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 BEB4DC38147 for ; Wed, 18 Jan 2023 20:21:47 +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=F8kjayYMPh50inL1oONMcAiahdoBY4AD8x/AtC0C+5g=; b=o+QkNqLVGefYJj 9U2NdnZuBWcxvILafDxS9jLc20v1xeHH52vANjJEoMB5j14S9IHlwHnLK9xnFiHqXlustrZY6hL4n eBLdfwHOFhlXxqquUW3PGAPqGtJymK3gqs65MlalDwd4dZhpmVBVmXPokju5XpoNvdLNZChx7ocR5 Oj8UzODdV1JQ492RH0g+u6FoZ7FbzMT1NvpA5cqFz6zDmIiJK6W9+aV1DHZu9pLBT3H1CGQv1QpPS BqmZuateqbc0u7jsB+sDaMJA16fSPRuRhUibKV0rICSuZA+kidUSGPcmFefnvvX/gL+lgubCP9IVq GPXJDQxSf7rgyVnc5m2g==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1pIEv5-002bDt-Sm; Wed, 18 Jan 2023 20:20:28 +0000 Received: from dfw.source.kernel.org ([139.178.84.217]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1pIEv2-002bCt-Fn for linux-arm-kernel@lists.infradead.org; Wed, 18 Jan 2023 20:20:26 +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 8FEA361A1E; Wed, 18 Jan 2023 20:20:21 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id E25FEC433D2; Wed, 18 Jan 2023 20:20:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1674073221; bh=LkDAI9qvKi0MAfsYbidawt8PxsElByOP+Pkz5GpU4hs=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=NSz4Cw/oXByK796oD3DytxHbEDXkhaciBuZCZt45dNnN2s0gmq78/PdAZYCRdE4OX gz3dDxoeuz+mKBLy2IgkAJzI+1iylam0JeecbKRS240Cf0ullxO+c5nmor/Gr5F/2J AwkG7qLuD7L71ncK3oGIUzOw2zE0s8dM/AYkWz+TxySO8nRHHCn4ks4FvNEXWGz3Ke CgbmmOlqZ+36yexqUqvhLyyj8u4HB/KHsj1pK62QWITX6NQM0KOXlOvkBMJjzlZhyh GfW49aeeMbUgY7ivyRdrvk4iT+2EsREynM2gVCKjqYcjsU3lveJY5nkdSQlBpQRhny p/yJPNrHzo9tg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 8482B5C0920; Wed, 18 Jan 2023 12:20:20 -0800 (PST) Date: Wed, 18 Jan 2023 12:20:20 -0800 From: "Paul E. McKenney" To: Suren Baghdasaryan Cc: Michal Hocko , 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: <20230118202020.GJ2948950@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-20230118_122024_619443_9B497054 X-CRM114-Status: GOOD ( 49.70 ) 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 Wed, Jan 18, 2023 at 11:01:08AM -0800, Suren Baghdasaryan wrote: > On Wed, Jan 18, 2023 at 10:34 AM Paul E. McKenney wrote: > > > > On Wed, Jan 18, 2023 at 10:04:39AM -0800, Suren Baghdasaryan wrote: > > > On Wed, Jan 18, 2023 at 1:49 AM Michal Hocko wrote: > > > > > > > > On Tue 17-01-23 17:19:46, Suren Baghdasaryan wrote: > > > > > On Tue, Jan 17, 2023 at 7:57 AM Michal Hocko wrote: > > > > > > > > > > > > On Mon 09-01-23 12:53:34, Suren Baghdasaryan wrote: > > > > > > > call_rcu() can take a long time when callback offloading is enabled. > > > > > > > Its use in the vm_area_free can cause regressions in the exit path when > > > > > > > multiple VMAs are being freed. > > > > > > > > > > > > What kind of regressions. > > > > > > > > > > > > > To minimize that impact, place VMAs into > > > > > > > a list and free them in groups using one call_rcu() call per group. > > > > > > > > > > > > Please add some data to justify this additional complexity. > > > > > > > > > > Sorry, should have done that in the first place. A 4.3% regression was > > > > > noticed when running execl test from unixbench suite. spawn test also > > > > > showed 1.6% regression. Profiling revealed that vma freeing was taking > > > > > longer due to call_rcu() which is slow when RCU callback offloading is > > > > > enabled. > > > > > > > > Could you be more specific? vma freeing is async with the RCU so how > > > > come this has resulted in a regression? Is there any heavy > > > > rcu_synchronize in the exec path? That would be an interesting > > > > information. > > > > > > No, there is no heavy rcu_synchronize() or any other additional > > > synchronous load in the exit path. It's the call_rcu() which can block > > > the caller if CONFIG_RCU_NOCB_CPU is enabled and there are lots of > > > other call_rcu()'s going on in parallel. Note that call_rcu() calls > > > rcu_nocb_try_bypass() if CONFIG_RCU_NOCB_CPU is enabled and profiling > > > revealed that this function was taking multiple ms (don't recall the > > > actual number, sorry). Paul's explanation implied that this happens > > > due to contention on the locks taken in this function. For more > > > in-depth details I'll have to ask Paul for help :) This code is quite > > > complex and I don't know all the details of RCU implementation. > > > > 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. Whew!!! ;-) > Thanks for clarifications, Paul! No problem! Thanx, Paul _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel