From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roman Gushchin Subject: Re: [PATCH v2 0/5] Introduce memcg_stock_pcp remote draining Date: Wed, 25 Jan 2023 15:14:48 -0800 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=linux.dev; s=key1; t=1674688503; 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=GQas9hJPH93Zr57s1dcc7T+g336s2ZkXNkxopiNZy5Y=; b=L++sVVFKgB2QgSfUln8N4pW6K99/uC71K7MHO0eTNd2gOjxQp1AktmI+V60pLnLZKq6KFC hACk9uz3DsoIitBsOXZt7BIu60/uMI/JLjE4PEEUb0PdbUDFsVn25UofpY3G6aCVyuS6ma 52wkIVgDt+qNHn0YlWdzI9qQ/UE4CgA= Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="iso-8859-1" To: Marcelo Tosatti Cc: Leonardo =?iso-8859-1?Q?Br=E1s?= , Michal Hocko , 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 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 confortable w= ith it. > > > > b - Performance numbers below did not include patch 5/5 (Remove fla= gs > > > > from memcg_stock_pcp), which could further improve performance = for > > > > drain_all_stock(), but I could only notice the optimization at = the > > > > last minute. > > > >=20 > > > >=20 > > > > 0 - Motivation: > > > > On current codebase, when drain_all_stock() is ran, it will schedul= e 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 shoul= d 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 mem= cg 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). Essentially each cpu will try to grab the remains of the= memory quota and move it locally. I wonder in such circumstances if we need to disable t= he pcp-caching on per-cgroup basis. Generally speaking, draining of pcpu stocks is useful only if an idle cpu i= s holding some charges/memcg references (it might be not completely idle, but running some= very special workload which is not doing any kernel allocations or a process belonging t= o the root memcg). In all other cases pcpu stock will be either drained naturally by an alloca= tion from another memcg or an allocation from the same memcg will "restore" it, making draini= ng useless. We also can into drain_all_pages() opportunistically, without waiting for t= he result. On a busy system it's most likely useless, we might oom before scheduled wo= rks will be executed. I admit I planned to do some work around and even started, but then never h= ad enough time to finish it. Overall I'm somewhat resistant to an idea of making generic allocation & fr= ee 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. Thanks!