Re: [RESEND RFC PATCH V3] sched: Improve scalability of select_idle_sibling using SMT balance

Subhra Mazumdar
2018-02-05T22:08:57Z

On 02/05/2018 04:19 AM, Peter Zijlstra wrote:
> On Fri, Feb 02, 2018 at 09:37:02AM -0800, Subhra Mazumdar wrote:
>> In the scheme of SMT balance, if the idle cpu search is done _not_ in the
>> last run core, then we need a random cpu to start from. If the idle cpu
>> search is done in the last run core we can start the search from last run
>> cpu. Since we need the random index for the first case I just did it for
>> both.
> That shouldn't be too hard to fix. I think we can simply transpose the
> CPU number. That is, something like:
>
> cpu' = core'_id + (cpu - core_id)
>
> should work for most sane cases. We don't give any guarantees this will
> in fact work, but (almost) all actual CPU enumeration schemes I've seen
> this should work for.
>
> And if it doesn't work, we're not worse of than we are now.
>
> I just couldn't readily find a place where we need to do this for cores
> with the current code. But I think we have one place between LLCs where
> it can be done:
>
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7b6535987500..eb8b8d0a026c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6109,7 +6109,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
> if (!static_branch_likely(&sched_smt_present))
> return -1;
>
> - for_each_cpu(cpu, cpu_smt_mask(target)) {
> + for_each_cpu_wrap(cpu, cpu_smt_mask(target), target) {
> if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> continue;
> if (idle_cpu(cpu))
> @@ -6357,8 +6357,17 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> if (cpu == prev_cpu)
> goto pick_cpu;
>
> - if (wake_affine(affine_sd, p, prev_cpu, sync))
> - new_cpu = cpu;
> + if (wake_affine(affine_sd, p, prev_cpu, sync)) {
> + /*
> + * Transpose prev_cpu's offset into this cpu's
> + * LLC domain to retain the 'random' search offset
> + * for for_each_cpu_wrap().
> + */
> + new_cpu = per_cpu(sd_llc_id, cpu) +
> + (prev_cpu - per_cpu(sd_llc_id, prev_cpu));
> + if (unlikely(!cpus_share_cache(new_cpu, cpu)))
> + new_cpu = cpu;
> + }
> }
>
> if (sd && !(sd_flag & SD_BALANCE_FORK)) {
The pseudo random is also used for choosing a random core to compare
with, how will transposing achieve that?

Thanks,
Subhra