Re: [PATCH v3 07/12] sched: test the cpu's capacity in wake affine

From: Vincent Guittot
Date: Thu Jul 10 2014 - 09:58:44 EST

On 10 July 2014 13:06, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Jun 30, 2014 at 06:05:38PM +0200, Vincent Guittot wrote:
>> Currently the task always wakes affine on this_cpu if the latter is idle.
>> Before waking up the task on this_cpu, we check that this_cpu capacity is not
>> significantly reduced because of RT tasks or irq activity.
>> Use case where the number of irq and/or the time spent under irq is important
>> will take benefit of this because the task that is woken up by irq or softirq
>> will not use the same CPU than irq (and softirq) but a idle one which share
>> its cache.
> The above, doesn't seem to explain:
>> + } else if (!(sd->flags & SD_SHARE_PKG_RESOURCES)) {
>> + this_eff_load = 0;
>> + }
>> +
>> + balanced = this_eff_load <= prev_eff_load;
> this. Why would you unconditionally allow wake_affine across cache
> domains?

The current policy is to use this_cpu if this_load <= 0. I want to
keep the current wake affine policy for all sched_domain that doesn't
share cache so if the involved CPUs don't share cache, I clear
this_eff_load to force balanced to be true. But when CPUs share their
cache, we not only look at idleness but also at available capacity of
prev and local CPUs.
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