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 822F3C004D4 for ; Thu, 19 Jan 2023 19:21:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231491AbjASTVY (ORCPT ); Thu, 19 Jan 2023 14:21:24 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59384 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231402AbjASTUz (ORCPT ); Thu, 19 Jan 2023 14:20:55 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 916B79DC87 for ; Thu, 19 Jan 2023 11:20:03 -0800 (PST) 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 7395161D23 for ; Thu, 19 Jan 2023 19:20:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C32D4C433F0; Thu, 19 Jan 2023 19:20:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1674156002; bh=sNtFF/n9MnZh/97jwulEfUXTbaxJQPrJ55JS7ZpWQ8A=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=GZFSCrODvWxbeYJIxgxvUyLH5Bhha6cEF9ov6eaWW972FVwxQeMbcwRyDHwfbqatd P+lNv8xWOCofVqyXl8boy2P+eIVwPkIxaXJx5Mnmck6tlpuHh3bCM08xuLF3UssXLw nyEJeHwbrLPEdL41bo1JPvo+KlSP+mGoRoqvqvXTnUuzNGWwFcHIUkEszYMJTrLIX8 m9v6OwccgV9ALZdgTrAmRxHhoaUxoeNrZ/jjI3dMs8rcbqYmMHmxZNqetFqn81O0HS lAGAVg0qzxG943YwiktwaXoRb8EQm5WIN9BqRqpuUPxzTkfw8PedrY4/ToF2MJHls3 o+f0eY20owrdQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 5E4BB5C1A49; Thu, 19 Jan 2023 11:20:02 -0800 (PST) Date: Thu, 19 Jan 2023 11:20:02 -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: <20230119192002.GX2948950@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org 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, Jan 19, 2023 at 10:52:03AM -0800, 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. WDYT about time domain throttling to > limit draining the list to say once per second like this: > > void vm_area_free(struct vm_area_struct *vma) > { > struct mm_struct *mm = vma->vm_mm; > bool drain; > > free_anon_vma_name(vma); > > spin_lock(&mm->vma_free_list.lock); > list_add(&vma->vm_free_list, &mm->vma_free_list.head); > mm->vma_free_list.size++; > - drain = mm->vma_free_list.size > VM_AREA_FREE_LIST_MAX; > + drain = jiffies > mm->last_drain_tm + HZ; > > spin_unlock(&mm->vma_free_list.lock); > > - if (drain) > + if (drain) { > drain_free_vmas(mm); > + mm->last_drain_tm = jiffies; > + } > } > > Ultimately we want to prevent very frequent call_rcu() calls, so > throttling in the time domain seems appropriate. That's the simplest > way I can think of to address your concern about a quick spike in VMA > freeing. It does not place any restriction on the list size and we > might have excessive dead vm_area_structs if after a large spike there > are no vm_area_free() calls but I don't know if that's a real problem, > so not sure we should be addressing it at this time. WDYT? Just to double-check, we really did try the very frequent call_rcu() invocations and we really did see a problem, correct? Although it is not perfect, call_rcu() is designed to take a fair amount of abuse. So if we didn't see a real problem, the frequent call_rcu() invocations might be a bit simpler. Thanx, Paul