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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id E559BC05027 for ; Fri, 20 Jan 2023 16:51:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229993AbjATQu7 (ORCPT ); Fri, 20 Jan 2023 11:50:59 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33222 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230504AbjATQuw (ORCPT ); Fri, 20 Jan 2023 11:50:52 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id AD2A8C79C0 for ; Fri, 20 Jan 2023 08:50:33 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=F1zALDE7aTxyrS74rgv02kuygXmkrK+dwh/wuZBtihs=; b=jpqicOFLnybxU1J/sYTsyjmHPn IEXlj2YUuwcpXrOpxj/kOzWraZQxHDLwL/HfUlrGfp7en08E+Avdj99YfFJk4Z/yRrQePX17QhoS+ FA2mD3Kigz+l/dmYCChZGbrulREuzox3D1s6THtew9Ls+d3Svi+wdKOQf0y9RCMaKD+n77iffBNvp LpDQp1f8VOeerGgi1fQvCHNbTXO5MvvYGluxo1BA+N2fFNzNVGFgtHSziBUaY4aCKyT0hN7n+V2Nj ZzpoLyWOq2pnrBe+A3C34mNKkV5vKx/QB8xoHfHIH4J4YXe4tRXqut4+NIv0Sx5IOGnAQhLoMyFGW luSl9FIg==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1pIuaE-002ElZ-3c; Fri, 20 Jan 2023 16:49:42 +0000 Date: Fri, 20 Jan 2023 16:49:42 +0000 From: Matthew Wilcox 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, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, 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: References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-40-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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().