Re: [PATCH] sched/fair: Do not decay new task load on first enqueue

[Vincent Guittot]

On 18 October 2016 at 13:09, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, Oct 12, 2016 at 09:41:36AM +0200, Vincent Guittot wrote:
>
>> ok. In fact, I have noticed another regression with tip/sched/core and
>> hackbench while looking at yours.
>> I have bisect to :
>> 10e2f1acd0 ("sched/core: Rewrite and improve select_idle_siblings")
>>
>> hackbench -P -g 1
>>
>>        v4.8        tip/sched/core  tip/sched/core+revert 10e2f1acd010
>> and 1b568f0aabf2
>> min 0.051       0,052               0.049
>> avg 0.057(0%)   0,062(-7%)   0.056(+1%)
>> max 0.070       0,073      0.067
>> stdev  +/-8%       +/-10%    +/-9%
>>
>> The issue seems to be that it prevents some migration at wake up at
>> the end of hackbench test so we have last tasks that compete for the
>> same CPU whereas other CPUs are idle in the same MC domain. I haven't
>> to look more deeply which part of the patch do the regression yet
>
> So select_idle_cpu(), which does the LLC wide CPU scan is now throttled
> by a comparison between avg_cost and avg_idle; where avg_cost is a
> historical measure of how costly it was to scan the entire LLC domain
> and avg_idle is our current idle time guestimate (also a historical
> average).
>
> The problem was that a number of workloads were spending quite a lot of
> time here scanning CPUs while they could be doing useful work (esp.
> since newer parts have silly amounts of CPUs per LLC).

make sense

>
> The toggle is a heuristic with a random number in.. we could see if
> there's anything better we can do. I know some people take the toggle
> out entirely, but that will regress other workloads.

ok. so removing the toggle fixes the problem in this test case too.

may be we can take into account the sd_llc_size in the toggle