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).
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.
