From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Rafael J. Wysocki" Subject: Re: [PATCH] sched/cpufreq_schedutil: use now as reference when aggregating shared policy requests Date: Thu, 04 May 2017 16:29:13 +0200 Message-ID: <1597987.Ujfy2HKTBn@aspire.rjw.lan> References: <20170503133048.8742-1-juri.lelli@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7Bit Return-path: Received: from cloudserver094114.home.net.pl ([79.96.170.134]:46655 "EHLO cloudserver094114.home.net.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752850AbdEDOff (ORCPT ); Thu, 4 May 2017 10:35:35 -0400 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, viresh.kumar@linaro.org, peterz@infradead.org, vincent.guittot@linaro.org On Wednesday, May 03, 2017 02:30:48 PM 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 OK I'll queue this up if there are no objections from the people in the CC. > --- > kernel/sched/cpufreq_schedutil.c | 7 +++---- > 1 file changed, 3 insertions(+), 4 deletions(-) > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > index 76877a62b5fa..622eed1b7658 100644 > --- a/kernel/sched/cpufreq_schedutil.c > +++ b/kernel/sched/cpufreq_schedutil.c > @@ -245,11 +245,10 @@ static void sugov_update_single(struct update_util_data *hook, u64 time, > sugov_update_commit(sg_policy, time, next_f); > } > > -static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu) > +static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu, u64 time) > { > struct sugov_policy *sg_policy = sg_cpu->sg_policy; > struct cpufreq_policy *policy = sg_policy->policy; > - u64 last_freq_update_time = sg_policy->last_freq_update_time; > unsigned long util = 0, max = 1; > unsigned int j; > > @@ -265,7 +264,7 @@ static unsigned int sugov_next_freq_shared(struct sugov_cpu *sg_cpu) > * enough, don't take the CPU into account as it probably is > * idle now (and clear iowait_boost for it). > */ > - delta_ns = last_freq_update_time - j_sg_cpu->last_update; > + delta_ns = time - j_sg_cpu->last_update; > if (delta_ns > TICK_NSEC) { > j_sg_cpu->iowait_boost = 0; > continue; > @@ -309,7 +308,7 @@ static void sugov_update_shared(struct update_util_data *hook, u64 time, > if (flags & SCHED_CPUFREQ_RT_DL) > next_f = sg_policy->policy->cpuinfo.max_freq; > else > - next_f = sugov_next_freq_shared(sg_cpu); > + next_f = sugov_next_freq_shared(sg_cpu, time); > > sugov_update_commit(sg_policy, time, next_f); > } > Thanks, Rafael