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 416F3C46467 for ; Fri, 20 Jan 2023 05:30:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230343AbjATFaN (ORCPT ); Fri, 20 Jan 2023 00:30:13 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:32902 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231501AbjATF2x (ORCPT ); Fri, 20 Jan 2023 00:28:53 -0500 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 420E872C1F for ; Thu, 19 Jan 2023 21:23:54 -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 ams.source.kernel.org (Postfix) with ESMTPS id E5918B82723 for ; Thu, 19 Jan 2023 19:55:04 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7FE89C433EF; Thu, 19 Jan 2023 19:55:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1674158103; bh=EMPBQ4pP3jV2vAFvJnKq8YQhPGbB7BbH0D7DPSSWiE0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=NXfYK9b7Bf1duGFORs3XSGIs+UMUQmhuwi4YVxvT7G/a3nyhvq5WGGqDvi2Jt4yEC CYTOZC7SC2s5sbih3/HU7vME2OyQq2gao04lm/woIzKWR9ZAuME5yoiZT76tvEu8hY KD/2TqtFC+45YbMMbghKuKW2NZDEpeKMkn5IPB9C6zT63HjYC3lvSzCL46WofWmxqR IbepnruhsHrtTmpmHMTT97T4P70VZLJSmePvbr104ujcjZtUyjrMyTAdUN4QDVPhe4 ZwN+eCEq65ETEcXVtoQ149M5NPDMN14ArNCMZH1F+kzZfsaR/80+AdiIBYkJGMpyEW WAxZ65/uHOQrQ== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 18A4D5C1A49; Thu, 19 Jan 2023 11:55:03 -0800 (PST) Date: Thu, 19 Jan 2023 11:55:03 -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: <20230119195503.GY2948950@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-40-surenb@google.com> <20230119192002.GX2948950@paulmck-ThinkPad-P17-Gen-1> 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 11:47:36AM -0800, Suren Baghdasaryan wrote: > On Thu, Jan 19, 2023 at 11:20 AM Paul E. McKenney wrote: > > > > 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? > > Correct. More specifically with CONFIG_RCU_NOCB_CPU=y we saw > regressions when a process exits and all its VMAs get destroyed, > causing a flood of call_rcu()'s. Thank you for the reminder, real problem needs solution. ;-) Thanx, Paul > > 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.