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

[Yicong Yang]

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@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 */
> 
> 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.

Thanks.

> Tim
> 
> 
>> +	if ((unsigned int)idle_cpu < nr_cpumask_bits)
>> +		return idle_cpu;
>> +
>>  	if (sched_feat(SIS_PROP) && !has_idle_core) {
>>  		u64 avg_cost, avg_idle, span_avg;
>>  		unsigned long now = jiffies;
>> @@ -6416,7 +6454,7 @@ 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;
>> @@ -6442,7 +6480,7 @@ 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)) {
>> @@ -6483,7 +6521,7 @@ static int select_idle_sibling(struct
>> task_struct *p, int prev, int target)
>>  		}
>>  	}
>>  
>> -	i = select_idle_cpu(p, sd, has_idle_core, target);
>> +	i = select_idle_cpu(p, sd, has_idle_core, prev, target);
>>  	if ((unsigned)i < nr_cpumask_bits)
>>  		return i;
>>  
> 
> .
>