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SubjectRe: [PATCH v11 2/2] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()
On Fri, Aug 25, 2017 at 03:40:51PM +0900, Byungchul Park wrote:
> It would be better to try to check other siblings first if
> SD_PREFER_SIBLING is flaged when pushing tasks - migration.

Any opinions?

> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> kernel/sched/rt.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 77 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 979b734..a32e36d 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1618,12 +1618,35 @@ static struct task_struct *pick_highest_pushable_task(struct rq *rq, int cpu)
>
> static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);
>
> +/*
> + * 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_lowest_rq(struct task_struct *task)
> {
> - struct sched_domain *sd;
> + struct sched_domain *sd, *prefer = NULL;
> struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask);
> int this_cpu = smp_processor_id();
> int cpu = task_cpu(task);
> + int fallback_cpu = -1;
>
> /* Make sure the mask is initialized first */
> if (unlikely(!lowest_mask))
> @@ -1668,9 +1691,37 @@ static int find_lowest_rq(struct task_struct *task)
> return this_cpu;
> }
>
> - best_cpu = cpumask_first_and(lowest_mask,
> - sched_domain_span(sd));
> + /*
> + * If a cpu exists that is in the lowest_mask and
> + * the current sd span, but not in the prefer sd
> + * span, then that becomes our choice.
> + *
> + * Of course, the lowest possible cpu is already
> + * under consideration through lowest_mask.
> + */
> + best_cpu = find_cpu(lowest_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;
> }
> @@ -1679,6 +1730,29 @@ static int find_lowest_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, cpu4 would have been a better choice, since
> + * cpu3 is a (SMT) thread of an already loaded core.
> + *
> + * Doing it 'right' is difficult and expensive. The current
> + * solution is an acceptable approximation.
> + */
> + if (fallback_cpu != -1)
> + return fallback_cpu;
> +
> + /*
> * And finally, if there were no matches within the domains
> * just give the caller *something* to work with from the compatible
> * locations.
> --
> 1.9.1

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