linux-kernel - Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity

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Date:	Tue, 15 Sep 2015 10:11:41 -0700
From:	bsegall@...gle.com
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Steve Muckle <steve.muckle@...aro.org>,
	"mingo\@redhat.com" <mingo@...hat.com>,
	"daniel.lezcano\@linaro.org" <daniel.lezcano@...aro.org>,
	"mturquette\@baylibre.com" <mturquette@...libre.com>,
	"rjw\@rjwysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"sgurrappadi\@nvidia.com" <sgurrappadi@...dia.com>,
	"pang.xunlei\@zte.com.cn" <pang.xunlei@....com.cn>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

Yuyang Du <yuyang.du@...el.com> writes:

> On Mon, Sep 14, 2015 at 10:34:00AM -0700, bsegall@...gle.com wrote:
>> >> SCHED_LOAD_RESOLUTION and the non-SLR part of SCHED_LOAD_SHIFT are not
>> >> required to be the same value and should not be conflated.
>> >> 
>> >> In particular, since cgroups are on the same timeline as tasks and their
>> >> shares are not scaled by SCHED_LOAD_SHIFT in any way (but are scaled so
>> >> that SCHED_LOAD_RESOLUTION is invisible), changing that part of
>> >> SCHED_LOAD_SHIFT would cause issues, since things can assume that nice-0
>> >> = 1024. However changing SCHED_LOAD_RESOLUTION would be fine, as that is
>> >> an internal value to the kernel.
>> >> 
>> >> In addition, changing the non-SLR part of SCHED_LOAD_SHIFT would require
>> >> recomputing all of prio_to_weight/wmult for the new NICE_0_LOAD.
>> >
>> > I think I follow, but doesn't that mean that the current code is broken
>> > too? NICE_0_LOAD changes if you change SCHED_LOAD_RESOLUTION:
>> >
>> > #define SCHED_LOAD_SHIFT        (10 + SCHED_LOAD_RESOLUTION)
>> > #define SCHED_LOAD_SCALE        (1L << SCHED_LOAD_SHIFT)
>> >
>> > #define NICE_0_LOAD             SCHED_LOAD_SCALE
>> > #define NICE_0_SHIFT            SCHED_LOAD_SHIFT
>> >
>> > To me it sounds like we need to define it the other way around:
>> >
>> > #define NICE_0_SHIFT            10
>> > #define NICE_0_LOAD             (1L << NICE_0_SHIFT)
>> >
>> > and then add any additional resolution bits from there to ensure that
>> > NICE_0_LOAD and the prio_to_weight/wmult tables are unchanged.
>> 
>> No, NICE_0_LOAD is supposed to be scale_load(prio_to_weight[nice_0]),
>> ie including SLR. It has never been clear to me what
>> SCHED_LOAD_SCALE/SCHED_LOAD_SHIFT were for as opposed to NICE_0_LOAD,
>> and the new utilization uses of it are entirely unlinked to 1024 == NICE_0
>
> Presume your SLR means SCHED_LOAD_RESOLUTION:
>
> 1) The introduction of (not redefinition of) SCHED_RESOLUTION_SHIFT does not
> change anything after macro expansion.
>
> 2) The constants in prio_to_weight[] and prio_to_wmult[] are tied to a
> resolution of 10bits NICE_0, i.e., 1024, I guest it is the user visible
> part you mentioned, so is the cgroup share.
>
> To me, it is all ok. With the SCHED_RESOLUTION_SHIFT, the basic resolution
> unit, it is just for us to state clearly, the NICE_0's weight has a fixed
> resolution of SCHED_RESOLUTION_SHIFT, or even add this:
>
> #if prio_to_weight[20] != 1 << SCHED_RESOLUTION_SHIFT
> error "NICE_0 weight not calibrated"
> #endif
> /* I can learn, Peter */
>
> I guess you are saying we are conflating NICE_0 with NICE_0_LOAD. But to me,
> they are just integer metrics, needing a resolution respectively. That is it.

Yes this would change nothing at the moment post-expansion, that's not
the point. SLR being 10 bits and the nice-0 being 1024 are completely
and utterly unrelated and the headers should not pretend they need to be
the same value, any more than there should be a #define that is shared
with every other use of 1024 in the kernel.
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