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 22F61C433EF for ; Sat, 5 Mar 2022 04:33:47 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id A39428D000C; Fri, 4 Mar 2022 23:33:46 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 9E7A68D0001; Fri, 4 Mar 2022 23:33:46 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 8D6848D000C; Fri, 4 Mar 2022 23:33:46 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (relay.hostedemail.com [64.99.140.25]) by kanga.kvack.org (Postfix) with ESMTP id 7F8C28D0001 for ; Fri, 4 Mar 2022 23:33:46 -0500 (EST) Received: from smtpin06.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 5B1E5DE9 for ; Sat, 5 Mar 2022 04:33:46 +0000 (UTC) X-FDA: 79209064452.06.5DD10F3 Received: from sin.source.kernel.org (sin.source.kernel.org [145.40.73.55]) by imf04.hostedemail.com (Postfix) with ESMTP id 6ADC440002 for ; Sat, 5 Mar 2022 04:33:45 +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 sin.source.kernel.org (Postfix) with ESMTPS id 053A7CE2E16; Sat, 5 Mar 2022 04:33:43 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 51468C004E1; Sat, 5 Mar 2022 04:33:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1646454821; bh=YvJWYgita7RiF0BJsjp7/gHtf6Fs5XQSkvkSiE00kJE=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=LwCADQBWieEzNr83jw6YYACNrZNkiUdmsVH181i7vK3M4ZwdA14v0yD2fpDRxxnG8 +k/ChKJ9pTQyshV1V7JGwFLn66oqPhc0kFgpHcM12gNLE5hrW1rTEGjN9ODTXmDMXg O8jgZxQH1XbI48yl/MmRlzct7fzUTqTTfYt8/7NXokrwdbwQMS7lVGz0RaNzLz2C3x GL8Z0Ls1Da9bdJiQzyoVxyczkFr3iDqQGakfAJCHktQVTt5wvn3NN1d5HJkzRaDcD5 uWswzH3u5JJhejC0EJ+lrubuk4yzMmkxSkOY1U/yfy1B69yFkAixlz/egnStn/Kz+s 2cGCEN2FUqYWA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id D9E5C5C0920; Fri, 4 Mar 2022 20:33:40 -0800 (PST) Date: Fri, 4 Mar 2022 20:33:40 -0800 From: "Paul E. McKenney" 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 , Andrew Morton Subject: Re: [patch v4] mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu Message-ID: <20220305043340.GU4285@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 6ADC440002 X-Stat-Signature: sddsyyun3za3ydhhnbbjar37g8whck15 X-Rspam-User: Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=LwCADQBW; spf=pass (imf04.hostedemail.com: domain of "SRS0=QET2=TQ=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org" designates 145.40.73.55 as permitted sender) smtp.mailfrom="SRS0=QET2=TQ=paulmck-ThinkPad-P17-Gen-1.home=paulmck@kernel.org"; dmarc=pass (policy=none) header.from=kernel.org X-Rspamd-Server: rspam07 X-HE-Tag: 1646454825-831620 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: On Fri, Mar 04, 2022 at 01:29:31PM -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 > > Signed-off-by: Marcelo Tosatti Given the explanation and the comments below, this does look plausible to me. Thanx, Paul > --- > > v4: improve comment clarify, mention synchronize_rcu guarantees > on v5.1 (Andrew Morton / > Paul E. McKenney) > v3: update stale comment (Nicolas Saenz Julienne) > v2: rt_spin_lock calls rcu_read_lock, no need > to add it before local_lock on swap.c (Nicolas Saenz Julienne) > > diff --git a/mm/swap.c b/mm/swap.c > index bcf3ac288b56..b5ee163daa66 100644 > --- 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)) || > @@ -876,15 +875,21 @@ atomic_t lru_disable_count = ATOMIC_INIT(0); > void lru_cache_disable(void) > { > atomic_inc(&lru_disable_count); > -#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. > + * Readers of lru_disable_count are protected by either disabling > + * preemption or rcu_read_lock: > + * > + * preempt_disable, local_irq_disable [bh_lru_lock()] > + * rcu_read_lock [rt_spin_lock CONFIG_PREEMPT_RT] > + * preempt_disable [local_lock !CONFIG_PREEMPT_RT] > + * > + * Since v5.1 kernel, synchronize_rcu() is guaranteed to wait on > + * preempt_disable() regions of code. So any CPU which sees > + * lru_disable_count = 0 will have exited the critical > + * section when synchronize_rcu() returns. > */ > + synchronize_rcu(); > +#ifdef CONFIG_SMP > __lru_add_drain_all(true); > #else > lru_add_and_bh_lrus_drain(); >