Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH 6/8] sched/fair: Filter out locally-unsolvable misfit imbalances | Date | Wed, 03 Feb 2021 18:43:06 +0000 |
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On 03/02/21 15:16, Qais Yousef wrote: > On 01/28/21 18:31, Valentin Schneider wrote: >> Consider the following (hypothetical) asymmetric CPU capacity topology, >> with some amount of capacity pressure (RT | DL | IRQ | thermal): >> >> DIE [ ] >> MC [ ][ ] >> 0 1 2 3 >> >> | CPU | capacity_orig | capacity | >> |-----+---------------+----------| >> | 0 | 870 | 860 | >> | 1 | 870 | 600 | >> | 2 | 1024 | 850 | >> | 3 | 1024 | 860 | >> >> If CPU1 has a misfit task, then CPU0, CPU2 and CPU3 are valid candidates to >> grant the task an uplift in CPU capacity. Consider CPU0 and CPU3 as >> sufficiently busy, i.e. don't have enough spare capacity to accommodate >> CPU1's misfit task. This would then fall on CPU2 to pull the task. > > I think this scenario would be hard in practice, but not impossible. Maybe > gaming could push the system that hard. >
Actually I wouldn't be surprised if a moderatly busy Android environment could hit this - slight thermal pressure on the bigs, RT pressure because we know folks love (ab)using RT, a pinch of IRQs in the mix...
>> @@ -8450,11 +8457,21 @@ static inline void update_sg_lb_stats(struct lb_env *env, >> continue; >> >> /* Check for a misfit task on the cpu */ >> - if (sd_has_asym_cpucapacity(env->sd) && >> - sgs->group_misfit_task_load < rq->misfit_task_load) { >> - sgs->group_misfit_task_load = rq->misfit_task_load; >> - *sg_status |= SG_OVERLOAD; >> - } >> + if (!sd_has_asym_cpucapacity(env->sd) || >> + !rq->misfit_task_load) >> + continue; >> + >> + *sg_status |= SG_OVERLOAD; >> + sgs->group_has_misfit_task = true; >> + >> + /* >> + * Don't attempt to maximize load for misfit tasks that can't be >> + * granted a CPU capacity uplift. >> + */ >> + if (cpu_capacity_greater(env->dst_cpu, i)) >> + sgs->group_misfit_task_load = max( >> + sgs->group_misfit_task_load, >> + rq->misfit_task_load); > > nit: missing curly braces around the if. >
Ack.
>> @@ -8504,7 +8521,7 @@ static bool update_sd_pick_busiest(struct lb_env *env, >> /* Don't try to pull misfit tasks we can't help */ >> if (static_branch_unlikely(&sched_asym_cpucapacity) && >> sgs->group_type == group_misfit_task && >> - (!capacity_greater(capacity_of(env->dst_cpu), sg->sgc->max_capacity) || >> + (!sgs->group_misfit_task_load || >> sds->local_stat.group_type != group_has_spare)) >> return false; >> >> @@ -9464,15 +9481,18 @@ static struct rq *find_busiest_queue(struct lb_env *env, >> case migrate_misfit: >> /* >> * For ASYM_CPUCAPACITY domains with misfit tasks we >> - * simply seek the "biggest" misfit task. >> + * simply seek the "biggest" misfit task we can >> + * accommodate. >> */ >> + if (!cpu_capacity_greater(env->dst_cpu, i)) >> + continue; > > Both this hunk and the one above mean we will end up searching harder to pull > the task into the right cpu taking actual capacity into account. Which is > a good improvement. >
Note that those extra checks are to make sure we *don't* downmigrate tasks (as stated somewhere above, this change lets find_busiest_queue() iterate over CPUs bigger than the local CPU's, which wasn't the case before). A "big" CPU will still get the chance to pull a "medium" task, even if a "medium" CPU would have been a "better" choice.
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