Re: [PATCH v4 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

From: Chen Yu
Date: Sat Jun 11 2022 - 05:05:33 EST

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In reply to: Yicong Yang: "Re: [PATCH v4 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path"
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On Sat, Jun 11, 2022 at 03:40:23PM +0800, Yicong Yang wrote:
> On 2022/6/11 11:03, Chen Yu wrote:
> > Hi Yicong,
> > On Fri, Jun 10, 2022 at 02:39:38PM +0800, Yicong Yang wrote:
> >> On 2022/6/10 6:47, Tim Chen wrote:
> >>> On Thu, 2022-06-09 at 20:06 +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.
> >>>>
> >>>> Note neither Kunpeng920 nor x86 Jacobsville supports SMT, so this patch
> >>>> doesn't consider SMT for this moment.
> >>>>
> >>>> 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:
> >>>> 5.19-rc1 patched
> >>>> Hmean 1 350.27 ( 0.00%) 406.88 * 16.16%*
> >>>> Hmean 2 702.01 ( 0.00%) 808.22 * 15.13%*
> >>>> Hmean 4 1405.14 ( 0.00%) 1614.34 * 14.89%*
> >>>> Hmean 8 2830.53 ( 0.00%) 3169.02 * 11.96%*
> >>>> Hmean 16 5597.95 ( 0.00%) 6224.20 * 11.19%*
> >>>> Hmean 32 10537.38 ( 0.00%) 10524.97 * -0.12%*
> >>>> Hmean 64 8366.04 ( 0.00%) 8437.41 * 0.85%*
> >>>> Hmean 128 7060.87 ( 0.00%) 7150.25 * 1.27%*
> >>>>
> >>>> On numa 0-1:
> >>>> 5.19-rc1 patched
> >>>> Hmean 1 346.11 ( 0.00%) 408.47 * 18.02%*
> >>>> Hmean 2 693.34 ( 0.00%) 805.78 * 16.22%*
> >>>> Hmean 4 1384.96 ( 0.00%) 1602.49 * 15.71%*
> >>>> Hmean 8 2699.45 ( 0.00%) 3069.98 * 13.73%*
> >>>> Hmean 16 5327.11 ( 0.00%) 5688.19 * 6.78%*
> >>>> Hmean 32 10019.10 ( 0.00%) 11862.56 * 18.40%*
> >>>> Hmean 64 13850.57 ( 0.00%) 17748.54 * 28.14%*
> >>>> Hmean 128 12498.25 ( 0.00%) 15541.59 * 24.35%*
> >>>> Hmean 256 11195.77 ( 0.00%) 13854.06 * 23.74%*
> >>>
> >>> Yicong,
> >>>
> >>> Have you tried any workload where tasks don't share data
> >>> with each other but have sleep/wakeup? That's the case
> >>> where we actually want to spread the tasks out among the clusters
> >>> to void contention for L2 cache.
> >>>
> >>> Will be nice to make sure there's no regression there for
> >>> such workload.
> >>>
> >>
> >> Any certain workload you'd like me test? I'm willing to do :)
> >>
> >> I've tested this patch with MySQL as well (like in v2). This won't hurt
> >> the MySQL case with SIS_PROP but observed some improvement with SIS_UTIL
> >> posted in [1]. We leverage the nr to suppress redundant scanning in the
> >> current approach and seems SIS_UTIL is more efficient in this case.
> >>
> >> 5.19-rc1 patched patched+SIS_UTIL[1]
> >> TPS-16threads 6215.11 6172.74 (-0.68%) 6217.33 (0.04%)
> >> QPS-16threads 124302.21 123454.68 (-0.68%) 124346.52 (0.04%)
> >> avg-lat-16threads 2.57 2.59 (-0.65%) 2.57 (0.00%)
> >> TPS-24threads 8726.40 8690.87 (-0.41%) 8833.08 (1.22%)
> >> QPS-24threads 174527.88 173817.42 (-0.41%) 176661.54 (1.21%)
> >> avg-lat-24threads 2.75 2.76 (-0.36%) 2.71 (1.33%)
> >> TPS-32threads 9555.42 9514.86 (-0.42%) 10010.87 (4.77%)
> >> QPS-32threads 191108.37 190297.28 (-0.42%) 200217.35 (4.55%)
> >> avg-lat-32threads 3.35 3.36 (-0.30%) 3.20 (4.58%)
> >> TPS-64threads 10290.10 10324.75 (0.34%) 10819.77 (5.15%)
> >> QPS-64threads 205802.05 206494.95 (0.34%) 216395.40 (4.90%)
> >> avg-lat-64threads 6.22 6.20 (0.38%) 5.92 (4.88%)
> >>
> >>
> > Thanks for the testings. I'm refining the patch according to Mel's suggestion and
> > will send it out later. Some minor nits below.
> >>> Code itself looks good.
> >>>
> >>> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> >>>
> >>
> >> Thanks.
> >>
> >> [1] https://lore.kernel.org/lkml/20220428182442.659294-1-yu.c.chen@xxxxxxxxx/
> >>
> >>>>
> >>>> Tested-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> >>>> Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> >>>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> >>>> ---
> >>>> kernel/sched/fair.c | 44 +++++++++++++++++++++++++++++++++++++++++---
> >>>> 1 file changed, 41 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >>>> index 77b2048a9326..6d173e196ad3 100644
> >>>> --- a/kernel/sched/fair.c
> >>>> +++ b/kernel/sched/fair.c
> >>>> @@ -6327,6 +6327,40 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd
> >>>>
> >>>> #endif /* CONFIG_SCHED_SMT */
> >>>>
> >>>> +#ifdef CONFIG_SCHED_CLUSTER
> >>>> +/*
> >>>> + * Scan the cluster domain for idle CPUs and clear cluster cpumask after scanning
> >>>> + */
> >>>> +static inline int scan_cluster(struct task_struct *p, struct cpumask *cpus,
> >>>> + int target, int *nr)
> >>>> +{
> >>>> + struct sched_domain *sd = rcu_dereference(per_cpu(sd_cluster, target));
> >>>> + int cpu, idle_cpu;
> >>>> +
> >>>> + /* TODO: Support SMT system with cluster topology */
> >>>> + if (!sched_smt_active() && sd) {
> >>>> + for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> >>>> + if (!--*nr)
> >>>> + break;
> > Maybe I understand it incorrectly, here we changed the value of nr. Then if
> > we did not find any idle CPU in the cluster, nr is set to 0. Then in the LLC scan,
> > since there is 'if (--nr <= 0)', the LLC scan terminates.
> > Is the policy like: if we can not find one idle CPU in the cluster, we give
> > up scan for one in the LLC?
> > [cut]
>
> We're still scanning in the LLC since cluster is within the LLC (on Kunpeng 920 cluster cpus
> share L3 Tag and on Jacobsville cluster cpus share L2, while on both platform LLC should be
> the L3 cache). So if we've run out of nr we should stop scanning the rest of LLC as well
> and ...
>
I see, the budget has been exhausted and we don't spend more time on LLC.
> >>>> +
> >>>> /*
> >>>> * Scan the LLC domain for idle CPUs; this is dynamically regulated by
> > If we add logic for cluster scan, might need to update the comment as well.
>
> ...the comment here still stands.
>
> >>>> * comparing the average scan cost (tracked in sd->avg_scan_cost) against the
> >>>> @@ -6375,6 +6409,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >>>> time = cpu_clock(this);
> >>>> }
> >>>>
> >>>> + idle_cpu = scan_cluster(p, cpus, target, &nr);
> >>>> + if ((unsigned int)idle_cpu < nr_cpumask_bits)
> >>>> + return idle_cpu;
> >>>> +
> >>>> for_each_cpu_wrap(cpu, cpus, target + 1) {
> >>>> if (has_idle_core) {
> >>>> i = select_idle_core(p, cpu, cpus, &idle_cpu);
> >>>> @@ -6382,7 +6420,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> >>>> return i;
> >>>>
> >>>> } else {
> >>>> - if (!--nr)
> >>>> + if (--nr <= 0)
> > May I know the reason to change this? I thought this could be a seperate patch.
> >
>
> scan_cluster() may run out of nr (on Kunpeng 920 there're 4 cpus in one cluster and nr will be at least 4 with
> SIS_PROP) and we cannot stop scanning correctly without this change. So the change should be in this patch.
>
So when nr reaches 0, the original loop could not stop. Thanks for the explanation.

