From mboxrd@z Thu Jan 1 00:00:00 1970 From: Viresh Kumar Subject: Re: [PATCH] sched/cpufreq_schedutil: use now as reference when aggregating shared policy requests Date: Fri, 5 May 2017 11:36:37 +0530 Message-ID: <20170505060637.GA5892@vireshk-i7> References: <20170503133048.8742-1-juri.lelli@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail-pg0-f50.google.com ([74.125.83.50]:34917 "EHLO mail-pg0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755110AbdEEGGl (ORCPT ); Fri, 5 May 2017 02:06:41 -0400 Received: by mail-pg0-f50.google.com with SMTP id o3so3800491pgn.2 for ; Thu, 04 May 2017 23:06:41 -0700 (PDT) Content-Disposition: inline In-Reply-To: <20170503133048.8742-1-juri.lelli@arm.com> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: Juri Lelli Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, rjw@rjwysocki.net, peterz@infradead.org, vincent.guittot@linaro.org, "Rafael J . Wysocki" On 03-05-17, 14:30, Juri Lelli wrote: > Currently, sugov_next_freq_shared() uses last_freq_update_time as a > reference to decide when to start considering CPU contributions as > stale. > > However, since last_freq_update_time is set by the last CPU that issued > a frequency transition, this might cause problems in certain cases. In > practice, the detection of stale utilization values fails whenever the > CPU with such values was the last to update the policy. For example (and > please note again that the SCHED_CPUFREQ_RT flag is not the problem > here, but only the detection of after how much time that flag has to be > considered stale), suppose a policy with 2 CPUs: > > CPU0 | CPU1 > | > | RT task scheduled > | SCHED_CPUFREQ_RT is set > | CPU1->last_update = now > | freq transition to max > | last_freq_update_time = now > | > > more than TICK_NSEC nsecs > > | > a small CFS wakes up | > CPU0->last_update = now1 | > delta_ns(CPU0) < TICK_NSEC* | > CPU0's util is considered | > delta_ns(CPU1) = | > last_freq_update_time - | > CPU1->last_update = 0 | > < TICK_NSEC | > CPU1 is still considered | > CPU1->SCHED_CPUFREQ_RT is set | > we stay at max (until CPU1 | > exits from idle) | > > * delta_ns is actually negative as now1 > last_freq_update_time > > While last_freq_update_time is a sensible reference for rate limiting, > it doesn't seem to be useful for working around stale CPU states. > > Fix the problem by always considering now (time) as the reference for > deciding when CPUs have stale contributions. > > Signed-off-by: Juri Lelli > Cc: Rafael J. Wysocki > Cc: Viresh Kumar > --- > kernel/sched/cpufreq_schedutil.c | 7 +++---- > 1 file changed, 3 insertions(+), 4 deletions(-) Acked-by: Viresh Kumar -- viresh