[PATCH v2 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

[Barry Song]

On Fri, Jan 28, 2022 at 4:41 AM Gautham R. Shenoy
<gautham.shenoy at amd.com> wrote:
>
> On Wed, Jan 26, 2022 at 04:09:47PM +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:
> >                             5.17-rc1                patched
> > Hmean     1        324.73 (   0.00%)      378.01 *  16.41%*
> > Hmean     2        645.36 (   0.00%)      754.63 *  16.93%*
> > Hmean     4       1302.09 (   0.00%)     1507.54 *  15.78%*
> > Hmean     8       2612.03 (   0.00%)     2982.57 *  14.19%*
> > Hmean     16      5307.12 (   0.00%)     5886.66 *  10.92%*
> > Hmean     32      9354.22 (   0.00%)     9908.13 *   5.92%*
> > Hmean     64      7240.35 (   0.00%)     7278.78 *   0.53%*
> > Hmean     128     6186.40 (   0.00%)     6187.85 (   0.02%)
> >
> > On numa 0-1:
> >                             5.17-rc1                patched
> > Hmean     1        320.01 (   0.00%)      378.44 *  18.26%*
> > Hmean     2        643.85 (   0.00%)      752.52 *  16.88%*
> > Hmean     4       1287.36 (   0.00%)     1505.62 *  16.95%*
> > Hmean     8       2564.60 (   0.00%)     2955.29 *  15.23%*
> > Hmean     16      5195.69 (   0.00%)     5814.74 *  11.91%*
> > Hmean     32      9769.16 (   0.00%)    10872.63 *  11.30%*
> > Hmean     64     15952.50 (   0.00%)    17281.98 *   8.33%*
> > Hmean     128    13113.77 (   0.00%)    13895.20 *   5.96%*
> > Hmean     256    10997.59 (   0.00%)    11244.69 *   2.25%*
> > Hmean     512    14623.60 (   0.00%)    15526.25 *   6.17%*
> >
> > This will also help to improve the MySQL. With MySQL server
> > running on numa 0 and client running on numa 1, both QPS and
> > latency is imporved on read-write case:
> >                         5.17-rc1        patched
> > QPS-16threads        143333.2633    145077.4033(+1.22%)
> > QPS-24threads        195085.9367    202719.6133(+3.91%)
> > QPS-32threads        241165.6867      249020.74(+3.26%)
> > QPS-64threads        244586.8433    253387.7567(+3.60%)
> > avg-lat-16threads           2.23           2.19(+1.19%)
> > avg-lat-24threads           2.46           2.36(+3.79%)
> > avg-lat-36threads           2.66           2.57(+3.26%)
> > avg-lat-64threads           5.23           5.05(+3.44%)
> >
> > 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 | 46 +++++++++++++++++++++++++++++++++++++++++----
> >  1 file changed, 42 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 5146163bfabb..2f84a933aedd 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6262,12 +6262,46 @@ 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, int prev_cpu, int target)
> > +{
> > +     struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
> > +     struct sched_domain *sd = rcu_dereference(per_cpu(sd_cluster, target));
> > +     int cpu, idle_cpu;
> > +
> > +     /* TODO: Support SMT case while a machine with both cluster and SMT born */
> > +     if (!sched_smt_active() && sd) {
> > +             for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> > +                     idle_cpu = __select_idle_cpu(cpu, p);
> > +                     if ((unsigned int)idle_cpu < nr_cpumask_bits)
> > +                             return idle_cpu;
> > +             }
> > +
> > +             /* Don't ping-pong tasks in and out cluster frequently */
> > +             if (cpus_share_resources(target, prev_cpu))
> > +                     return target;
>
> We reach here when there aren't any idle CPUs within the
> cluster. However there might be idle CPUs in the MC domain. Is a busy
> @target preferable to a potentially idle CPU within the larger domain
> ?

Hi Gautham,

My benchmark showed some performance regression while load was medium or above
if we grabbed idle cpu in and out the cluster. it turned out the
regression disappeared if
we blocked the ping-pong. so the logic here is that if we have scanned
and found an
idle cpu within the cluster before, we don't let the task jumping back
and forth frequently
as cache synchronization is higher cost. but the code still allows
scanning out of the cluster
if we haven't packed waker and wakee together yet.

it might not be a universal win in all kinds of workload. we saw
tbench, mysql benefit from
the whole change. but pgbench seems not always. so we are still on the
way to make possible
further tuning here.

>
>
> --
> Thanks and Regards
> gautham.

Thanks
Barry