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Date: Thu, 22 Oct 2020 17:29:43 +0100 From: Mel Gorman <mgorman@...e.de> To: Peter Zijlstra <peterz@...radead.org> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>, Giovanni Gherdovich <ggherdovich@...e.com>, Viresh Kumar <viresh.kumar@...aro.org>, Julia Lawall <julia.lawall@...ia.fr>, Ingo Molnar <mingo@...hat.com>, kernel-janitors@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Daniel Bristot de Oliveira <bristot@...hat.com>, linux-kernel@...r.kernel.org, Valentin Schneider <valentin.schneider@....com>, Gilles Muller <Gilles.Muller@...ia.fr>, srinivas.pandruvada@...ux.intel.com, Linux PM <linux-pm@...r.kernel.org>, Len Brown <len.brown@...el.com> Subject: Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core On Thu, Oct 22, 2020 at 05:25:14PM +0200, Peter Zijlstra wrote: > On Thu, Oct 22, 2020 at 03:52:50PM +0100, Mel Gorman wrote: > > > There are some questions > > currently on whether schedutil is good enough when HWP is not available. > > Srinivas and Rafael will know better, but Intel does run a lot of tests > and IIRC it was found that schedutil was on-par for !HWP. That was the > basis for commit: > > 33aa46f252c7 ("cpufreq: intel_pstate: Use passive mode by default without HWP") > > But now it turns out that commit results in running intel_pstate-passive > on ondemand, which is quite horrible. > I know Intel ran a lot of tests, no question about it and no fingers are being pointed. I know I've had enough bugs patches tested with a battery of tests on various machines and still ended up with bug reports :) > > There was some evidence (I don't have the data, Giovanni was looking into > > it) that HWP was a requirement to make schedutil work well. > > That seems to be the question; Rafael just said the opposite. > > > For distros, switching to schedutil by default would be nice because > > frequency selection state would follow the task instead of being per-cpu > > and we could stop worrying about different HWP implementations but it's > > s/HWP/cpufreq-governors/ ? But yes. > I've seen cases where HWP had variable behaviour between CPU generations. It was hard to quantify and/or figure out because HWP is a black box. > > not at the point where the switch is advisable. I would expect hard data > > before switching the default and still would strongly advise having a > > period of time where we can fall back when someone inevitably finds a > > new corner case or exception. > > Which is why I advocated to make it 'difficult' to use the old ones and > only later remove them. > That's fair. -- Mel Gorman SUSE Labs
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