Re: [PATCH v4 2/2] sched/fair: Scan cluster before scanning LLC in wake-up path

[Yicong Yang]

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@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:
>>                             5.19-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%)


> Code itself looks good.
> 
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> 

Thanks.

[1] https://lore.kernel.org/lkml/20220428182442.659294-1-yu.c.chen@xxxxxxxxx/

>>
>> 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 | 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;
>> +
>> +			idle_cpu = __select_idle_cpu(cpu, p);
>> +			if ((unsigned int)idle_cpu < nr_cpumask_bits)
>> +				return idle_cpu;
>> +		}
>> +
>> +		cpumask_andnot(cpus, cpus, sched_domain_span(sd));
>> +	}
>> +
>> +	return -1;
>> +}
>> +#else
>> +static inline int scan_cluster(struct task_struct *p, struct cpumask *cpus,
>> +			       int target, int *nr)
>> +{
>> +	return -1;
>> +}
>> +#endif
>> +
>>  /*
>>   * Scan the LLC domain for idle CPUs; this is dynamically regulated by
>>   * 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)
>>  				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)) {
> 
> 
> .
>