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SubjectRe: [PATCH v9 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()
Hi,

On 22/08/17 17:30, Byungchul Park wrote:
> It would be better to try to check other siblings first if
> SD_PREFER_SIBLING is flaged when pushing tasks - migration.
>
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
^
This has to come before your SoB.

> ---
> kernel/sched/deadline.c | 80 ++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 73 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0223694..b6b3855 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1319,12 +1319,35 @@ static struct task_struct *pick_earliest_pushable_dl_task(struct rq *rq, int cpu
>
> static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
>
> +/*
> + * Find the first cpu in: mask & sd & ~prefer
> + */
> +static int find_cpu(const struct cpumask *mask,
> + const struct sched_domain *sd,
> + const struct sched_domain *prefer)
> +{
> + const struct cpumask *sds = sched_domain_span(sd);
> + const struct cpumask *ps = prefer ? sched_domain_span(prefer) : NULL;
> + int cpu;
> +
> + for_each_cpu(cpu, mask) {
> + if (!cpumask_test_cpu(cpu, sds))
> + continue;
> + if (ps && cpumask_test_cpu(cpu, ps))
> + continue;
> + break;
> + }
> +
> + return cpu;
> +}
> +
> static int find_later_rq(struct task_struct *task)
> {
> - struct sched_domain *sd;
> + struct sched_domain *sd, *prefer = NULL;
> struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
> int this_cpu = smp_processor_id();
> int cpu = task_cpu(task);
> + int fallback_cpu = -1;
>
> /* Make sure the mask is initialized first */
> if (unlikely(!later_mask))
> @@ -1376,15 +1399,35 @@ static int find_later_rq(struct task_struct *task)
> return this_cpu;
> }
>
> - best_cpu = cpumask_first_and(later_mask,
> - sched_domain_span(sd));
> /*
> - * Last chance: if a cpu being in both later_mask
> - * and current sd span is valid, that becomes our
> - * choice. Of course, the latest possible cpu is
> - * already under consideration through later_mask.
> + * If a cpu being in later_mask & current sd &
> + * ~prefer sd is valid, that becomes our choice.
> + * Of course, the latest possible cpu is already
> + * under consideration through later_mask.
> */
> + best_cpu = find_cpu(later_mask, sd, prefer);
> +
> if (best_cpu < nr_cpu_ids) {
> + /*
> + * If current domain is SD_PREFER_SIBLING
> + * flaged, we have to try to check other
> + * siblings first.
> + */
> + if (sd->flags & SD_PREFER_SIBLING) {
> + prefer = sd;
> +
> + /*
> + * fallback_cpu should be one
> + * in the closest domain among
> + * SD_PREFER_SIBLING domains,
> + * in case that more than one
> + * SD_PREFER_SIBLING domains
> + * exist in the hierachy.
> + */
> + if (fallback_cpu == -1)
> + fallback_cpu = best_cpu;
> + continue;
> + }
> rcu_read_unlock();
> return best_cpu;
> }
> @@ -1393,6 +1436,29 @@ static int find_later_rq(struct task_struct *task)
> rcu_read_unlock();
>
> /*
> + * If fallback_cpu is valid, all our guesses failed *except* for
> + * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
> + *
> + * XXX: Consider the following example, 4 cores SMT2 system:
> + *
> + * LLC [0 - 7]
> + * SMT [0 1][2 3][4 5][6 7]
> + * o x o x x x x x
> + *
> + * where 'o': occupied and 'x': empty.
> + *
> + * A wakeup on cpu0 will exclude cpu1 and choose cpu3, since
> + * cpu1 is in a SD_PREFER_SIBLING sd and cpu3 is not. However,
> + * in this case, we have to choose cpu4 for better work, instead

... in this case cpu4 would have been a better choice, since cpu3 is a
(SMT) thread of an already loaded core.

> + * of cpu3 that is fully loaded.
> + *
> + * We have to do the best if possible, but choose the second
> + * best here since that is too expensive to adopt.
> + */

I'd also modify this last paragraph with something like:

"Doing it 'right' is difficult and expensive. The current solution is an
acceptable approximation."

Apart from these minor points, patch looks ok to me.

Acked-by: Juri Lelli <juri.lelli@arm.com>

Best,

- Juri

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