Hi,
On 06/02/2019 16:04, Peter Zijlstra wrote:
[...]
>> @@ -9561,6 +9573,14 @@ static void nohz_balancer_kick(struct rq *rq)
>
> sd = rcu_dereference(rq->sd);
> if (sd) {
> if ((rq->cfs.h_nr_running >= 1) &&
> check_cpu_capacity(rq, sd)) {
> flags = NOHZ_KICK_MASK;
> goto unlock;
>> }
>> }
>>
>> + sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
>> + if (sd) {
>> + if (check_misfit_status(rq, sd)) {
>> + flags = NOHZ_KICK_MASK;
>> + goto unlock;
>> + }
>> + }
>
> So while the exact @sd to use for check_cpu_capacity() likely doesn't
> matter; this is a 'implicit' test for actually having asym_capacity.
>
I did feel compelled to use the "right" @sd to have a coherent
imbalance_pct being used - on big.LITTLE, the @sd above this hunk would be
MC (117 imbalance_pct) whereas the sd_asym_cpucapacity one would be
DIE (125 imbalance_pct).
> Fair enough I suppose. However, now that you wrote such a nice comment
> for the sd_llc_shared case, these other two cases are sad to not have a
> comment.
>
> So how about you add something like:
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9589,8 +9589,12 @@ static void nohz_balancer_kick(struct rq
>
> sd = rcu_dereference(rq->sd);
> if (sd) {
> - if ((rq->cfs.h_nr_running >= 1) &&
> - check_cpu_capacity(rq, sd)) {
> + /*
> + * If there's a CFS task and the current CPU has reduced
> + * capacity; kick the ILB to see if there's a better CPU to run
> + * on.
> + */
> + if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) {
> flags = NOHZ_KICK_MASK;
> goto unlock;
> }
> @@ -9598,6 +9602,10 @@ static void nohz_balancer_kick(struct rq
>
> sd = rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu));
> if (sd) {
> + /*
> + * When ASYM_CAPACITY; see if there's a higher capacity CPU to
> + * run the misfit task on.
> + */
> if (check_misfit_status(rq, sd)) {
> flags = NOHZ_KICK_MASK;
> goto unlock;
> @@ -9606,6 +9614,10 @@ static void nohz_balancer_kick(struct rq
>
> sd = rcu_dereference(per_cpu(sd_asym_packing, cpu));
> if (sd) {
> + /*
> + * When ASYM_PACKING; see if there's a more preferred CPU going
> + * idle; in which case, kick the ILB to move tasks around.
> + */
s/going idle/currently idle/, no?
> for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask)
> {
> if (sched_asym_prefer(i, cpu)) {
> flags = NOHZ_KICK_MASK;
>
