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

[Yicong Yang]

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@xxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxx>
>> [https://lore.kernel.org/lkml/Ytfjs+m1kUs0ScSn@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx]
>> Tested-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
>> ---
>>  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@xxxxxxxxx>
> 

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