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 0B617C05027 for ; Fri, 20 Jan 2023 17:22:59 +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=Px+4xwrLUluk0iVNc6cuF1tLG9EPVZuR3fZbBq8h2sk=; b=wgbaro2B46X4G+ TCNjy4wWfR0bMneSIUSsYhnGq7vrT9okfjXO7CFb3DEr/fvwPD5TD9x3+vxS8jP2fwngCOSa9LpR7 ARaVef2gvjUGmrUbK7oJ7uXmVbyXqlXgSjSiM1Y98v05KEimZ0/2pPpgU31W7rCUo+eQFyvtD+oxx rOIoLCizvyTX4P57HxmRrIvAn+B1fVUgDPnFejkAoL8d0JIQXR5SNTOrK0aTY0cpETYe9xAwIg3bz 3hfyypjCgTPLQ/UL5qFagdimlFG/xp7204QhXZyPEXBqVhjUQm0gOY1WWazM/WUhd2eaLK26R7mIL WeZfANipYVEwjKFo307A==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1pIv5I-00BXCw-B2; Fri, 20 Jan 2023 17:21:49 +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 1pIv5C-00BXA2-M6 for linux-arm-kernel@lists.infradead.org; Fri, 20 Jan 2023 17:21:44 +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 D486D62024; Fri, 20 Jan 2023 17:21:41 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id F3F22C433A1; Fri, 20 Jan 2023 17:21:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1674235301; bh=I1qTWw2OMTpCQGNS0oCnz+DeLugW7m0MkCxFsFb/Dak=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=Kfdej4NjVMobPhJ3rE/PMAzza8oADmNlkGqiZYGqRvTSIUQdTHKRd6NHGGwNKg8g4 Wv67aWSm+XPbi1RBoo3IcSzQxQNxls+vkqagf4zovgJuG5e5YtyX6zoONtyfcnsHr/ MmszMAR/PoZVZaQ2ZUmMiK+VbMH8Nxupenpd4yEa4t3V07RSClxZbnlZ9l7Bxy/Eha 7l4dxYe+UKqgg6a7G6pQcoxZWuDHr9mOJgi4qoCp2cwXyt1YcH92lwoAsO8udPKTIu N9GMcWwfBWm9lgbgj4KzGHOF1wWRo+bYyHu/V0BAlDo1pzAH3QCCu2hHfcUhjy3Azr AmgM+CiHvme9Q== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 8BB745C17DC; Fri, 20 Jan 2023 09:21:40 -0800 (PST) Date: Fri, 20 Jan 2023 09:21:40 -0800 From: "Paul E. McKenney" To: Matthew Wilcox Cc: Suren Baghdasaryan , Michal Hocko , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, 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: <20230120172140.GL2948950@paulmck-ThinkPad-P17-Gen-1> References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-40-surenb@google.com> 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-20230120_092142_872341_D9428BC6 X-CRM114-Status: GOOD ( 41.50 ) 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 Fri, Jan 20, 2023 at 04:49:42PM +0000, Matthew Wilcox wrote: > On Fri, Jan 20, 2023 at 08:45:21AM -0800, Suren Baghdasaryan wrote: > > On Fri, Jan 20, 2023 at 8:20 AM Suren Baghdasaryan wrote: > > > > > > On Fri, Jan 20, 2023 at 12:52 AM Michal Hocko wrote: > > > > > > > > On Thu 19-01-23 10:52:03, Suren Baghdasaryan wrote: > > > > > On Thu, Jan 19, 2023 at 4:59 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. To minimize that impact, place VMAs into > > > > > > > a list and free them in groups using one call_rcu() call per group. > > > > > > > > > > > > After some more clarification I can understand how call_rcu might not be > > > > > > super happy about thousands of callbacks to be invoked and I do agree > > > > > > that this is not really optimal. > > > > > > > > > > > > On the other hand I do not like this solution much either. > > > > > > VM_AREA_FREE_LIST_MAX is arbitrary and it won't really help all that > > > > > > much with processes with a huge number of vmas either. It would still be > > > > > > in housands of callbacks to be scheduled without a good reason. > > > > > > > > > > > > Instead, are there any other cases than remove_vma that need this > > > > > > batching? We could easily just link all the vmas into linked list and > > > > > > use a single call_rcu instead, no? This would both simplify the > > > > > > implementation, remove the scaling issue as well and we do not have to > > > > > > argue whether VM_AREA_FREE_LIST_MAX should be epsilon or epsilon + 1. > > > > > > > > > > Yes, I agree the solution is not stellar. I wanted something simple > > > > > but this is probably too simple. OTOH keeping all dead vm_area_structs > > > > > on the list without hooking up a shrinker (additional complexity) does > > > > > not sound too appealing either. > > > > > > > > I suspect you have missed my idea. I do not really want to keep the list > > > > around or any shrinker. It is dead simple. Collect all vmas in > > > > remove_vma and then call_rcu the whole list at once after the whole list > > > > (be it from exit_mmap or remove_mt). See? > > > > > > Yes, I understood your idea but keeping dead objects until the process > > > exits even when the system is low on memory (no shrinkers attached) > > > seems too wasteful. If we do this I would advocate for attaching a > > > shrinker. > > > > Maybe even simpler, since we are hit with this VMA freeing flood > > during exit_mmap (when all VMAs are destroyed), we pass a hint to > > vm_area_free to batch the destruction and all other cases call > > call_rcu()? I don't think there will be other cases of VMA destruction > > floods. > > ... or have two different call_rcu functions; one for munmap() and > one for exit. It'd be nice to use kmem_cache_free_bulk(). Good point, kfree_rcu(p, r) where "r" is the name of the rcu_head structure's field, is much more cache-efficient. The penalty is that there is no callback function to do any cleanup. There is just a kfree()/kvfree (bulk version where applicable), nothing else. Thanx, Paul _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel