On 10/05/20 16:57, Peter Zijlstra wrote:
> Replace a bunch of cpumask_any*() instances with
> cpumask_any*_distribute(), by injecting this little bit of random in
> cpu selection, we reduce the chance two competing balance operations
> working off the same lowest_mask pick the same CPU.
>
> Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
> ---
> include/linux/cpumask.h | 6 ++++++
> kernel/sched/cpupri.c | 4 ++--
> kernel/sched/deadline.c | 2 +-
> kernel/sched/rt.c | 6 +++---
> lib/cpumask.c | 18 ++++++++++++++++++
> 5 files changed, 30 insertions(+), 6 deletions(-)
>
[...]
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1752,8 +1752,8 @@ static int find_lowest_rq(struct task_st
> return this_cpu;
> }
>
> - best_cpu = cpumask_first_and(lowest_mask,
> - sched_domain_span(sd));
> + best_cpu = cpumask_any_and_distribute(lowest_mask,
> +
> sched_domain_span(sd));
I guess I should have done this 6 months ago and just got done with it :)
[email protected]
> if (best_cpu < nr_cpu_ids) {
> rcu_read_unlock();
> return best_cpu;
> @@ -1770,7 +1770,7 @@ static int find_lowest_rq(struct task_st
> if (this_cpu != -1)
> return this_cpu;
>
> - cpu = cpumask_any(lowest_mask);
> + cpu = cpumask_any_distribute(lowest_mask);
> if (cpu < nr_cpu_ids)
> return cpu;
>
> --- a/lib/cpumask.c
> +++ b/lib/cpumask.c
> @@ -267,3 +267,21 @@ int cpumask_any_and_distribute(const str
> return next;
> }
> EXPORT_SYMBOL(cpumask_any_and_distribute);
> +
> +int cpumask_any_distribute(const struct cpumask *srcp)
> +{
> + int next, prev;
> +
> + /* NOTE: our first selection will skip 0. */
> + prev = __this_cpu_read(distribute_cpu_mask_prev);
We had a discussion then that __this_cpu*() variant assumes preemption being
disabled and it's safer to use this_cpu*() variant instead. Still holds true
here?
Thanks
--
Qais Yousef
> +
> + next = cpumask_next(prev, srcp);
> + if (next >= nr_cpu_ids)
> + next = cpumask_first(srcp);
> +
> + if (next < nr_cpu_ids)
> + __this_cpu_write(distribute_cpu_mask_prev, next);
> +
> + return next;
> +}
> +EXPORT_SYMBOL(cpumask_any_distribute);
>
>
