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

Yicong Yang
2022-08-23T07:50:13Z

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.

> 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
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));
>> + }
>> + }
> .