From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [PATCH v2 0/5] Introduce memcg_stock_pcp remote draining Date: Thu, 26 Jan 2023 21:32:18 -0300 Message-ID: References: <20230125073502.743446-1-leobras@redhat.com> <9e61ab53e1419a144f774b95230b789244895424.camel@redhat.com> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1674794293; 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=WkUjKQ0TOKG6Ayu8Csy4oLzrPAc76BS3zy8eIJBaab7Ffx7FVFmBRdnkJT/nPEmVwubue/ OtNZVftjzSYyWUL3Oep5OODYzNFbldAi4EKxIQiNcy+d0FKAyPJfseolxJ925k2vsxB5nO ANBBEJym0EV5cnGWvcHsKbmBBmLhHSY= Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="iso-8859-1" To: Michal Hocko Cc: Roman Gushchin , Leonardo =?iso-8859-1?Q?Br=E1s?= , Johannes Weiner , Shakeel Butt , Muchun Song , Andrew Morton , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.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 Br=E1s 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 confo= rtable with it. > > > > > > > > b - Performance numbers below did not include patch 5/5 (Re= move flags > > > > > > > > from memcg_stock_pcp), which could further improve perf= ormance for > > > > > > > > drain_all_stock(), but I could only notice the optimiza= tion at the > > > > > > > > last minute. > > > > > > > >=20 > > > > > > > >=20 > > > > > > > > 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 as= sociated with a > > > > > > > > descendant of a given root_memcg. > > > >=20 > > > > 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? > > > >=20 > > > > 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 c= alls can be > > > > scheduled without a good reason (assuming the cgroup contain multip= le tasks running > > > > on multiple cpus). > > >=20 > > > I believe I've never got a specific answer to that. We > > > have discussed that in the previous version submission > > > (20221102020243.522358-1-leobras-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.= org and specifically > > > Y2TQLavnLVd4qHMT-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org). Leonardo h= as 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.=20 > >=20 > > 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=20 > > non realtime applications on the same system. > >=20 > > > Then an RT task or whatever task that is running on an isolated > > > cpu can have pcp charges. > >=20 > > 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. >=20 > 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 =3D smp_processor_id(); for_each_online_cpu(cpu) { struct memcg_stock_pcp *stock =3D &per_cpu(memcg_stock, cpu= ); struct mem_cgroup *memcg; bool flush =3D false; rcu_read_lock(); memcg =3D stock->cached; if (memcg && stock->nr_pages && mem_cgroup_is_descendant(memcg, root_memcg)) flush =3D true; else if (obj_stock_flush_required(stock, root_memcg)) flush =3D true; rcu_read_unlock(); if (flush && !test_and_set_bit(FLUSHING_CACHED_CHARGE, &stock->flags= )) { if (cpu =3D=3D 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 allocat= ion & 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. > > >=20 > > > 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. > >=20 > > 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=20 > > more realtime apps are required, another where less=20 > > realtime apps are required). >=20 > 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. >=20 > > Concrete example: think of a computer handling vRAN traffic near a=20 > > cell tower. The traffic (therefore amount of processing required > > by realtime applications) might vary during the day. > >=20 > > User might want to run containers that depend on good memcg charging > > performance on isolated CPUs. >=20 > What kind of role would memcg play here? Do you want to memory constrain > that workload? See example above. > --=20 > Michal Hocko > SUSE Labs >=20 >=20