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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1BA1EC38142 for ; Fri, 27 Jan 2023 04:39:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229629AbjA0EjF (ORCPT ); Thu, 26 Jan 2023 23:39:05 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34414 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229448AbjA0EjD (ORCPT ); Thu, 26 Jan 2023 23:39:03 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5C2DE39B9E for ; Thu, 26 Jan 2023 20:38:10 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1674794290; 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=8CP3YQJy/1BqI0NOeozKEh6zQUilSGJ56jPhvOdxgCg=; b=JznjTcYp6it8SZ82QS+BgPzJO/VDSHyRTlK8q2jAQln/zJRPHXlamzPJ+Kj00AoZ0siKUq ZoPhzzmLrnjAJPcjOqQJeijhCyTgn53d5ASMNs4josDLnb13EzaA/M72FAAnvS4CRFIiK8 TiQuXRfloS8kDXRkTCNnxoc/+y9KNbo= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-434-5yczKWEpPFuzhKGqukOaPg-1; Thu, 26 Jan 2023 23:38:06 -0500 X-MC-Unique: 5yczKWEpPFuzhKGqukOaPg-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 2981C80D0E6; Fri, 27 Jan 2023 04:38:06 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-2.gru2.redhat.com [10.97.112.2]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B27824014CE2; Fri, 27 Jan 2023 04:38:05 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 9D38042627607; Thu, 26 Jan 2023 21:32:18 -0300 (-03) Date: Thu, 26 Jan 2023 21:32:18 -0300 From: Marcelo Tosatti To: Michal Hocko Cc: Roman Gushchin , Leonardo =?iso-8859-1?Q?Br=E1s?= , Johannes Weiner , Shakeel Butt , Muchun Song , Andrew Morton , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 0/5] Introduce memcg_stock_pcp remote draining Message-ID: References: <20230125073502.743446-1-leobras@redhat.com> <9e61ab53e1419a144f774b95230b789244895424.camel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 26, 2023 at 08:20:46PM +0100, Michal Hocko wrote: > On Thu 26-01-23 15:03:43, Marcelo Tosatti wrote: > > On Thu, Jan 26, 2023 at 08:41:34AM +0100, Michal Hocko wrote: > > > On Wed 25-01-23 15:14:48, Roman Gushchin wrote: > > > > On Wed, Jan 25, 2023 at 03:22:00PM -0300, Marcelo Tosatti wrote: > > > > > On Wed, Jan 25, 2023 at 08:06:46AM -0300, Leonardo Brás wrote: > > > > > > On Wed, 2023-01-25 at 09:33 +0100, Michal Hocko wrote: > > > > > > > On Wed 25-01-23 04:34:57, Leonardo Bras wrote: > > > > > > > > Disclaimer: > > > > > > > > a - The cover letter got bigger than expected, so I had to split it in > > > > > > > > sections to better organize myself. I am not very confortable with it. > > > > > > > > b - Performance numbers below did not include patch 5/5 (Remove flags > > > > > > > > from memcg_stock_pcp), which could further improve performance for > > > > > > > > drain_all_stock(), but I could only notice the optimization at the > > > > > > > > last minute. > > > > > > > > > > > > > > > > > > > > > > > > 0 - Motivation: > > > > > > > > On current codebase, when drain_all_stock() is ran, it will schedule a > > > > > > > > drain_local_stock() for each cpu that has a percpu stock associated with a > > > > > > > > descendant of a given root_memcg. > > > > > > > > Do you know what caused those drain_all_stock() calls? I wonder if we should look > > > > into why we have many of them and whether we really need them? > > > > > > > > It's either some user's actions (e.g. reducing memory.max), either some memcg > > > > is entering pre-oom conditions. In the latter case a lot of drain calls can be > > > > scheduled without a good reason (assuming the cgroup contain multiple tasks running > > > > on multiple cpus). > > > > > > I believe I've never got a specific answer to that. We > > > have discussed that in the previous version submission > > > (20221102020243.522358-1-leobras@redhat.com and specifically > > > Y2TQLavnLVd4qHMT@dhcp22.suse.cz). Leonardo has mentioned a mix of RT and > > > isolcpus. I was wondering about using memcgs in RT workloads because > > > that just sounds weird but let's say this is the case indeed. > > > > This could be the case. You