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From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Tue, 8 Sep 2015 17:17:03 +0200
Message-ID: <CAKfTPtDAr3dbB008Kc09HoKXtwy8i+=nnyN6721Fks-u7A7XZA@mail.gmail.com>
Subject: Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steve Muckle <steve.muckle@linaro.org>,
        Morten Rasmussen <Morten.Rasmussen@arm.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "daniel.lezcano@linaro.org" <daniel.lezcano@linaro.org>,
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        Juri Lelli <Juri.Lelli@arm.com>,
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On 8 September 2015 at 16:10, Peter Zijlstra <peterz@infradead.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.