thanks,
Chenyu
> Thanks,
> Yicong
>
> > thanks,
> > Chenyu
> >>>> return -1;
> >>>> idle_cpu = __select_idle_cpu(cpu, p);
> >>>> if ((unsigned int)idle_cpu < nr_cpumask_bits)
> >>>> @@ -6481,7 +6519,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >>>> /*
> >>>> * If the previous CPU is cache affine and idle, don't be stupid:
> >>>> */
> >>>> - if (prev != target && cpus_share_cache(prev, target) &&
> >>>> + if (prev != target && cpus_share_resources(prev, target) &&
> >>>> (available_idle_cpu(prev) || sched_idle_cpu(prev)) &&
> >>>> asym_fits_capacity(task_util, prev))
> >>>> return prev;
> >>>> @@ -6507,7 +6545,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> >>>> p->recent_used_cpu = prev;
> >>>> if (recent_used_cpu != prev &&
> >>>> recent_used_cpu != target &&
> >>>> - cpus_share_cache(recent_used_cpu, target) &&
> >>>> + cpus_share_resources(recent_used_cpu, target) &&
> >>>> (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) &&
> >>>> cpumask_test_cpu(p->recent_used_cpu, p->cpus_ptr) &&
> >>>> asym_fits_capacity(task_util, recent_used_cpu)) {
> >>>
> >>>
> >>> .
> >>>
> > .
> >

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Previous message: Shawn Guo: "Re: [PATCH v3 0/4] ARM: dts: imx6dl-colibri: Unify with changes to Apalis iMX6 device trees."
In reply to: Yicong Yang: "Re: [PATCH v4 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path"
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