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 0EE8DC433EF for ; Wed, 18 May 2022 12:52:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237070AbiERMwD (ORCPT ); Wed, 18 May 2022 08:52:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42322 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236836AbiERMwA (ORCPT ); Wed, 18 May 2022 08:52:00 -0400 Received: from outbound-smtp25.blacknight.com (outbound-smtp25.blacknight.com [81.17.249.193]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BCB3218E19 for ; Wed, 18 May 2022 05:51:57 -0700 (PDT) Received: from mail.blacknight.com (pemlinmail03.blacknight.ie [81.17.254.16]) by outbound-smtp25.blacknight.com (Postfix) with ESMTPS id 90070CAB78 for ; Wed, 18 May 2022 13:51:55 +0100 (IST) Received: (qmail 19066 invoked from network); 18 May 2022 12:51:55 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.198.246]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 18 May 2022 12:51:54 -0000 Date: Wed, 18 May 2022 13:51:52 +0100 From: Mel Gorman To: Qian Cai Cc: Andrew Morton , Nicolas Saenz Julienne , Marcelo Tosatti , Vlastimil Babka , Michal Hocko , LKML , Linux-MM Subject: Re: [PATCH 0/6] Drain remote per-cpu directly v3 Message-ID: <20220518125152.GQ3441@techsingularity.net> References: <20220512085043.5234-1-mgorman@techsingularity.net> <20220517233507.GA423@qian> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20220517233507.GA423@qian> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 17, 2022 at 07:35:07PM -0400, Qian Cai wrote: > On Thu, May 12, 2022 at 09:50:37AM +0100, Mel Gorman wrote: > > Changelog since v2 > > o More conversions from page->lru to page->[pcp_list|buddy_list] > > o Additional test results in changelogs > > > > Changelog since v1 > > o Fix unsafe RT locking scheme > > o Use spin_trylock on UP PREEMPT_RT > > > > This series has the same intent as Nicolas' series "mm/page_alloc: Remote > > per-cpu lists drain support" -- avoid interference of a high priority > > task due to a workqueue item draining per-cpu page lists. While many > > workloads can tolerate a brief interruption, it may be cause a real-time > > task runnning on a NOHZ_FULL CPU to miss a deadline and at minimum, > > the draining in non-deterministic. > > > > Currently an IRQ-safe local_lock protects the page allocator per-cpu lists. > > The local_lock on its own prevents migration and the IRQ disabling protects > > from corruption due to an interrupt arriving while a page allocation is > > in progress. The locking is inherently unsafe for remote access unless > > the CPU is hot-removed. > > > > This series adjusts the locking. A spinlock is added to struct > > per_cpu_pages to protect the list contents while local_lock_irq continues > > to prevent migration and IRQ reentry. This allows a remote CPU to safely > > drain a remote per-cpu list. > > > > This series is a partial series. Follow-on work should allow the > > local_irq_save to be converted to a local_irq to avoid IRQs being > > disabled/enabled in most cases. Consequently, there are some TODO comments > > highlighting the places that would change if local_irq was used. However, > > there are enough corner cases that it deserves a series on its own > > separated by one kernel release and the priority right now is to avoid > > interference of high priority tasks. > > Reverting the whole series fixed an issue that offlining a memory > section blocking for hours on today's linux-next tree. > > __wait_rcu_gp > synchronize_rcu at kernel/rcu/tree.c:3915 > lru_cache_disable at mm/swap.c:886 > __alloc_contig_migrate_range at mm/page_alloc.c:9078 > isolate_single_pageblock at mm/page_isolation.c:405 > start_isolate_page_range > offline_pages > memory_subsys_offline > device_offline > online_store > dev_attr_store > sysfs_kf_write > kernfs_fop_write_iter > new_sync_write > vfs_write > ksys_write > __arm64_sys_write > invoke_syscall > el0_svc_common.constprop.0 > do_el0_svc > el0_svc > el0t_64_sync_handler > el0t_64_sync > > For full disclosure, I have also reverted the commit 0d523026abd4 > ("mm/page_alloc: fix tracepoint mm_page_alloc_zone_locked()"), so the > series can be reverted cleanly. But, I can't see how the commit > 0d523026abd4 could cause this issue at all. This is halting in __lru_add_drain_all where it calls synchronize_rcu before the drain even happens. It's also an LRU drain and not PCP which is what the series affects and the allocator doesn't use rcu. In a KVM machine, I can do $ for BANK in `(for i in {1..20}; do echo $((RANDOM%416)); done) | sort -n | uniq`; do BEFORE=`cat /sys/devices/system/memory/memory$BANK/online`; echo 0 > /sys/devices/system/memory/memory$BANK/online; AFTER=`cat /sys/devices/system/memory/memory$BANK/online`; printf "%4d %d -> %d\n" $BANK $BEFORE $AFTER; done 3 1 -> 0 57 1 -> 0 74 1 -> 0 93 1 -> 0 101 1 -> 0 128 1 -> 0 133 1 -> 0 199 1 -> 0 223 1 -> 0 225 1 -> 0 229 1 -> 0 243 1 -> 0 263 1 -> 0 300 1 -> 0 309 1 -> 0 329 1 -> 0 355 1 -> 0 365 1 -> 0 372 1 -> 0 383 1 -> 0 It offlines 20 sections although after several attempts free -m starts reporting negative used memory so there is a bug of some description. How are you testing this exactly? Is it every time or intermittent? Are you confident that reverting the series makes the problem go away? -- Mel Gorman SUSE Labs