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 39758C05027 for ; Fri, 20 Jan 2023 16:45:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230401AbjATQpg (ORCPT ); Fri, 20 Jan 2023 11:45:36 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56304 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229612AbjATQpf (ORCPT ); Fri, 20 Jan 2023 11:45:35 -0500 Received: from mail-yb1-xb2b.google.com (mail-yb1-xb2b.google.com [IPv6:2607:f8b0:4864:20::b2b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1F7C73A83 for ; Fri, 20 Jan 2023 08:45:34 -0800 (PST) Received: by mail-yb1-xb2b.google.com with SMTP id x4so5693457ybp.1 for ; Fri, 20 Jan 2023 08:45:34 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=z0OLQmsl59ql36Nt1ul7FYpKxJlgxRM2tCRyP08DhEY=; b=W59IhQSf4UTrvQ9bLPz6u3uc2BOxq0pMkHivnPC4k1cSc2+k+CVf+Uivommx3eFJkW L+t7MF2Jc6+x6P0uFcZalXlqTevHxQZvn7lftwlbuUrFIHwrq3DZtce5+tlWZuePmwkV 7XHIXDsM+/2s1DblH06GNCxqbkR9jxOA3O7ePu4LYLjnHSJ8UMoYYWWRtoKf1BEg/ucc TXY+NthBUNgrQf7FMhko2QUPSs6XH/Al/mWvHn66Wc85et242ge36hUadSNtnfCITKxx 8LWJghm6q4oVA5OlR2EsyU3muu3fvKhvD4yUWo8QOVtco0VBZfJbGHmGxiP5DDDldNDl 0+yQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=z0OLQmsl59ql36Nt1ul7FYpKxJlgxRM2tCRyP08DhEY=; b=T+MgvkvgXMq7uVP3XxxzxEKypabYmSvUfRPiIb80OigL7rkuPk+LQPmsz0yY51bRh+ Htl9S02BYUR92T9wGcb1/ta3mmexc0j/GNsxgT8yn9z6gIyA1zs16DeZBlWJeex8zHD0 jhDRZFS+xzOylDh6yyWaGzLw1PFNc1BJxHNlqEsqH+Kfr4FjtuGgGBFar4D+jgB/dGLb Hg+oKhQ+4PvsoEFLKaoMI+icNSB3vwpsFqElWSJNWV7s4Dwm9Jwmvh4B3gl6xQZRNVMZ D6WkLnggIFo4CbhcIf01sd06LIDSaJ5UGpUEkRR3HSTFtnAzed71/vx0nagNgy6IBiYZ /OoA== X-Gm-Message-State: AFqh2kriwwDFvXCokwZFpGLNsEKRzKvwQL7n/1VoUzQYVtTPPoO858Gl DgG23GO6rp47m0mzkJqknTHl/0VC/NJ90GzkSaSWMw== X-Google-Smtp-Source: AMrXdXvO2ujCwBBpTmun7yGb1xblSnuWHOwAyGX/hTpcV4QP0YJTe4uFn59cJoGH58RwqZy6oUCBBus40+natizZJ5U= X-Received: by 2002:a25:9801:0:b0:7d5:b884:3617 with SMTP id a1-20020a259801000000b007d5b8843617mr1424736ybo.380.1674233133069; Fri, 20 Jan 2023 08:45:33 -0800 (PST) MIME-Version: 1.0 References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-40-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Fri, 20 Jan 2023 08:45:21 -0800 Message-ID: Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free To: Michal Hocko 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 Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > > > > -- > > Michal Hocko > > SUSE Labs