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([2804:1b3:a802:1099:7cb2:3a49:6197:5307]) by smtp.gmail.com with ESMTPSA id cm3-20020a056830650300b006618586b850sm410907otb.46.2022.11.03.07.59.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 03 Nov 2022 07:59:27 -0700 (PDT) Message-ID: <07810c49ef326b26c971008fb03adf9dc533a178.camel@redhat.com> Subject: Re: [PATCH v1 0/3] Avoid scheduling cache draining to isolated cpus From: Leonardo =?ISO-8859-1?Q?Br=E1s?= To: Michal Hocko Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider , Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Frederic Weisbecker , Phil Auld , Marcelo Tosatti , linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, linux-mm@kvack.org Date: Thu, 03 Nov 2022 11:59:20 -0300 In-Reply-To: References: <20221102020243.522358-1-leobras@redhat.com> User-Agent: Evolution 3.46.1 MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1667487571; h=from:from:sender: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:dkim-signature; bh=tQYuzqixH+Fy3EOo9WqVlwQ+6mailyHi9cBj8lDCjt4=; b=5D82ol2Pr+WwElQVEyHgMFk2KXd8i9w3vgd0g7QhQU+SY4fldWSeO9CATe30t9Zo4nloZu WFWWgdw8X8UdmAxmZSpJFQCHzjAGdWRP4KRHHw/7CFsOXbQGsSVU5Hk2Yf0LO4/tVGS8qJ TPfBebTwClkqBWB591DythJdCx6YjRw= ARC-Authentication-Results: i=1; imf27.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="dlNLu3e/"; spf=pass (imf27.hostedemail.com: domain of leobras@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=leobras@redhat.com; dmarc=pass (policy=none) header.from=redhat.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1667487571; a=rsa-sha256; cv=none; b=IX5haFEp1IT7FsfhaY+8RT+4BwhMA5s5jf4deHe5uk7xfRuVfUDml5bwY6n6AqsQbzlNz2 Cf6qU3j5cSSI3kenk7O48qwx9P3XdOneBUZXr6qXtrzMIfTl1ntHjSpl0HZ6UDZ56VczT/ 8SaRbxE74pSfmQdW34+Ak6v7j5szzL8= X-Stat-Signature: dxbaydakx5dg5woh4ob6mj49y7sxg94h X-Rspamd-Queue-Id: 8106B40007 X-Rspamd-Server: rspam06 X-Rspam-User: Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b="dlNLu3e/"; spf=pass (imf27.hostedemail.com: domain of leobras@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=leobras@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-HE-Tag: 1667487571-40784 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 Wed, 2022-11-02 at 09:53 +0100, Michal Hocko wrote: > On Tue 01-11-22 23:02:40, Leonardo Bras wrote: > > Patch #1 expands housekeep=C3=ADng_any_cpu() so we can find housekeepin= g cpus > > closer (NUMA) to any desired CPU, instead of only the current CPU. > >=20 > > ### Performance argument that motivated the change: > > There could be an argument of why would that be needed, since the curre= nt > > CPU is probably acessing the current cacheline, and so having a CPU clo= ser > > to the current one is always the best choice since the cache invalidati= on > > will take less time. OTOH, there could be cases like this which uses > > perCPU variables, and we can have up to 3 different CPUs touching the > > cacheline: > >=20 > > C1 - Isolated CPU: The perCPU data 'belongs' to this one > > C2 - Scheduling CPU: Schedule some work to be done elsewhere, current c= pu > > C3 - Housekeeping CPU: This one will do the work > >=20 > > Most of the times the cacheline is touched, it should be by C1. Some ti= mes > > a C2 will schedule work to run on C3, since C1 is isolated. > >=20 > > If C1 and C2 are in different NUMA nodes, we could have C3 either in > > C2 NUMA node (housekeeping_any_cpu()) or in C1 NUMA node=20 > > (housekeeping_any_cpu_from(C1).=20 > >=20 > > If C3 is in C2 NUMA node, there will be a faster invalidation when C3 > > tries to get cacheline exclusivity, and then a slower invalidation when > > this happens in C1, when it's working in its data. > >=20 > > If C3 is in C1 NUMA node, there will be a slower invalidation when C3 > > tries to get cacheline exclusivity, and then a faster invalidation when > > this happens in C1. > >=20 > > The thing is: it should be better to wait less when doing kernel work > > on an isolated CPU, even at the cost of some housekeeping CPU waiting > > a few more cycles. > > ### > >=20 > > Patch #2 changes the locking strategy of memcg_stock_pcp->stock_lock fr= om > > local_lock to spinlocks, so it can be later used to do remote percpu > > cache draining on patch #3. Most performance concerns should be pointed > > in the commit log. > >=20 > > Patch #3 implements the remote per-CPU cache drain, making use of both= =20 > > patches #2 and #3. Performance-wise, in non-isolated scenarios, it shou= ld > > introduce an extra function call and a single test to check if the CPU = is > > isolated.=20 > >=20 > > On scenarios with isolation enabled on boot, it will also introduce an > > extra test to check in the cpumask if the CPU is isolated. If it is, > > there will also be an extra read of the cpumask to look for a > > housekeeping CPU. >=20 Hello Michael, thanks for reviewing! > This is a rather deep dive in the cache line usage but the most > important thing is really missing. Why do we want this change? From the > context it seems that this is an actual fix for isolcpu=3D setup when > remote (aka non isolated activity) interferes with isolated cpus by > scheduling pcp charge caches on those cpus. >=20 > Is this understanding correct? That's correct! The idea is to avoid scheduling work to isolated CPUs. > If yes, how big of a problem that is? The use case I have been following requires both isolcpus=3D and PREEMPT_RT= , since the isolated CPUs will be running a real-time workload. In this scenario, getting any work done instead of the real-time workload may cause the syste= m to miss a deadline, which can be bad.=20 > If you want a remote draining then > you need some sort of locking (currently we rely on local lock). How > come this locking is not going to cause a different form of disturbance? If I did everything right, most of the extra work should be done either in = non- isolated (housekeeping) CPUs, or during a syscall. I mean, the pcp charge c= aches will be happening on a housekeeping CPU, and the locking cost should be pai= d there as we want to avoid doing that in the isolated CPUs.=20 I understand there will be a locking cost being paid in the isolated CPUs w= hen: a) The isolated CPU is requesting the stock drain, b) When the isolated CPUs do a syscall and end up using the protected struc= ture the first time after a remote drain. Both (a) and (b) should happen during a syscall, and IIUC the a rt workload should not expect the syscalls to be have a predictable time, so it should = be fine. Thanks for helping me explain the case! Best regards, Leo