linux-kernel - Re: sched-freq locking

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Message-ID: <24262322.WeTSTa6SpV@vostro.rjw.lan>
Date:	Wed, 27 Jan 2016 09:03:01 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	Juri Lelli <Juri.Lelli@....com>,
	Michael Turquette <mturquette@...libre.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Javi Merino <Javi.Merino@....com>,
	Punit Agrawal <punit.agrawal@....com>
Subject: Re: sched-freq locking

On Tuesday, January 26, 2016 05:54:13 PM Steve Muckle wrote:
> On 01/21/2016 05:21 PM, Rafael J. Wysocki wrote:
> > On Thursday, January 21, 2016 10:49:58 AM Juri Lelli wrote:
> >> [+Punit, Javi]
> >>
> >> Hi Rafael,
> >>
> >> On 21/01/16 02:46, Rafael J. Wysocki wrote:
> >>> On Wednesday, January 20, 2016 05:39:14 PM Steve Muckle wrote:
> >>>> On 01/20/2016 05:22 PM, Rafael J. Wysocki wrote:
> >>>>> One comment here (which may be a bit off in which case please ignore it).
> >>>>>
> >>>>> You seem to be thinking that sched-freq needs to be a cpufreq governor
> >>>>> and thus be handled in the same way as ondemand, for example.
> >>>>
> >>>> That's true, I hadn't really given much thought to the alternative you
> >>>> mention below.
> >>>>
> >>>>>
> >>>>> However, this doesn't have to be the case in principle.  For example,
> >>>>> if we have a special driver callback specifically to work with sched-freq,
> >>>>> it may just use that callback and bypass (almost) all of the usual
> >>>>> cpufreq mechanics.  This way you may avoid worrying about the governor
> >>>>> locking and related ugliness entirely.
> >>>>
> >>>> That sounds good but I'm worried about other consequences of taking
> >>>> cpufreq out of the loop. For example wouldn't we need a new way for
> >>>> something like thermal to set frequency limits?
> >>>
> >>> I don't know from the top of my head, but that's at least worth investigating.
> >>>
> >>
> >> Yes, that's an interesting alternative that we have to think through.
> >>
> >>> Maybe we can keep the interface for those things unchanged, but handle it
> >>> differently under the hood?
> >>>
> >>
> >> Let me see if I understand what you are proposing :). If we don't want
> >> to duplicate too many things, maybe it is still feasible to just use
> >> existing cpufreq mechanics to handle hotplug, sysfs, thermal, etc. (with
> >> possibly minor modifications to be notified of events) and only create a
> >> new method to ask the driver for frequency changes, since we will have
> >> replicated policy and freq_table information inside sched-freq.  Is that
> >> what you were also thinking of by saying "bypass (almost) all the usual
> >> cpufreq mechanics"? :)
> > 
> > Yes, it is.
> 
> I've been working on the locking in schedfreq (governor) and believe it
> is now functionally correct. It'll get sent out in another RFC soon.
> 
> Having wrestled with that locking a bit I can appreciate the value of
> potentially deprecating some or all of the cpufreq core. I'm also
> fearful though of making the current task (creating a scheduler-based
> CPU frequency scaling algorithm) more complex than it is already.
> 
> For that reason my preference would be to get the thing to a viable
> state as a governor first, assuming that's possible, and then take on
> restructuring to eliminate/deprecate unnecessary infrastructure. Does
> this seem reasonable?

Quite frankly, it depends on what's in the code.

I may not like that approach in principle, but then if the resulting
code turns out to be clean and nice and has no unnecessary overhead and
so on, it may very well work for me.

That said I have some simple patches to replace timer functions with
scheduler callbacks in cpufreq as a whole, which to be honest I'd prefer
to do first, because it would allow us to eliminate the heavy use of
timers that was complained about during the Kernel Summit last year.

I haven't posted them yest as they have been under some preliminary
testing and I'm still traveling today.  However, since the patches don't
seem to break stuff and I'll be back home tomorrow, I think I'll just
post them then.

Thanks,
Rafael