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

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Date:	Tue, 8 Sep 2015 17:17:03 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"yuyang.du@...el.com" <yuyang.du@...el.com>,
	"mturquette@...libre.com" <mturquette@...libre.com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"sgurrappadi@...dia.com" <sgurrappadi@...dia.com>,
	"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

On 8 September 2015 at 16:10, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Sep 08, 2015 at 03:39:37PM +0200, Vincent Guittot wrote:
>> > Now, given all that, units are a complete mess here, and I'd not mind
>> > something like:
>> >
>> > #if (SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) != SCHED_CAPACITY_SHIFT
>> > #error "something usefull"
>> > #endif
>>
>> In this case why not simply doing
>> #define SCHED_CAPACITY_SHIFT SCHED_LOAD_SHIFT
>> or the opposite ?
>
> Sadly not enough; aside from the fact that we really should do !0
> LOAD_RESOLUTION on 64bit, the whole magic tables (runnable_avg_yN_*[])
> and LOAD_AVG_MAX* values rely on the unit being 1<<10.

ah yes, i forgot to take into account the LOAD_RESOLUTION.

So after some more thinking,  i finally don't see where in the code,
we will have a issue if SCHED_CAPACITY_SHIFT is not equal to
(SCHED_LOAD_SHIFT - SCHED_LOAD_RESOLUTION) or not equal to 10 with the
respect of using a value that doesn't overflow the variables

Regards,
Vincent

>
> So regardless of defining one in terms of the other, we should check
> both are in fact 10 and error out otherwise.
>
> Changing them must involve recomputing these numbers or otherwise
> mucking about with shifts to ensure its back to 10 when we do this load
> muck.
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