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 CE67CC6FD1C for ; Wed, 22 Mar 2023 10:04:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229789AbjCVKEQ (ORCPT ); Wed, 22 Mar 2023 06:04:16 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:58056 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229750AbjCVKDu (ORCPT ); Wed, 22 Mar 2023 06:03:50 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 24E912E826 for ; Wed, 22 Mar 2023 03:03:49 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id D7C153399A; Wed, 22 Mar 2023 10:03:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1679479427; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=2MLTpDoYnLV5dF3NCYU327C0XL13UyAq3sNf1Gle6b8=; b=Evl84Jii4HGa0Ifhe7gJbp2u2F2WV1Utfwi7n6ACtiWkwMPUgbuKWTl2AQ1GE3ZQFX0CME 8d5wMMmcmAsRdSe2ohqlG6lMPYlAspsUtdmTcDscph5GngssyE98ECFSahGILxV++pjcoZ 1tRg+sNtZihsdjODCmpHr6kyyDZhSI0= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id B3F1613416; Wed, 22 Mar 2023 10:03:47 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id ZsqpKYPSGmSUBAAAMHmgww (envelope-from ); Wed, 22 Mar 2023 10:03:47 +0000 Date: Wed, 22 Mar 2023 11:03:47 +0100 From: Michal Hocko To: Marcelo Tosatti Cc: Christoph Lameter , Aaron Tomlin , Frederic Weisbecker , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Russell King , Huacai Chen , Heiko Carstens , x86@kernel.org, Vlastimil Babka Subject: Re: [PATCH v7 01/13] vmstat: allow_direct_reclaim should use zone_page_state_snapshot Message-ID: References: <20230320180332.102837832@redhat.com> <20230320180745.556821285@redhat.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 Mon 20-03-23 15:32:15, Marcelo Tosatti wrote: > On Mon, Mar 20, 2023 at 07:21:04PM +0100, Michal Hocko wrote: > > On Mon 20-03-23 15:03:33, Marcelo Tosatti wrote: > > > A customer provided evidence indicating that a process > > > was stalled in direct reclaim: > > > > > > - The process was trapped in throttle_direct_reclaim(). > > > The function wait_event_killable() was called to wait condition > > > allow_direct_reclaim(pgdat) for current node to be true. > > > The allow_direct_reclaim(pgdat) examined the number of free pages > > > on the node by zone_page_state() which just returns value in > > > zone->vm_stat[NR_FREE_PAGES]. > > > > > > - On node #1, zone->vm_stat[NR_FREE_PAGES] was 0. > > > However, the freelist on this node was not empty. > > > > > > - This inconsistent of vmstat value was caused by percpu vmstat on > > > nohz_full cpus. Every increment/decrement of vmstat is performed > > > on percpu vmstat counter at first, then pooled diffs are cumulated > > > to the zone's vmstat counter in timely manner. However, on nohz_full > > > cpus (in case of this customer's system, 48 of 52 cpus) these pooled > > > diffs were not cumulated once the cpu had no event on it so that > > > the cpu started sleeping infinitely. > > > I checked percpu vmstat and found there were total 69 counts not > > > cumulated to the zone's vmstat counter yet. > > > > > > - In this situation, kswapd did not help the trapped process. > > > In pgdat_balanced(), zone_wakermark_ok_safe() examined the number > > > of free pages on the node by zone_page_state_snapshot() which > > > checks pending counts on percpu vmstat. > > > Therefore kswapd could know there were 69 free pages correctly. > > > Since zone->_watermark = {8, 20, 32}, kswapd did not work because > > > 69 was greater than 32 as high watermark. > > > > > > Change allow_direct_reclaim to use zone_page_state_snapshot, which > > > allows a more precise version of the vmstat counters to be used. > > > > > > allow_direct_reclaim will only be called from try_to_free_pages, > > > which is not a hot path. > > > > Have you managed to test this patch to confirm it addresses the above > > issue? It should but better double check that. > > > > > Suggested-by: Michal Hocko > > > Signed-off-by: Marcelo Tosatti > > > > The patch makes sense regardless but a note about testing should be > > added. > > > > Acked-by: Michal Hocko > > Michal, > > The patch has not been tested in the original setup where the problem > was found, however i don't think its easy to do that validation > (checking with the reporter anyway). This is a fair point and I would just add it to the changelog for the future reference. -- Michal Hocko SUSE Labs