On Fri, Jan 08, 2021 at 09:21:48PM +0100, Peter Zijlstra wrote: > On Fri, Jan 08, 2021 at 10:27:38AM +0000, Mel Gorman wrote: > > > 1. avg_scan_cost is now based on the average scan cost of a rq but > > avg_idle is still scaled to the domain size. This is a bit problematic > > because it's comparing scan cost of a single rq with the estimated > > average idle time of a domain. As a result, the scan depth can be much > > larger than it was before the patch and led to some regressions. > > > @@ -6164,25 +6164,25 @@ static int select_idle_cpu(struct task_struct *p, > > struct sched_domain *sd, int t > > */ > > avg_idle = this_rq()->avg_idle / 512; > > avg_cost = this_sd->avg_scan_cost + 1; > > - > > - span_avg = sd->span_weight * avg_idle; > > - if (span_avg > 4*avg_cost) > > - nr = div_u64(span_avg, avg_cost); > > - else > > + nr = div_u64(avg_idle, avg_cost); > > + if (nr < 4) > > nr = 4; > > Oooh, could it be I simply didn't remember how that code was supposed to > work and should kick my (much) younger self for not writing a comment? > > Consider: > > span_weight * avg_idle avg_cost > nr = ---------------------- = avg_idle / ---------- > avg_cost span_weigt > > Where: avg_cost / span_weight ~= cost-per-rq >
This would definitely make sense and I even evaluated it but the nature of avg_idle and the scale it works at (up to 2*sched_migration_cost) just ended up generating lunatic values far outside the size of the domain size. Fitting that to the domain size just ended up looking silly too and avg_cost does not decay. Still, in principle, it's the right direction, it's just not what the code does right now. -- Mel Gorman SUSE Labs
