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Date:   Tue, 23 Aug 2022 16:09:54 +0800
From:   Chen Yu <yu.c.chen@intel.com>
To:     Yicong Yang <yangyicong@huawei.com>
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Subject: Re: [PATCH v7 2/2] sched/fair: Scan cluster before scanning LLC in
 wake-up path
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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));
> >> +		}
> >> +	}
> > .
> >