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 0EFA0C25B50 for ; Fri, 20 Jan 2023 08:52:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229528AbjATIwt (ORCPT ); Fri, 20 Jan 2023 03:52:49 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35370 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229447AbjATIwr (ORCPT ); Fri, 20 Jan 2023 03:52:47 -0500 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CDF801816A for ; Fri, 20 Jan 2023 00:52:45 -0800 (PST) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id D4440228BD; Fri, 20 Jan 2023 08:52:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1674204763; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=NPjXf88ZU8FVuBa4GWfXY3q6lH6kqoJUTnCqbetZU5Y=; b=rVOftAenNnA8PiL17Z4Pvgxr3PGI5zIZDNJZhTWOuwjNH7ZF8duaDITjsCCPVYUUl+4mAF SsZLXg+YuWZWmpQYKnBigdv+oOwpwP8b3sMrIOZpOAVevFZTdzxUdshJBulqHxA0CeorJN JV/cNcPxjm+c/YD6MkjUZzLm2MN4Cno= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id B24EA13251; Fri, 20 Jan 2023 08:52:43 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id hsl6K1tWymP+HQAAMHmgww (envelope-from ); Fri, 20 Jan 2023 08:52:43 +0000 Date: Fri, 20 Jan 2023 09:52:43 +0100 From: Michal Hocko To: Suren Baghdasaryan Cc: 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, 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 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? -- Michal Hocko SUSE Labs