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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9750EC433F5 for ; Fri, 4 Mar 2022 03:52:34 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 349F58D0002; Thu, 3 Mar 2022 22:52:34 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 2FA368D0001; Thu, 3 Mar 2022 22:52:34 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 1E9638D0002; Thu, 3 Mar 2022 22:52:34 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0079.hostedemail.com [216.40.44.79]) by kanga.kvack.org (Postfix) with ESMTP id 12A7C8D0001 for ; Thu, 3 Mar 2022 22:52:34 -0500 (EST) Received: from smtpin21.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id B2E3696F1D for ; Fri, 4 Mar 2022 03:52:33 +0000 (UTC) X-FDA: 79205331786.21.79A0314 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf29.hostedemail.com (Postfix) with ESMTP id 1661F12000A for ; Fri, 4 Mar 2022 03:52:32 +0000 (UTC) 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 C9C55B826EC; Fri, 4 Mar 2022 01:03:25 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 28BF4C340E9; Fri, 4 Mar 2022 01:03:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1646355804; bh=kEGxW7YmAqNPZGYRTWmVUsbA8UegX6jJvMlZtWicT4M=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=YpLXx7TWbtRmK31E33qnQgUjEcp2DYpVsFjNH1B1BDsAynYPFzUtaD1cnW5n1ULIb +r3jqEystMhLPKLO4Va6NbWAdz02KdTsLFdH0PHQJwPNbvEwXDczzcf1SiuaGpWemh A1IrjbEYeJ081+qenPQIToO+qNaetbNeWn+PqlC8= Date: Thu, 3 Mar 2022 17:03:23 -0800 From: Andrew Morton To: Marcelo Tosatti Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Minchan Kim , Matthew Wilcox , Mel Gorman , Nicolas Saenz Julienne , Juri Lelli , Thomas Gleixner , Sebastian Andrzej Siewior , "Paul E. McKenney" Subject: Re: [patch v3] mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu Message-Id: <20220303170323.82d8424d214fcb3a32155952@linux-foundation.org> In-Reply-To: References: X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: ty3gf33nsoxcbke9szqk6gwsy7557i8n Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=linux-foundation.org header.s=korg header.b=YpLXx7TW; spf=pass (imf29.hostedemail.com: domain of akpm@linux-foundation.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org; dmarc=none X-Rspamd-Queue-Id: 1661F12000A X-HE-Tag: 1646365952-787488 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: (Question for paulmck below, please) On Tue, 22 Feb 2022 13:07:35 -0300 Marcelo Tosatti wrote: > > On systems that run FIFO:1 applications that busy loop > on isolated CPUs, executing tasks on such CPUs under > lower priority is undesired (since that will either > hang the system, or cause longer interruption to the > FIFO task due to execution of lower priority task > with very small sched slices). > > Commit d479960e44f27e0e52ba31b21740b703c538027c ("mm: disable LRU > pagevec during the migration temporarily") relies on > queueing work items on all online CPUs to ensure visibility > of lru_disable_count. > > However, its possible to use synchronize_rcu which will provide the same > guarantees (see comment this patch modifies on lru_cache_disable). > > Fixes: > > [ 1873.243925] INFO: task kworker/u160:0:9 blocked for more than 622 seconds. > [ 1873.243927] Tainted: G I --------- --- 5.14.0-31.rt21.31.el9.x86_64 #1 > [ 1873.243929] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > [ 1873.243929] task:kworker/u160:0 state:D stack: 0 pid: 9 ppid: 2 flags:0x00004000 > [ 