* Vincent Guittot <[email protected]> [2021-03-08 14:52:39]:
> On Fri, 26 Feb 2021 at 17:41, Srikar Dronamraju > <[email protected]> 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 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? 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/ 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. 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
