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 5F4C2ECAAA3 for ; Fri, 26 Aug 2022 02:43:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S244869AbiHZCnp (ORCPT ); Thu, 25 Aug 2022 22:43:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50346 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S244883AbiHZCnd (ORCPT ); Thu, 25 Aug 2022 22:43:33 -0400 Received: from out30-57.freemail.mail.aliyun.com (out30-57.freemail.mail.aliyun.com [115.124.30.57]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0E41ECD50E for ; Thu, 25 Aug 2022 19:43:09 -0700 (PDT) X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R771e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=ay29a033018045192;MF=rocking@linux.alibaba.com;NM=1;PH=DS;RN=11;SR=0;TI=SMTPD_---0VNHDA4g_1661481784; Received: from IT-C02ZG85SLVDL.local(mailfrom:rocking@linux.alibaba.com fp:SMTPD_---0VNHDA4g_1661481784) by smtp.aliyun-inc.com; Fri, 26 Aug 2022 10:43:06 +0800 Subject: Re: [PATCH] sched/fair: select waker's cpu for wakee on sync wakeup To: Mel Gorman Cc: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org References: <1508aa17d1a169077c8d8d8c22d2bd529101af0e.1661313074.git.rocking@linux.alibaba.com> <20220824084614.kykmtbhyawcei67z@suse.de> <093acf40-9e57-d011-d90b-ea216700edc3@linux.alibaba.com> <20220825090932.4cfxvt4ktm24guur@suse.de> From: Peng Wang Message-ID: <3e248fab-9ee3-3041-f4d5-99d6313f018b@linux.alibaba.com> Date: Fri, 26 Aug 2022 10:43:04 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <20220825090932.4cfxvt4ktm24guur@suse.de> Content-Type: text/plain; charset=iso-8859-15; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 25/08/2022 17:09, , Mel Gorman wrote: > On Thu, Aug 25, 2022 at 02:45:05PM +0800, Peng Wang wrote: >> On 24/08/2022 16:46, , Mel Gorman wrote: >>> On Wed, Aug 24, 2022 at 12:37:50PM +0800, Peng Wang wrote: >>>> On sync wakeup, waker is about to sleep, and if it is the only >>>> running task, wakee can get warm data on waker's cpu. >>>> >>>> Unixbench, schbench, and hackbench are tested on >>>> Intel(R) Xeon(R) Platinum 8163 CPU @ 2.50GHz (192 logic CPUs) >>>> >>>> Unixbench get +20.7% improvement with full threads mainly >>>> because of the pipe-based context switch and fork test. >>>> >>>> No obvious impact on schbench. >>>> >>>> This change harms hackbench with lower concurrency, while gets improvement >>>> when concurrency increases. >>>> >>> >>> Note that historically patches in this direction have been hazardous because >>> it makes a key assumption "sync wakers always go to sleep in the near future" >>> when the sync hint is not that reliable. Networking from a brief glance >>> still uses sync wakeups where wakers could have a 1:N relationship between >>> work producers and work consumers that would then stack multiple tasks on >>> one CPU for multiple consumers. The workloads mentioned in the changelog >>> are mostly strictly-synchronous wakeups (i.e. the waker definitely goes >>> to sleep almost immediately) and benefit from this sort of patch but it's >>> not necessarily a universal benefit. >> >> Hi, Mel >> >> Thanks for your clarification. >> >> Besides these benchmarks, I also find a similar strictly-synchronous wakeup >> case [1]. >> >> [1]https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1478754.html >> > > Yep, but it falls under the same heading, sometimes the caller knows it's > a strict sync wakeup but not always. > >>> >>> Note that most of these hazards occurred *LONG* before I was paying much >>> attention to how the scheduler behaved so I cannot state "sync is still >>> unreliable" with absolute certainty. However, long ago there was logic >>> that tried to track the accuracy of the sync hint that was ultimately >>> abandoned by commit e12f31d3e5d3 ("sched: Remove avg_overlap"). AFAIK, >>> the sync hint is still not 100% reliable and while stacking sync works >>> for some workloads, it's likely to be a regression magnet for network >>> intensive workloads or client/server workloads like databases where >>> "synchronous wakeups are not always synchronous". >>> >> Yes, you are right. Perhaps in such situation, a strong contract from user >> is a better alternative than struggling with the weak hint in kernel. >> > > Even the kernel doesn't always know if a wakeup is really sync or not > because it lacks valuable context and the number of tasks on the runqueue is > insufficient if there are multiple wakeups in quick succession. At best, > there could be two WF_SYNC hints and hope every caller gets it right > (hint, they won't because even if it's right once, cargo cult copying > will eventually get it wrong and there is an API explosion issue such as > wake_up_interruptible_*). A user hint would be tricky. Core libraries > couldn't use it because it has no idea if the linked application wants > a strictly sync wakeup or not, a core library couldn't tell given just > a pthread_mute_t for example. Even if it was true at one point in time, OK, I get it now, thanks! If we passed more information dealing with pthread_mute_t, it would bring too much changes through user core libraries to this kernel scheduling decision. And the current weak sync-wakeup hint can only bring us a candidate in the same LLC cache domain at most. > it might not be true later if the application design changed leading to > application bugs being blamed on the kernel for poor placement decisions. >