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 B3B75CD6E5D for ; Wed, 11 Oct 2023 11:47:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345944AbjJKLrK (ORCPT ); Wed, 11 Oct 2023 07:47:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49592 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231601AbjJKLrI (ORCPT ); Wed, 11 Oct 2023 07:47:08 -0400 Received: from desiato.infradead.org (desiato.infradead.org [IPv6:2001:8b0:10b:1:d65d:64ff:fe57:4e05]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B59C18F for ; Wed, 11 Oct 2023 04:47:07 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Transfer-Encoding: Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To:Content-ID:Content-Description; bh=RSsV3tzE81yhMrJ2OAEkPBSGqOhOdmB831LXDp5PvM8=; b=D6O30peoybOa+iTaf2UQhdZU0s kpT3cNMQkH2f5dT8iFXvz7/4WP4IAfW+lJe8X38+4NOkKSz9wXr7WTSa+fxGQzUslfvJAhtpwYhOU +yhWzE6xPEPeFQieevpx8CZ6ndWYWvw54QGbp52lKRFbp5izL/h1m0mTFrM9UkAFtlIlA7ha9MoMS vBMkaBbHH8QUrVWJHye6n5nvlSp2N0P/RhjK6aWuu9pXEa6oAVKOaUVHIQUjLL8S+agrVT1guRgW3 naz9KZ7wsUwiBDz9XhegVDDSTgenVsITNszY7p59JdrswuJaiAsf9d0W8jHocBu1dSnUmlb6g/Upn S9eK0L5A==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.96 #2 (Red Hat Linux)) id 1qqXfl-000BmS-0e; Wed, 11 Oct 2023 11:46:42 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 2273130036C; Wed, 11 Oct 2023 13:46:42 +0200 (CEST) Date: Wed, 11 Oct 2023 13:46:42 +0200 From: Peter Zijlstra To: Ankit Jain Cc: yury.norov@gmail.com, andriy.shevchenko@linux.intel.com, linux@rasmusvillemoes.dk, qyousef@layalina.io, pjt@google.com, joshdon@google.com, bristot@redhat.com, vschneid@redhat.com, linux-kernel@vger.kernel.org, namit@vmware.com, amakhalov@vmware.com, srinidhir@vmware.com, vsirnapalli@vmware.com, vbrahmajosyula@vmware.com, akaher@vmware.com, srivatsa@csail.mit.edu Subject: Re: [PATCH RFC] cpumask: Randomly distribute the tasks within affinity mask Message-ID: <20231011114642.GA36521@noisy.programming.kicks-ass.net> References: <20231011071925.761590-1-ankitja@vmware.com> <20231011105329.GA17066@noisy.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20231011105329.GA17066@noisy.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 11, 2023 at 12:53:29PM +0200, Peter Zijlstra wrote: > On Wed, Oct 11, 2023 at 12:49:25PM +0530, Ankit Jain wrote: > > commit 46a87b3851f0 ("sched/core: Distribute tasks within affinity masks") > > and commit 14e292f8d453 ("sched,rt: Use cpumask_any*_distribute()") > > introduced the logic to distribute the tasks at initial wakeup on cpus > > where load balancing works poorly or disabled at all (isolated cpus). > > > > There are cases in which the distribution of tasks > > that are spawned on isolcpus does not happen properly. > > In production deployment, initial wakeup of tasks spawn from > > housekeeping cpus to isolcpus[nohz_full cpu] happens on first cpu > > within isolcpus range instead of distributed across isolcpus. > > > > Usage of distribute_cpu_mask_prev from one processes group, > > will clobber previous value of another or other groups and vice-versa. > > > > When housekeeping cpus spawn multiple child tasks to wakeup on > > isolcpus[nohz_full cpu], using cpusets.cpus/sched_setaffinity(), > > distribution is currently performed based on per-cpu > > distribute_cpu_mask_prev counter. > > At the same time, on housekeeping cpus there are percpu > > bounded timers interrupt/rcu threads and other system/user tasks > > would be running with affinity as housekeeping cpus. In a real-life > > environment, housekeeping cpus are much fewer and are too much loaded. > > So, distribute_cpu_mask_prev value from these tasks impacts > > the offset value for the tasks spawning to wakeup on isolcpus and > > thus most of the tasks end up waking up on first cpu within the > > isolcpus set. > > > > Steps to reproduce: > > Kernel cmdline parameters: > > isolcpus=2-5 skew_tick=1 nohz=on nohz_full=2-5 > > rcu_nocbs=2-5 rcu_nocb_poll idle=poll irqaffinity=0-1 > > > > * pid=$(echo $$) > > * taskset -pc 0 $pid > > * cat loop-normal.c > > int main(void) > > { > > while (1) > > ; > > return 0; > > } > > * gcc -o loop-normal loop-normal.c > > * for i in {1..50}; do ./loop-normal & done > > * pids=$(ps -a | grep loop-normal | cut -d' ' -f5) > > * for i in $pids; do taskset -pc 2-5 $i ; done > > > > Expected output: > > * All 50 “loop-normal” tasks should wake up on cpu2-5 > > equally distributed. > > * ps -eLo cpuid,pid,tid,ppid,cls,psr,cls,cmd | grep "^ [2345]" > > > > Actual output: > > * All 50 “loop-normal” tasks got woken up on cpu2 only > > Your expectation is wrong. Things work as advertised. That is, isolcpus results in single CPU balance domains and as such we must not distribute -- there is no load balancing. Ideally we'd reject setting cpumasks with multi bits set on domains like that, but alas, that would break historical behaviour :/ Now, looking at the code, I don't think the current code actually behaves correct in this case :-(, somewhere along the line we should truncate cpu_valid_mask to a single bit. Let me see where the sane place is to do that.