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

Chen Yu
2022-08-23T08:12:35Z

On 2022-08-23 at 15:48:00 +0800, Yicong Yang wrote:
> On 2022/8/23 11:45, Chen Yu wrote:
> > 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?
>
> Since we'll need to recalculate the mask of rest CPUs to test in the LLC after scanning the cluster CPUs.
>
I was thinking of introducing a temporary variable
cpumask_and(cpus_cluster, sched_domain_span(sdc), cpus);
and iterate this cpus_cluster in the loop. But since the
cpus is reused, it is ok to be as it is.
> > 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:
> >
>
> It's on 6.0-rc1 when sent but can be cleanly rebased on rc2:
>
> yangyicong@ubuntu:~/mainline_linux/linux_sub_workspace$ git log --oneline -3
> 0079c27ba265 (HEAD -> topost-cls-v7, topost-cls-v6) sched/fair: Scan cluster before scanning LLC in wake-up path
> 1ecb9e322bd7 sched: Add per_cpu cluster domain info and cpus_share_lowest_cache API
I did not apply 1/2, and that was why it failed I think. Thanks for explaination.

Thanks,
Chenyu
> 1c23f9e627a7 (tag: v6.0-rc2, origin/master, origin/HEAD, master) Linux 6.0-rc2
>
> So I'm not sure where's the problem...
>
> > Reviewed-by: Chen Yu <yu.c.chen@intel.com>
> >
>
> Thanks!
>
> > 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));
> >> + }
> >> + }
> > .
> >