[PATCH v4 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path
Chen Yu
yu.c.chen at intel.com
Fri Jun 10 20:03:22 PDT 2022
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 at 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.
> >>
> >> 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 at linux.intel.com>
> >
>
> Thanks.
>
> [1] https://lore.kernel.org/lkml/20220428182442.659294-1-yu.c.chen@intel.com/
>
> >>
> >> Tested-by: Yicong Yang <yangyicong at hisilicon.com>
> >> Signed-off-by: Barry Song <song.bao.hua at hisilicon.com>
> >> Signed-off-by: Yicong Yang <yangyicong at hisilicon.com>
> >> ---
> >> 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]
> >> +
> >> /*
> >> * 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.
> >> * 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.
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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