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 X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7DCA9C433FE for ; Mon, 20 Sep 2021 19:54:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 61D076121D for ; Mon, 20 Sep 2021 19:54:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1351285AbhITTym (ORCPT ); Mon, 20 Sep 2021 15:54:42 -0400 Received: from outbound-smtp29.blacknight.com ([81.17.249.32]:56904 "EHLO outbound-smtp29.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1351015AbhITTwj (ORCPT ); Mon, 20 Sep 2021 15:52:39 -0400 Received: from mail.blacknight.com (pemlinmail04.blacknight.ie [81.17.254.17]) by outbound-smtp29.blacknight.com (Postfix) with ESMTPS id 63272BEFF1 for ; Mon, 20 Sep 2021 20:51:11 +0100 (IST) Received: (qmail 25265 invoked from network); 20 Sep 2021 19:51:11 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.17.29]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 20 Sep 2021 19:51:11 -0000 Date: Mon, 20 Sep 2021 20:51:09 +0100 From: Mel Gorman To: Matthew Wilcox Cc: Linux-MM , NeilBrown , Theodore Ts'o , Andreas Dilger , "Darrick J . Wong" , Michal Hocko , Dave Chinner , Rik van Riel , Vlastimil Babka , Johannes Weiner , Jonathan Corbet , Linux-fsdevel , LKML Subject: Re: [RFC PATCH 0/5] Remove dependency on congestion_wait in mm/ Message-ID: <20210920195109.GJ3959@techsingularity.net> References: <20210920085436.20939-1-mgorman@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 20, 2021 at 12:42:44PM +0100, Matthew Wilcox wrote: > On Mon, Sep 20, 2021 at 09:54:31AM +0100, Mel Gorman wrote: > > This has been lightly tested only and the testing was useless as the > > relevant code was not executed. The workload configurations I had that > > used to trigger these corner cases no longer work (yey?) and I'll need > > to implement a new synthetic workload. If someone is aware of a realistic > > workload that forces reclaim activity to the point where reclaim stalls > > then kindly share the details. > > The stereeotypical "stalling on I/O" problem is to plug in one of the > crap USB drives you were given at a trade show and simply > dd if=/dev/zero of=/dev/sdb > sync > > You can also set up qemu to have extremely slow I/O performance: > https://serverfault.com/questions/675704/extremely-slow-qemu-storage-performance-with-qcow2-images > Ok, I managed to get something working and nothing blew up. The workload was similar to what I described except the dirty file data is related to dirty_ratio, the memory hogs no longer sleep and I disabled the parallel readers. There is still a configuration with the parallel readers but I won't have the results till tomorrow. Surprising no one, vanilla kernel throttling barely works. 1 writeback_wait_iff_congested: usec_delayed=4000 3 writeback_congestion_wait: usec_delayed=108000 196 writeback_congestion_wait: usec_delayed=104000 16697 writeback_wait_iff_congested: usec_delayed=0 too_many_isolated it not tracked at all so we don't know what that looks like but kswapd "blocking" on dirty pages at the tail basically never stalls. The few congestion_wait's that did happen stalled for the full duration as the bdi is not tracking congestion at all. With the series, the breakdown of reasons to stall were 5703 reason=VMSCAN_THROTTLE_WRITEBACK 29644 reason=VMSCAN_THROTTLE_NOPROGRESS 1979999 reason=VMSCAN_THROTTLE_ISOLATED kswapd stalls were rare but they did happen and surprise surprise, it was dirty pages 914 reason=VMSCAN_THROTTLE_WRITEBACK All of them stalled for the full timeout so there might be a bug in patch 1 because that sounds suspicious. As "too many pages isolated" was the top reason, the frequency of each stall time is as follows 1 usect_delayed=164000 1 usect_delayed=192000 1 usect_delayed=200000 1 usect_delayed=208000 1 usect_delayed=220000 1 usect_delayed=244000 1 usect_delayed=308000 1 usect_delayed=312000 1 usect_delayed=316000 1 usect_delayed=332000 1 usect_delayed=588000 1 usect_delayed=620000 1 usect_delayed=836000 3 usect_delayed=116000 4 usect_delayed=124000 4 usect_delayed=128000 6 usect_delayed=120000 9 usect_delayed=112000 11 usect_delayed=100000 13 usect_delayed=48000 13 usect_delayed=96000 14 usect_delayed=40000 15 usect_delayed=88000 15 usect_delayed=92000 16 usect_delayed=80000 18 usect_delayed=68000 19 usect_delayed=76000 22 usect_delayed=84000 23 usect_delayed=108000 23 usect_delayed=60000 25 usect_delayed=44000 25 usect_delayed=52000 29 usect_delayed=36000 30 usect_delayed=56000 30 usect_delayed=64000 33 usect_delayed=72000 57 usect_delayed=32000 91 usect_delayed=20000 107 usect_delayed=24000 125 usect_delayed=28000 131 usect_delayed=16000 180 usect_delayed=12000 186 usect_delayed=8000 1379 usect_delayed=104000 16493 usect_delayed=4000 1960837 usect_delayed=0 In other words, the vast majority of stalls were for 0 time and the task was immediately woken again. The next most common stall time was 1 tick but a sizable number reach the full timeout. Everything else is somewhere in between so the event trigger appears to be ok. I don't know how the application itself performed as I still have to write the analysis script and assuming I can look at this tomorrow, I'll probably start with why VMSCAN_THROTTLE_WRITEBACK always stalled for the full timeout. -- Mel Gorman SUSE Labs