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

[Tim Chen <tim.c.chen@linux.intel.com>]

On Thu, 2022-01-27 at 10:02 +0800, Yicong Yang wrote:
> On 2022/1/27 9:14, Tim Chen wrote:
> > On Wed, 2022-01-26 at 16:09 +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.
> > > 
> > > 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@hisilicon.com>
> > > Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> > > Signed-off-by: Yicong Yang <yangyicong@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 */
> > 
> > This is probably a clearer comment
> > 
> > 	/* TODO: Support SMT system with cluster topology */
> > 
> > > +	if (!sched_smt_active() && sd) {
> > > +		for_each_cpu_and(cpu, cpus, sched_domain_span(sd)) {
> > > +			idle_cpu = __select_idle_cpu(cpu, p);
> > >   */
> > > -static int select_idle_cpu(struct task_struct *p, struct
> > > sched_domain *sd, bool has_idle_core, int target)
> > > +static int select_idle_cpu(struct task_struct *p, struct
> > > sched_domain *sd, bool has_idle_core, int prev_cpu, int target)
> > >  {
> > >  	struct cpumask *cpus =
> > > this_cpu_cpumask_var_ptr(select_idle_mask);
> > >  	int i, cpu, idle_cpu = -1, nr = INT_MAX;
> > > @@ -6282,6 +6316,10 @@ static int select_idle_cpu(struct
> > > task_struct
> > > *p, struct sched_domain *sd, bool
> > >  
> > >  	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> > >  
> > > +	idle_cpu = scan_cluster(p, prev_cpu, target);
> > 
> > Shouldn't "cpus" from 
> > 
> > cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> > 
> > be passed to scan_cluster, to make sure that the cpu returned is 
> > in the affinity mask of the task? I don't see p->cpus_ptr
> > being checked in scan_cluster to make sure the cpu found is in the
> > affinity mask.
> > 
> 
> The cpus scanned in scan_cluster() is the intersection of
> select_idle_mask and sched_domain_span(cluster_sd), and
> we limited the select_idle_mask in the tasks' affinity mask
> before we enter scan_cluster() here.

Ah, I missed the fact that cpus point to the select_idle_mask.

Thanks.

Tim