko , LKML , Linux-MM Subject: Re: [PATCH 6/6] mm/page_alloc: Remotely drain per-cpu lists Message-ID: <20220513182301.GK3441@techsingularity.net> References: <20220512085043.5234-1-mgorman@techsingularity.net> <20220512085043.5234-7-mgorman@techsingularity.net> <20220512123743.5be26b3ad4413f20d5f46564@linux-foundation.org> <20220513150402.GJ3441@techsingularity.net> <167d30f439d171912b1ef584f20219e67a009de8.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <167d30f439d171912b1ef584f20219e67a009de8.camel@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 1AF40400BB Authentication-Results: imf01.hostedemail.com; dkim=none; spf=pass (imf01.hostedemail.com: domain of mgorman@techsingularity.net designates 46.22.136.242 as permitted sender) smtp.mailfrom=mgorman@techsingularity.net; dmarc=none X-Rspam-User: X-Stat-Signature: 8r6gimdhcm1h3818or7jiwfuppdfpo9e X-HE-Tag: 1652466171-191405 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, May 13, 2022 at 05:19:18PM +0200, Nicolas Saenz Julienne wrote: > On Fri, 2022-05-13 at 16:04 +0100, Mel Gorman wrote: > > On Thu, May 12, 2022 at 12:37:43PM -0700, Andrew Morton wrote: > > > On Thu, 12 May 2022 09:50:43 +0100 Mel Gorman wrote: > > > > > > > From: Nicolas Saenz Julienne > > > > > > > > Some setups, notably NOHZ_FULL CPUs, are too busy to handle the per-cpu > > > > drain work queued by __drain_all_pages(). So introduce a new mechanism to > > > > remotely drain the per-cpu lists. It is made possible by remotely locking > > > > 'struct per_cpu_pages' new per-cpu spinlocks. A benefit of this new scheme > > > > is that drain operations are now migration safe. > > > > > > > > There was no observed performance degradation vs. the previous scheme. > > > > Both netperf and hackbench were run in parallel to triggering the > > > > __drain_all_pages(NULL, true) code path around ~100 times per second. > > > > The new scheme performs a bit better (~5%), although the important point > > > > here is there are no performance regressions vs. the previous mechanism. > > > > Per-cpu lists draining happens only in slow paths. > > > > > > > > Minchan Kim tested this independently and reported; > > > > > > > > My workload is not NOHZ CPUs but run apps under heavy memory > > > > pressure so they goes to direct reclaim and be stuck on > > > > drain_all_pages until work on workqueue run. > > > > > > > > unit: nanosecond > > > > max(dur) avg(dur) count(dur) > > > > 166713013 487511.77786438033 1283 > > > > > > > > From traces, system encountered the drain_all_pages 1283 times and > > > > worst case was 166ms and avg was 487us. > > > > > > > > The other problem was alloc_contig_range in CMA. The PCP draining > > > > takes several hundred millisecond sometimes though there is no > > > > memory pressure or a few of pages to be migrated out but CPU were > > > > fully booked. > > > > > > > > Your patch perfectly removed those wasted time. > > > > > > I'm not getting a sense here of the overall effect upon userspace > > > performance. As Thomas said last year in > > > https://lkml.kernel.org/r/87v92sgt3n.ffs@tglx > > > > > > : The changelogs and the cover letter have a distinct void vs. that which > > > : means this is just another example of 'scratch my itch' changes w/o > > > : proper justification. > > > > > > Is there more to all of this than itchiness and if so, well, you know > > > the rest ;) > > > > > > > I think Minchan's example is clear-cut. The draining operation can take > > an arbitrary amount of time waiting for the workqueue to run on each CPU > > and can cause severe delays under reclaim or CMA and the patch fixes > > it. Maybe most users won't even notice but I bet phone users do if a > > camera app takes too long to open. > > > > The first paragraphs was written by Nicolas and I did not want to modify > > it heavily and still put his Signed-off-by on it. Maybe it could have > > been clearer though because "too busy" is vague when the actual intent > > is to avoid interfering with RT tasks. Does this sound better to you? > > > > Some setups, notably NOHZ_FULL CPUs, may be running realtime or > > latency-sensitive applications that cannot tolerate interference > > due to per-cpu drain work queued by __drain_all_pages(). Introduce > > a new mechanism to remotely drain the per-cpu lists. It is made > > possible by remotely locking 'struct per_cpu_pages' new per-cpu > > spinlocks. This has two advantages, the time to drain is more > > predictable and other unrelated tasks are not interrupted. > > > > You raise a very valid point with Thomas' mail and it is a concern that > > the local_lock is no longer strictly local. We still need preemption to > > be disabled between the percpu lookup and the lock acquisition but that > > can be done with get_cpu_var() to make the scope clear. > > This isn't going to work in RT :( > > get_cpu_var() disables preemption hampering RT spinlock use. There is more to > it in Documentation/locking/locktypes.rst. > Bah, you're right. A helper that called preempt_disable() on !RT and migrate_disable() on RT would work although similar to local_lock with a different name. I'll look on Monday to see how the code could be restructured to always have the get_cpu_var() call immediately before the lock acquisition. Once that is done, I'll look what sort of helper that "disables preempt/migration, lookup pcp structure, acquire lock, enable preempt/migration". It's effectively the magic trick that local_lock uses to always lock the right pcpu lock but we want the spinlock semantics for remote drain. -- Mel Gorman SUSE Labs