can consider an "edge device" where it is > > necessary to run a RT workload. It might also be useful to run > > non realtime applications on the same system. > > > > > Then an RT task or whatever task that is running on an isolated > > > cpu can have pcp charges. > > > > Usually the RT task (or more specifically the realtime sensitive loop > > of the application) runs entirely on userspace. But i suppose there > > could be charges on application startup. > > What is the role of memcg then? If the memory limit is in place and the > workload doesn't fit in then it will get reclaimed during start up and > memory would need to be refaulted if not mlocked. If it is mlocked then > the limit cannot be enforced and the start up would likely fail as a > result of the memcg oom killer. 1) Application which is not time sensitive executes on isolated CPU, with memcg control enabled. Per-CPU stock is created. 2) App with memcg control enabled exits, per-CPU stock is not drained. 3) Latency sensitive application starts, isolated per-CPU has stock to be drained, and: /* * Drains all per-CPU charge caches for given root_memcg resp. subtree * of the hierarchy under it. */ static void drain_all_stock(struct mem_cgroup *root_memcg) { int cpu, curcpu; /* If someone's already draining, avoid adding running more workers. */ if (!mutex_trylock(&percpu_charge_mutex)) return; /* * Notify other cpus that system-wide "drain" is running * We do not care about races with the cpu hotplug because cpu down * as well as workers from this path always operate on the local * per-cpu data. CPU up doesn't touch memcg_stock at all. */ migrate_disable(); curcpu = smp_processor_id(); for_each_online_cpu(cpu) { struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu); struct mem_cgroup *memcg; bool flush = false; rcu_read_lock(); memcg = stock->cached; if (memcg && stock->nr_pages && mem_cgroup_is_descendant(memcg, root_memcg)) flush = true; else if (obj_stock_flush_required(stock, root_memcg)) flush = true; rcu_read_unlock(); if (flush && !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags)) { if (cpu == curcpu) drain_local_stock(&stock->work); else schedule_work_on(cpu, &stock->work); } } migrate_enable(); mutex_unlock(&percpu_charge_mutex); } > [...] > > > > Overall I'm somewhat resistant to an idea of making generic allocation & free paths slower > > > > for an improvement of stock draining. It's not a strong objection, but IMO we should avoid > > > > doing this without a really strong reason. > > > > > > Are you OK with a simple opt out on isolated CPUs? That would make > > > charges slightly slower (atomic on the hierarchy counters vs. a single > > > pcp adjustment) but it would guarantee that the isolated workload is > > > predictable which is the primary objective AFAICS. > > > > This would make isolated CPUs "second class citizens": it would be nice > > to be able to execute non realtime apps on isolated CPUs as well > > (think of different periods of time during a day, one where > > more realtime apps are required, another where less > > realtime apps are required). > > An alternative requires to make the current implementation that is > lockless to use locks and introduce potential lock contention. This > could be harmful to regular workloads. Not using pcp caching would make > a fast path using few atomics rather than local pcp update. That is not > a terrible cost to pay for special cased workloads which use isolcpus. > Really we are not talking about a massive cost to be payed. At least > nobody has shown that in any numbers. > > > Concrete example: think of a computer handling vRAN traffic near a > > cell tower. The traffic (therefore amount of processing required > > by realtime applications) might vary during the day. > > > > User might want to run containers that depend on good memcg charging > > performance on isolated CPUs. > > What kind of role would memcg play here? Do you want to memory constrain > that workload? See example above. > -- > Michal Hocko > SUSE Labs > >