From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Down Subject: Re: [PATCH] mm, memcg: reclaim more aggressively before high allocator throttling Date: Thu, 28 May 2020 17:48:48 +0100 Message-ID: <20200528164848.GB839178@chrisdown.name> References: <20200520165131.GB630613@cmpxchg.org> <20200520170430.GG6462@dhcp22.suse.cz> <20200520175135.GA793901@cmpxchg.org> <20200521073245.GI6462@dhcp22.suse.cz> <20200521135152.GA810429@cmpxchg.org> <20200521143515.GU6462@dhcp22.suse.cz> <20200521163833.GA813446@cmpxchg.org> <20200521173701.GX6462@dhcp22.suse.cz> <20200521184505.GA815980@cmpxchg.org> <20200528163101.GJ27484@dhcp22.suse.cz> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chrisdown.name; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=UwZ28g34UZ/LmYps/esI5h2JZjKeclqR/inpTX02z2o=; b=PPb3j7a/BdD86VI8igVSltNvHQ9B8pjLLH1ePEwa/baG3Wzmss76Lv0gRQbqiB0y7I qB3fSv9TLfHEKw23N/5h80TiBDGczfNsmJpY+OMnOWzCIfO1yGUMNJi8SGt8vStQ6HYl zOW8eKaK+Jq3r13HH3nV3qAD0HpX1p6KWe31I= Content-Disposition: inline In-Reply-To: <20200528163101.GJ27484-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" Content-Transfer-Encoding: 7bit To: Michal Hocko Cc: Johannes Weiner , Andrew Morton , Tejun Heo , linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kernel-team-b10kYP2dOMg@public.gmane.org Michal Hocko writes: >> We send a simple bug fix: bring this instance of reclaim in line with >> how everybody else is using the reclaim API, to meet the semantics as >> they are intendend and documented. > >Here is where we are not on the same page though. Once you have identified >that the main problem is that the reclaim fails too early to meet the >target then the fix would be to enforce that target. I have asked why >this hasn't been done and haven't got any real answer for that. Instead >what you call "a simple bug fix" has larger consequences which are not >really explained in the changelog and they are also not really trivial >to see. If the changelog explicitly stated that the proportional memory >reclaim is not sufficient because XYZ and the implementation has been >changed to instead meet the high limit target then this would be a >completely different story and I believe we could have saved some >discussion. I agree that the changelog can be made more clear. Any objection if I send v2 with changelog changes to that effect, then? :-) >> And somehow this is controversial, and we're just changing around user >> promises as we see fit for our particular usecase? >> >> I don't even understand how the supposed alternate semantics you read >> between the lines in the documentation would make for a useful >> feature: It may fail to contain a group of offending tasks to the >> configured limit, but it will be fair to those tasks while doing so? >> >> > But if your really want to push this through then let's do it >> > properly at least. memcg->memcg_nr_pages_over_high has only very >> > vague meaning if the reclaim target is the high limit. >> >> task->memcg_nr_pages_over_high is not vague, it's a best-effort >> mechanism to distribute fairness. It's the current task's share of the >> cgroup's overage, and it allows us in the majority of situations to >> distribute reclaim work and sleeps in proportion to how much the task >> is actually at fault. > >Agreed. But this stops being the case as soon as the reclaim target has >been reached and new reclaim attempts are enforced because the memcg is >still above the high limit. Because then you have a completely different >reclaim target - get down to the limit. This would be especially visible >with a large memcg_nr_pages_over_high which could even lead to an over >reclaim. We actually over reclaim even before this patch -- this patch doesn't bring much new in that regard. Tracing try_to_free_pages for a cgroup at the memory.high threshold shows that before this change, we sometimes even reclaim on the order of twice the number of pages requested. For example, I see cases where we requested 1000 pages to be reclaimed, but end up reclaiming 2000 in a single reclaim attempt.