From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Mel Gorman <mgorman@suse.de>
Cc: Phil Auld <pauld@redhat.com>,
Colin Ian King <colin.king@canonical.com>,
Peter Zijlstra <peterz@infradead.org>,
"Rafael J. Wysocki" <rjw@rjwysocki.net>,
Giovanni Gherdovich <ggherdovich@suse.com>,
Viresh Kumar <viresh.kumar@linaro.org>,
Julia Lawall <julia.lawall@inria.fr>,
Ingo Molnar <mingo@redhat.com>,
kernel-janitors@vger.kernel.org,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Valentin Schneider <valentin.schneider@arm.com>,
Gilles Muller <Gilles.Muller@inria.fr>,
Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
Linux PM <linux-pm@vger.kernel.org>,
Len Brown <len.brown@intel.com>
Subject: Re: default cpufreq gov, was: [PATCH] sched/fair: check for idle core
Date: Thu, 22 Oct 2020 17:59:43 +0000 [thread overview]
Message-ID: <CAJZ5v0he839sJNh0xjmvLqzuE7X27PgJKxtSV8giZh004E7pXw@mail.gmail.com> (raw)
In-Reply-To: <20201022163509.GM32041@suse.de>
On Thu, Oct 22, 2020 at 6:35 PM Mel Gorman <mgorman@suse.de> wrote:
>
> On Thu, Oct 22, 2020 at 11:12:00AM -0400, Phil Auld wrote:
> > > > AFAIK, not quite (added Giovanni as he has been paying more attention).
> > > > Schedutil has improved since it was merged but not to the extent where
> > > > it is a drop-in replacement. The standard it needs to meet is that
> > > > it is at least equivalent to powersave (in intel_pstate language)
> > > > or ondemand (acpi_cpufreq) and within a reasonable percentage of the
> > > > performance governor. Defaulting to performance is a) giving up and b)
> > > > the performance governor is not a universal win. There are some questions
> > > > currently on whether schedutil is good enough when HWP is not available.
> > > > 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 is a
> > > > hazard in itself because someone could test on the latest gen Intel CPU
> > > > and conclude everything is fine and miss that Intel-specific technology
> > > > is needed to make it work well while throwing everyone else under a bus.
> > > > Giovanni knows a lot more than I do about this, I could be wrong or
> > > > forgetting things.
> > > >
> > > > 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
> > > > 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.
> > >
> > > ..and it would be really useful for distros to know when the hard data
> > > is available so that they can make an informed decision when to move to
> > > schedutil.
> > >
> >
> > I think distros are on the hook to generate that hard data themselves
> > with which to make such a decision. I don't expect it to be done by
> > someone else.
> >
>
> Yep, distros are on the hook. When I said "I would expect hard data",
> it was in the knowledge that for openSUSE/SLE, we (as in SUSE) would be
> generating said data and making a call based on it. I'd be surprised if
> Phil was not thinking along the same lines.
>
> > > > For reference, SLUB had the same problem for years. It was switched
> > > > on by default in the kernel config but it was a long time before
> > > > SLUB was generally equivalent to SLAB in terms of performance. Block
> > > > multiqueue also had vaguely similar issues before the default changes
> > > > and a period of time before it was removed removed (example whinging mail
> > > > https://lore.kernel.org/lkml/20170803085115.r2jfz2lofy5spfdb@techsingularity.net/)
> > > > It's schedutil's turn :P
> > > >
> > >
> >
> > Agreed. I'd like the option to switch back if we make the default change.
> > It's on the table and I'd like to be able to go that way.
> >
>
> Yep. It sounds chicken, but it's a useful safety net and a reasonable
> way to deprecate a feature. It's also useful for bug creation -- User X
> running whatever found that schedutil is worse than the old governor and
> had to temporarily switch back. Repeat until complaining stops and then
> tear out the old stuff.
>
> When/if there is a patch setting schedutil as the default, cc suitable
> distro people (Giovanni and myself for openSUSE).
So for the record, Giovanni was on the CC list of the "cpufreq:
intel_pstate: Use passive mode by default without HWP" patch that this
discussion resulted from (and which kind of belongs to the above
category).
> Other distros assuming they're watching can nominate their own victim.
