Re: [PATCH v2 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path
From: Barry Song
Date: Fri Jan 28 2022 - 02:40:43 EST
On Fri, Jan 28, 2022 at 8:13 PM Srikar Dronamraju
<srikar@xxxxxxxxxxxxxxxxxx> wrote:
>
> * Barry Song <21cnbao@xxxxxxxxx> [2022-01-28 09:21:08]:
>
> > On Fri, Jan 28, 2022 at 4:41 AM Gautham R. Shenoy
> > <gautham.shenoy@xxxxxxx> wrote:
> > >
> > > On Wed, Jan 26, 2022 at 04:09:47PM +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:
> > > > 5.17-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@xxxxxxxxxxxxx>
> > > > Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> > > > Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
> > > > ---
> > > > 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,
> >
>
> Hi Barry,
>
>
> > 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.
> >
>
> Like what Gautham said, should we choose the same cluster if we find that
> there are no idle-cpus in the LLC? This way we avoid ping-pong if there are
> no idle-cpus but we still pick an idle-cpu to a busy cpu?
Hi Srikar,
I am sorry I didn't get your question. Currently the code works as below:
if task A wakes up task B, and task A is in LLC0 and task B is in LLC1.
we will scan the cluster of A before scanning the whole LLC0, in this case,
cluster of A is the closest sibling, so it is the better choice than other CPUs
which are in LLC0 but not in the cluster of A. But we do scan all cpus of LLC0
afterwards if we fail to find an idle CPU in the cluster.
After a while, if the cluster of A gets an idle CPU and pulls B into the
cluster, we prefer not pushing B out of the cluster of A again though
there might be an idle CPU outside. as benchmark shows getting an
idle CPU out of the cluster of A doesn't bring performance improvement
but performance decreases as B might be getting in and getting out
the cluster of A very frequently, then cache coherence ping-pong.
Note we are only returning target while if
(cpus_share_resources(target, prev_cpu))
is true. So we are not losing chance to pull B to the LLC of A while LLC0 has
an idle one.
>
> > 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
>
> --
> Thanks and Regards
> Srikar Dronamraju
Thanks
Barry