Re: [PATCH v7 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

Chen Yu
2022-08-23T03:48:01Z

On 2022-08-22 at 15:36:10 +0800, Yicong Yang wrote:
> From: Barry Song <song.bao.hua@hisilicon.com>
>
> For platforms having clusters like Kunpeng920, CPUs within the same cluster
> have lower latency when synchronizing and accessing shared resources like
> cache. Thus, this patch tries to find an idle cpu within the cluster of the
> target CPU before scanning the whole LLC to gain lower latency.
>
> Testing has been done on Kunpeng920 by pinning tasks to one numa and two
> numa. On Kunpeng920, Each numa has 8 clusters and each cluster has 4 CPUs.
>
> With this patch, We noticed enhancement on tbench within one numa or cross
> two numa.
>
> On numa 0:
> 6.0-rc1 patched
> Hmean 1 351.20 ( 0.00%) 396.45 * 12.88%*
> Hmean 2 700.43 ( 0.00%) 793.76 * 13.32%*
> Hmean 4 1404.42 ( 0.00%) 1583.62 * 12.76%*
> Hmean 8 2833.31 ( 0.00%) 3147.85 * 11.10%*
> Hmean 16 5501.90 ( 0.00%) 6089.89 * 10.69%*
> Hmean 32 10428.59 ( 0.00%) 10619.63 * 1.83%*
> Hmean 64 8223.39 ( 0.00%) 8306.93 * 1.02%*
> Hmean 128 7042.88 ( 0.00%) 7068.03 * 0.36%*
>
> On numa 0-1:
> 6.0-rc1 patched
> Hmean 1 363.06 ( 0.00%) 397.13 * 9.38%*
> Hmean 2 721.68 ( 0.00%) 789.84 * 9.44%*
> Hmean 4 1435.15 ( 0.00%) 1566.01 * 9.12%*
> Hmean 8 2776.17 ( 0.00%) 3007.05 * 8.32%*
> Hmean 16 5471.71 ( 0.00%) 6103.91 * 11.55%*
> Hmean 32 10164.98 ( 0.00%) 11531.81 * 13.45%*
> Hmean 64 17143.28 ( 0.00%) 20078.68 * 17.12%*
> Hmean 128 14552.70 ( 0.00%) 15156.41 * 4.15%*
> Hmean 256 12827.37 ( 0.00%) 13326.86 * 3.89%*
>
> Note neither Kunpeng920 nor x86 Jacobsville supports SMT, so the SMT branch
> in the code has not been tested but it supposed to work.
>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> [https://lore.kernel.org/lkml/Ytfjs+m1kUs0ScSn@worktop.programming.kicks-ass.net]
> Tested-by: Yicong Yang <yangyicong@hisilicon.com>
> Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
> kernel/sched/fair.c | 30 +++++++++++++++++++++++++++---
> kernel/sched/sched.h | 2 ++
> kernel/sched/topology.c | 10 ++++++++++
> 3 files changed, 39 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 914096c5b1ae..6fa77610d0f5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6437,6 +6437,30 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> }
> }
>
> + if (static_branch_unlikely(&sched_cluster_active)) {
> + struct sched_domain *sdc = rcu_dereference(per_cpu(sd_cluster, target));
> +
> + if (sdc) {
> + for_each_cpu_wrap(cpu, sched_domain_span(sdc), target + 1) {
Looks good to me. One minor question, why don't we use
cpumask_and(cpus, sched_domain_span(sdc), cpus);
> + if (!cpumask_test_cpu(cpu, cpus))
> + continue;
so above check can be removed in each loop? Besides may I know what version this patch
is based on? since I failed to apply the patch on v6.0-rc2. Other than that:

Reviewed-by: Chen Yu <yu.c.chen@intel.com>

thanks,
Chenyu
> +
> + if (has_idle_core) {
> + i = select_idle_core(p, cpu, cpus, &idle_cpu);
> + if ((unsigned int)i < nr_cpumask_bits)
> + return i;
> + } else {
> + if (--nr <= 0)
> + return -1;
> + idle_cpu = __select_idle_cpu(cpu, p);
> + if ((unsigned int)idle_cpu < nr_cpumask_bits)
> + return idle_cpu;
> + }
> + }
> + cpumask_andnot(cpus, cpus, sched_domain_span(sdc));
> + }
> + }