Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | [PATCH v2 7/7] sched/fair: Relax task_hot() for misfit tasks | Date | Fri, 19 Feb 2021 13:00:03 +0000 |
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Consider the following topology:
DIE [ ] MC [ ][ ] 0 1 2 3
capacity_orig_of(x \in {0-1}) < capacity_orig_of(x \in {2-3})
w/ CPUs 2-3 idle and CPUs 0-1 running CPU hogs (util_avg=1024).
When CPU2 goes through load_balance() (via periodic / NOHZ balance), it should pull one CPU hog from either CPU0 or CPU1 (this is misfit task upmigration). However, should a e.g. pcpu kworker awake on CPU0 just before this load_balance() happens and preempt the CPU hog running there, we would have, for the [0-1] group at CPU2's DIE level:
o sgs->sum_nr_running > sgs->group_weight o sgs->group_capacity * 100 < sgs->group_util * imbalance_pct
IOW, this group is group_overloaded.
Considering CPU0 is picked by find_busiest_queue(), we would then visit the preempted CPU hog in detach_tasks(). However, given it has just been preempted by this pcpu kworker, task_hot() will prevent it from being detached. We then leave load_balance() without having done anything.
Long story short, preempted misfit tasks are affected by task_hot(), while currently running misfit tasks are intentionally preempted by the stopper task to migrate them over to a higher-capacity CPU.
Align detach_tasks() with the active-balance logic and let it pick a cache-hot misfit task when the destination CPU can provide a capacity uplift.
Reviewed-by: Qais Yousef <qais.yousef@arm.com> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> --- kernel/sched/fair.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index ee172b384e29..554430fd249c 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7448,6 +7448,17 @@ static int task_hot(struct task_struct *p, struct lb_env *env) if (env->sd->flags & SD_SHARE_CPUCAPACITY) return 0; + /* + * On a (sane) asymmetric CPU capacity system, the increase in compute + * capacity should offset any potential performance hit caused by a + * migration. + */ + if (sd_has_asym_cpucapacity(env->sd) && + env->idle != CPU_NOT_IDLE && + !task_fits_capacity(p, capacity_of(env->src_cpu)) && + cpu_capacity_greater(env->dst_cpu, env->src_cpu)) + return 0; + /* * Buddy candidates are cache hot: */ -- 2.27.0
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