Messages in this thread | | | Date | Wed, 24 Nov 2021 16:02:38 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] sched/fair: Remove the cost of a redundant cpumask_next_wrap in select_idle_cpu |
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On Thu, Nov 25, 2021 at 01:02:00AM +1300, Barry Song wrote: > On Thu, Nov 25, 2021 at 12:57 AM Barry Song <21cnbao@gmail.com> wrote:
> > Let me make it clearer. if nr=5, the original code will loop 5 times, > > but in the 5th loop, it returns directly, so __select_idle_cpu is > > only done 4 times. > > > > if nr=1, the original code will loop 1 time, but in the 1st loop, > > it returns directly, so __select_idle_cpu is done 0 times. > > this is also why in the first version of patch, i did this: > span_avg = sd->span_weight * avg_idle; > if (span_avg > 4*avg_cost) > - nr = div_u64(span_avg, avg_cost); > + nr = div_u64(span_avg, avg_cost) - 1; > else > - nr = 4; > + nr = 3; > > because we are actually scanning 3 times or div_u64(span_avg, avg_cost) - 1 > times but not 4 times or div_u64(span_avg, avg_cost) times.
It still is confusing, because > 4*span -> nr = avg/span, very much implies we want to bottom out at 4.
> this is not confusing at all. the only thing which is confusing is the original > code.
But yes, it seems a whole lot of confusion stacked together. Let make it sane and say that we do 'nr' iterations, because clearly that was the intent :-)
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