Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 10 Mar 2021 16:37:15 +0100 | Subject | Re: [PATCH] sched/fair: Prefer idle CPU to cache affinity |
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On Wed, 10 Mar 2021 at 06:53, Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote: > > * Vincent Guittot <vincent.guittot@linaro.org> [2021-03-08 14:52:39]: > > > On Fri, 26 Feb 2021 at 17:41, Srikar Dronamraju > > <srikar@linux.vnet.ibm.com> wrote: > > > > > Thanks Vincent for your review comments. > > > > +static int prefer_idler_llc(int this_cpu, int prev_cpu, int sync) > > > +{ > > > + struct sched_domain_shared *tsds, *psds; > > > + int pnr_busy, pllc_size, tnr_busy, tllc_size, diff; > > > + > > > + tsds = rcu_dereference(per_cpu(sd_llc_shared, this_cpu)); > > > + tnr_busy = atomic_read(&tsds->nr_busy_cpus); > > > + tllc_size = per_cpu(sd_llc_size, this_cpu); > > > + > > > + psds = rcu_dereference(per_cpu(sd_llc_shared, prev_cpu)); > > > + pnr_busy = atomic_read(&psds->nr_busy_cpus); > > > + pllc_size = per_cpu(sd_llc_size, prev_cpu); > > > + > > > + /* No need to compare, if both LLCs are fully loaded */ > > > + if (pnr_busy == pllc_size && tnr_busy == pllc_size) > > > + return nr_cpumask_bits; > > > + > > > + if (sched_feat(WA_WAKER) && tnr_busy < tllc_size) > > > + return this_cpu; > > > > Why have you chosen to favor this_cpu instead of prev_cpu unlike for wake_idle ? > > At this point, we know the waker running on this_cpu and wakee which was > running on prev_cpu are affine to each other and this_cpu and prev_cpu dont > share cache. I chose to move them close to each other to benefit from the > cache sharing. Based on feedback from Peter and Rik, I made the check more > conservative i.e tnr_busy <= tllc_size/smt_weight (where smt_weight is the > cpumask weight of smt domain for this_cpu) i.e if we have a free core in
yeah make sense
> this llc domain, chose this_cpu. select_idle_sibling() should pick an idle > cpu/core/smt within the llc domain for this_cpu. > > Do you feel, this may not be the correct option?
I was worried that we end up pulling tasks in same llc but the condition above and wake_wide should prevent such behavior
> > We are also experimenting with another option, were we call prefer_idler_cpu > after wa_weight. I.e > 1. if wake_affine_weight choses this_cpu but llc in prev_cpu has an idle > smt/CPU but there are no idle smt/CPU in this_cpu, then chose idle smt/CPU > in prev_cpu > 2. if wake_affine_weight choses nr_cpumask(aka prev_cpu) but llc in this_cpu > has an idle smt/CPU but there are no idle smt/CPU in prev_cpu, then chose > idle smt/CPU in this_cpu > > > > > + > > > + /* For better wakeup latency, prefer idler LLC to cache affinity */ > > > + diff = tnr_busy * pllc_size - sync - pnr_busy * tllc_size; > > > + if (!diff) > > > + return nr_cpumask_bits; > > > + if (diff < 0) > > > + return this_cpu; > > > + > > > + return prev_cpu; > > > +} > > > + > > > static int wake_affine(struct sched_domain *sd, struct task_struct *p, > > > int this_cpu, int prev_cpu, int sync) > > > { > > > @@ -5877,6 +5907,10 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, > > > if (sched_feat(WA_IDLE)) > > > target = wake_affine_idle(this_cpu, prev_cpu, sync); > > > > > > + if (sched_feat(WA_IDLER_LLC) && target == nr_cpumask_bits && > > > + !cpus_share_cache(this_cpu, prev_cpu)) > > > + target = prefer_idler_llc(this_cpu, prev_cpu, sync); > > > > could you use the same naming convention as others function ? > > wake_affine_llc as an example > > I guess you meant s/prefer_idler_llc/wake_affine_llc/
yes
> Sure. I can modify. > > > > > > + > > > if (sched_feat(WA_WEIGHT) && target == nr_cpumask_bits) > > > target = wake_affine_weight(sd, p, this_cpu, prev_cpu, sync); > > > > > > @@ -5884,8 +5918,11 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, > > > if (target == nr_cpumask_bits) > > > return prev_cpu; > > > > > > - schedstat_inc(sd->ttwu_move_affine); > > > - schedstat_inc(p->se.statistics.nr_wakeups_affine); > > > + if (target == this_cpu) { > > > > How is this condition related to $subject ? > > Before this change, wake_affine_weight and wake_affine_idle would either > return this_cpu or nr_cpumask_bits. Just before this check, we check if > target is nr_cpumask_bits and return prev_cpu. So the stats were only > incremented when target was this_cpu. > > However with prefer_idler_llc, we may return this_cpu, prev_cpu or > nr_cpumask_bits. Now we only to update stats when we have chosen to migrate > the task to this_cpu. Hence I had this check.
ok got it.
May be return earlier in this case like for if (target == nr_cpumask_bits) above
> > If we use the slightly lazier approach which is check for wa_weight first > before wa_idler_llc, then we may not need this change at all. > > -- > Thanks and Regards > Srikar Dronamraju
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