But no other victims had been nominated at that time.
next prev parent reply other threads:[~2020-10-22 17:59 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-20 16:37 [PATCH] sched/fair: check for idle core Julia Lawall
2020-10-21 7:29 ` Vincent Guittot
2020-10-21 11:13 ` Peter Zijlstra
2020-10-21 12:27 ` Vincent Guittot
2020-10-21 11:20 ` Mel Gorman
2020-10-21 11:56 ` Julia Lawall
2020-10-21 12:19 ` Peter Zijlstra
2020-10-21 12:42 ` Julia Lawall
2020-10-21 12:52 ` Peter Zijlstra
2020-10-21 18:18 ` Rafael J. Wysocki
2020-10-21 18:15 ` Rafael J. Wysocki
2020-10-21 19:47 ` Julia Lawall
2020-10-21 20:25 ` Rafael J. Wysocki
2020-10-21 13:10 ` Peter Zijlstra
2020-10-21 18:11 ` Rafael J. Wysocki
2020-10-22 4:53 ` Viresh Kumar
2020-10-22 7:11 ` Peter Zijlstra
2020-10-22 10:59 ` Viresh Kumar
2020-10-22 11:45 ` Rafael J. Wysocki
2020-10-22 12:02 ` default cpufreq gov, was: " Peter Zijlstra
2020-10-22 12:19 ` Rafael J. Wysocki
2020-10-22 12:29 ` Peter Zijlstra
2020-10-22 14:52 ` Mel Gorman
2020-10-22 14:58 ` Colin Ian King
2020-10-22 15:12 ` Phil Auld
2020-10-22 16:35 ` Mel Gorman
2020-10-22 17:59 ` Rafael J. Wysocki [this message]
2020-10-22 20:32 ` Mel Gorman
2020-10-22 20:39 ` Phil Auld
2020-10-22 15:25 ` Peter Zijlstra
2020-10-22 15:55 ` Rafael J. Wysocki
2020-10-22 16:29 ` Mel Gorman
2020-10-22 20:10 ` Giovanni Gherdovich
2020-10-22 20:16 ` Giovanni Gherdovich
2020-10-23 7:03 ` Peter Zijlstra
2020-10-23 17:46 ` Tom Lendacky
2020-10-26 19:52 ` Fontenot, Nathan
2020-10-22 15:45 ` A L
2020-10-22 15:55 ` Vincent Guittot
2020-10-23 5:23 ` Viresh Kumar
2020-10-22 16:23 ` [PATCH] cpufreq: Avoid configuring old governors as default with intel_pstate Rafael J. Wysocki
2020-10-23 6:29 ` Viresh Kumar
2020-10-23 11:59 ` Rafael J. Wysocki
2020-10-23 15:15 ` [PATCH v2] " Rafael J. Wysocki
2020-10-27 3:13 ` Viresh Kumar
2020-10-27 11:11 ` default cpufreq gov, was: [PATCH] sched/fair: check for idle core Qais Yousef
2020-10-27 11:26 ` Valentin Schneider
2020-10-27 11:42 ` Qais Yousef
2020-10-27 11:48 ` Viresh Kumar
2020-10-23 6:24 ` Viresh Kumar
2020-10-23 15:06 ` Rafael J. Wysocki
2020-10-27 3:13 ` Viresh Kumar
2020-10-22 11:21 ` AW: " Walter Harms
2020-10-21 12:28 ` Mel Gorman
2020-10-21 12:25 ` Vincent Guittot
2020-10-21 12:47 ` Mel Gorman
2020-10-21 12:56 ` Julia Lawall
2020-10-21 13:18 ` Mel Gorman
2020-10-21 13:24 ` Julia Lawall
2020-10-21 15:08 ` Mel Gorman
2020-10-21 15:18 ` Julia Lawall
2020-10-21 15:23 ` Vincent Guittot
2020-10-21 15:33 ` Julia Lawall
2020-10-21 15:19 ` Vincent Guittot
2020-10-21 17:00 ` Mel Gorman
2020-10-21 17:39 ` Julia Lawall
2020-10-21 13:48 ` Julia Lawall
2020-10-21 15:26 ` Mel Gorman
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