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

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Date:	Mon, 14 Sep 2015 10:34:00 -0700
From:	bsegall@...gle.com
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Yuyang Du <yuyang.du@...el.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

Morten Rasmussen <morten.rasmussen@....com> writes:

> On Fri, Sep 11, 2015 at 10:05:53AM -0700, bsegall@...gle.com wrote:
>> Morten Rasmussen <morten.rasmussen@....com> writes:
>> 
>> > On Fri, Sep 11, 2015 at 08:28:25AM +0800, Yuyang Du wrote:
>> >> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> >> index 119823d..55a7b93 100644
>> >> --- a/include/linux/sched.h
>> >> +++ b/include/linux/sched.h
>> >> @@ -912,7 +912,7 @@ enum cpu_idle_type {
>> >>  /*
>> >>   * Increase resolution of cpu_capacity calculations
>> >>   */
>> >> -#define SCHED_CAPACITY_SHIFT	10
>> >> +#define SCHED_CAPACITY_SHIFT	SCHED_RESOLUTION_SHIFT
>> >>  #define SCHED_CAPACITY_SCALE	(1L << SCHED_CAPACITY_SHIFT)
>> >>  
>> >>  /*
>> >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> >> index 68cda11..d27cdd8 100644
>> >> --- a/kernel/sched/sched.h
>> >> +++ b/kernel/sched/sched.h
>> >> @@ -40,6 +40,9 @@ static inline void update_cpu_load_active(struct rq *this_rq) { }
>> >>   */
>> >>  #define NS_TO_JIFFIES(TIME)	((unsigned long)(TIME) / (NSEC_PER_SEC / HZ))
>> >>  
>> >> +# define SCHED_RESOLUTION_SHIFT	10
>> >> +# define SCHED_RESOLUTION_SCALE	(1L << SCHED_RESOLUTION_SHIFT)
>> >> +
>> >>  /*
>> >>   * Increase resolution of nice-level calculations for 64-bit architectures.
>> >>   * The extra resolution improves shares distribution and load balancing of
>> >> @@ -53,16 +56,15 @@ static inline void update_cpu_load_active(struct rq *this_rq) { }
>> >>   * increased costs.
>> >>   */
>> >>  #if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power usage under light load  */
>> >> -# define SCHED_LOAD_RESOLUTION	10
>> >> -# define scale_load(w)		((w) << SCHED_LOAD_RESOLUTION)
>> >> -# define scale_load_down(w)	((w) >> SCHED_LOAD_RESOLUTION)
>> >> +# define SCHED_LOAD_SHIFT	(SCHED_RESOLUTION_SHIFT + SCHED_RESOLUTION_SHIFT)
>> >> +# define scale_load(w)		((w) << SCHED_RESOLUTION_SHIFT)
>> >> +# define scale_load_down(w)	((w) >> SCHED_RESOLUTION_SHIFT)
>> >>  #else
>> >> -# define SCHED_LOAD_RESOLUTION	0
>> >> +# define SCHED_LOAD_SHIFT	(SCHED_RESOLUTION_SHIFT)
>> >>  # define scale_load(w)		(w)
>> >>  # define scale_load_down(w)	(w)
>> >>  #endif
>> >>  
>> >> -#define SCHED_LOAD_SHIFT	(10 + SCHED_LOAD_RESOLUTION)
>> >>  #define SCHED_LOAD_SCALE	(1L << SCHED_LOAD_SHIFT)
>> >>  
>> >>  #define NICE_0_LOAD		SCHED_LOAD_SCALE
>> >
>> > I think this is pretty much the required relationship between all the
>> > SHIFTs and SCALEs that Peter checked for in his #if-#error thing
>> > earlier, so no disagreements from my side :-)
>> > --
>> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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>> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> > Please read the FAQ at  http://www.tux.org/lkml/
>> 
>> 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
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