1873.243932] Workqueue: cpuset_migrate_mm cpuset_migrate_mm_workfn > [ 1873.243936] Call Trace: > [ 1873.243938] __schedule+0x21b/0x5b0 > [ 1873.243941] schedule+0x43/0xe0 > [ 1873.243943] schedule_timeout+0x14d/0x190 > [ 1873.243946] ? resched_curr+0x20/0xe0 > [ 1873.243953] ? __prepare_to_swait+0x4b/0x70 > [ 1873.243958] wait_for_completion+0x84/0xe0 > [ 1873.243962] __flush_work.isra.0+0x146/0x200 > [ 1873.243966] ? flush_workqueue_prep_pwqs+0x130/0x130 > [ 1873.243971] __lru_add_drain_all+0x158/0x1f0 > [ 1873.243978] do_migrate_pages+0x3d/0x2d0 > [ 1873.243985] ? pick_next_task_fair+0x39/0x3b0 > [ 1873.243989] ? put_prev_task_fair+0x1e/0x30 > [ 1873.243992] ? pick_next_task+0xb30/0xbd0 > [ 1873.243995] ? __tick_nohz_task_switch+0x1e/0x70 > [ 1873.244000] ? raw_spin_rq_unlock+0x18/0x60 > [ 1873.244002] ? finish_task_switch.isra.0+0xc1/0x2d0 > [ 1873.244005] ? __switch_to+0x12f/0x510 > [ 1873.244013] cpuset_migrate_mm_workfn+0x22/0x40 > [ 1873.244016] process_one_work+0x1e0/0x410 > [ 1873.244019] worker_thread+0x50/0x3b0 > [ 1873.244022] ? process_one_work+0x410/0x410 > [ 1873.244024] kthread+0x173/0x190 > [ 1873.244027] ? set_kthread_struct+0x40/0x40 > [ 1873.244031] ret_from_fork+0x1f/0x30 > > ... > > --- a/mm/swap.c > +++ b/mm/swap.c > @@ -831,8 +831,7 @@ inline void __lru_add_drain_all(bool force_all_cpus) > for_each_online_cpu(cpu) { > struct work_struct *work = &per_cpu(lru_add_drain_work, cpu); > > - if (force_all_cpus || > - pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) || > + if (pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) || > data_race(pagevec_count(&per_cpu(lru_rotate.pvec, cpu))) || > pagevec_count(&per_cpu(lru_pvecs.lru_deactivate_file, cpu)) || > pagevec_count(&per_cpu(lru_pvecs.lru_deactivate, cpu)) || This change appears to be "don't queue work on CPUs which don't have any work to do". Correct? This isn't changelogged? > @@ -876,14 +875,19 @@ atomic_t lru_disable_count = ATOMIC_INIT(0); > void lru_cache_disable(void) > { > atomic_inc(&lru_disable_count); > + synchronize_rcu(); > #ifdef CONFIG_SMP > /* > - * lru_add_drain_all in the force mode will schedule draining on > - * all online CPUs so any calls of lru_cache_disabled wrapped by > - * local_lock or preemption disabled would be ordered by that. > - * The atomic operation doesn't need to have stronger ordering > - * requirements because that is enforced by the scheduling > - * guarantees. > + * synchronize_rcu() waits for preemption disabled > + * and RCU read side critical sections. > + * For the users of lru_disable_count: > + * > + * preempt_disable, local_irq_disable [bh_lru_lock()] > + * rcu_read_lock [rt_spin_lock CONFIG_PREEMPT_RT] > + * preempt_disable [local_lock !CONFIG_PREEMPT_RT] > + * > + * so any calls of lru_cache_disabled wrapped by local_lock or > + * preemption disabled would be ordered by that. > */ > __lru_add_drain_all(true); > #else Does this also work with CONFIG_TINY_RCU? This seems abusive of synchronize_rcu(). None of this code uses RCU, but it so happens that synchronize_rcu() happily provides the desired effects. Changes in RCU's happy side-effects might break this. Perhaps a formal API function which does whatever-you-want-it-to-do would be better. And... I really don't understand the fix. What is it about synchronize_rcu() which guarantees that a work function which is queued on CPU N will now get executed even if CPU N is spinning in SCHED_FIFO userspace?