Re: [PATCH RESEND v9 00/10] sched: consolidation of CPU capacity and usage

From: Vincent Guittot
Date: Fri Feb 20 2015 - 09:13:45 EST

On 20 February 2015 at 12:52, Morten Rasmussen <morten.rasmussen@xxxxxxx> wrote:
> On Fri, Feb 20, 2015 at 11:34:47AM +0000, Peter Zijlstra wrote:
>> On Thu, Feb 19, 2015 at 12:49:40PM +0000, Morten Rasmussen wrote:
>> > Also, it still not clear why patch 10 uses relative capacity reduction
>> > instead of absolute capacity available to CFS tasks.
>> As present in your asymmetric big and small systems? Yes it would be
>> unfortunate to migrate a task to an idle small core when the big core is
>> still faster, even if reduced by rt/irq work.
> Yes, exactly. I don't think it would cause any harm for symmetric cases
> to use absolute capacity instead. Am I missing something?

If absolute capacity is used, we will trig an active load balance from
little to big core each time a little has got 1 task and a big core is
idle whereas we only want to trig an active migration is the src_cpu's
capacity that is available for the cfs task is significantly reduced
by rt tasks.

I can mix absolute and relative tests by 1st testing that the capacity
of the src is reduced and then ensure that the dst_cpu has more
absolute capacity than src_cpu
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