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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id DAC0AC004D4 for ; Thu, 19 Jan 2023 19:23:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:Reply-To:List-Subscribe:List-Help: List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=8Cvn1GWnyN9WYyhx8+eCLs8S1dxAuDdgcGdc1gzMEjg=; b=edWaqgitXvks9B IwqZyHhKpuOwILymYS5O5MzceEl6rx/tXGFQyjToDJbLoF3NLTb/CMnTTtrMBj4FyR5wQkbZKu4EP pnUbMEABI+SSUppS+2gQp6tVYR8AFfWb+yTmQL7dzUF7c6F9oNhhtKwO80xByBHTk84qr6d4C5DC7 zrtL6S0q0zNadEKEbddX/JC1XzZU9m3iEPcc0DhjB9eSCNoyMQt/eI+9JqYJCYaZhlzl0Dnskl5i/ 5jNV07f81OqW7jv7ljl/kPywt19ldEe6pHFr1kpflM3DXyK3c5jMjJYpMFdzuf0IvahX0Wl58ojfz z17ZYrC1UqEP5jmCkqEw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1pIaU9-006u7z-DU; Thu, 19 Jan 2023 19:22:08 +0000 Received: from dfw.source.kernel.org ([2604:1380:4641:c500::1]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1pIaPN-006rki-6R for linux-arm-kernel@lists.infradead.org; Thu, 19 Jan 2023 19:17:14 +0000 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 23BC461D52; Thu, 19 Jan 2023 19:17:08 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6C3F5C433F0; Thu, 19 Jan 2023 19:17:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1674155827; bh=eCVbGXT27QnDo1PpAtkdYdCPSqT1Q5ve44Q8Q75yHjw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=FCre4bIyUsGr3UbsqKMkAQvOnNpSp+ZWq+NbqqKwwWefnRM01gepbNKLipNzquSuy TS1GmlOYCg6zD4+UEVOtBJY+0ZPzqe/LmfVDG1bJ9ZhVV0p32LzzMAiWRBMPn1T1h4 lAj0M0gK0MPei3TtqZWQkMcaTNS1rzDw3cKDdRJFes7uSpMhuW4+YiOaItA+N+gKAc JMMK/ovArgjTbmyUfDHcWXuHF2hwBrtaAlfxcWTYx4Ccah0CVipkCZ8Vylnq9UGQx7 NSQGdLJX6rKSEaJIAZ+2hMcEPpSmmYmmGZ10dwS+6DGIGltSWvjltMSHYoKUe32kIM fDrsAF3VjIYdg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 0FE685C1A49; Thu, 19 Jan 2023 11:17:07 -0800 (PST) Date: Thu, 19 Jan 2023 11:17:07 -0800 From: "Paul E. McKenney" To: Michal Hocko Cc: Suren Baghdasaryan , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free Message-ID: <20230119191707.GW2948950@paulmck-ThinkPad-P17-Gen-1> References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-40-surenb@google.com> <20230118183447.GG2948950@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230119_111709_449142_18F4B962 X-CRM114-Status: GOOD ( 46.31 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: paulmck@kernel.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Jan 19, 2023 at 01:52:14PM +0100, Michal Hocko wrote: > On Wed 18-01-23 11:01:08, Suren Baghdasaryan wrote: > > On Wed, Jan 18, 2023 at 10:34 AM Paul E. McKenney wrote: > [...] > > > There are a couple of possibilities here. > > > > > > First, if I am remembering correctly, the time between the call_rcu() > > > and invocation of the corresponding callback was taking multiple seconds, > > > but that was because the kernel was built with CONFIG_LAZY_RCU=y in > > > order to save power by batching RCU work over multiple call_rcu() > > > invocations. If this is causing a problem for a given call site, the > > > shiny new call_rcu_hurry() can be used instead. Doing this gets back > > > to the old-school non-laziness, but can of course consume more power. > > > > That would not be the case because CONFIG_LAZY_RCU was not an option > > at the time I was profiling this issue. > > Laxy RCU would be a great option to replace this patch but > > unfortunately it's not the default behavior, so I would still have to > > implement this batching in case lazy RCU is not enabled. > > > > > > > > Second, there is a much shorter one-jiffy delay between the call_rcu() > > > and the invocation of the corresponding callback in kernels built with > > > either CONFIG_NO_HZ_FULL=y (but only on CPUs mentioned in the nohz_full > > > or rcu_nocbs kernel boot parameters) or CONFIG_RCU_NOCB_CPU=y (but only > > > on CPUs mentioned in the rcu_nocbs kernel boot parameters). The purpose > > > of this delay is to avoid lock contention, and so this delay is incurred > > > only on CPUs that are queuing callbacks at a rate exceeding 16K/second. > > > This is reduced to a per-jiffy limit, so on a HZ=1000 system, a CPU > > > invoking call_rcu() at least 16 times within a given jiffy will incur > > > the added delay. The reason for this delay is the use of a separate > > > ->nocb_bypass list. As Suren says, this bypass list is used to reduce > > > lock contention on the main ->cblist. This is not needed in old-school > > > kernels built without either CONFIG_NO_HZ_FULL=y or CONFIG_RCU_NOCB_CPU=y > > > (including most datacenter kernels) because in that case the callbacks > > > enqueued by call_rcu() are touched only by the corresponding CPU, so > > > that there is no need for locks. > > > > I believe this is the reason in my profiled case. > > > > > > > > Third, if you are instead seeing multiple milliseconds of CPU consumed by > > > call_rcu() in the common case (for example, without the aid of interrupts, > > > NMIs, or SMIs), please do let me know. That sounds to me like a bug. > > > > I don't think I've seen such a case. > > Thanks for clarifications, Paul! > > Thanks for the explanation Paul. I have to say this has caught me as a > surprise. There are just not enough details about the benchmark to > understand what is going on but I find it rather surprising that > call_rcu can induce a higher overhead than the actual kmem_cache_free > which is the callback. My naive understanding has been that call_rcu is > really fast way to defer the execution to the RCU safe context to do the > final cleanup. If I am following along correctly (ha!), then your "induce a higher overhead" should be something like "induce a higher to-kfree() latency". Of course, there already is a higher latency-to-kfree via call_rcu() than via a direct call to kfree(), and callback-offload CPUs that are being flooded with callbacks raise that latency a jiffy or so more in order to avoid lock contention. If this becomes a problem, the callback-offloading code can be a bit smarter about avoiding lock contention, but need to see a real problem before I make that change. But if there is a real problem I will of course fix it. Or did I miss a turn in this discussion? Thanx, Paul _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel