Re: [PATCH 4/4] sched: bias to target cpu load to reduce task moving

[Alex Shi]

>> From 5cd67d975001edafe2ee820e0be5d86881a23bd6 Mon Sep 17 00:00:00 2001
>> From: Alex Shi <alex.shi@xxxxxxxxxx>
>> Date: Sat, 23 Nov 2013 23:18:09 +0800
>> Subject: [PATCH 4/4] sched: bias to target cpu load to reduce task moving
>>
>> Task migration happens when target just a bit less then source cpu load.
>> To reduce such situation happens, aggravate the target cpu load with
>> sd->imbalance_pct/100 in wake_affine.
>>
>> In find_idlest/busiest_group, change the aggravate to local cpu only
>> from old group aggravation.
>>
>> on my pandaboard ES.
>>
>> 	latest kernel 527d1511310a89		+ whole patchset
>> hackbench -T -g 10 -f 40
>> 	23.25"					21.99"
>> 	23.16"					21.20"
>> 	24.24"					21.89"
>> hackbench -p -g 10 -f 40
>> 	26.52"					21.46"
>> 	23.89"					22.96"
>> 	25.65"					22.73"
>> hackbench -P -g 10 -f 40
>> 	20.14"					19.72"
>> 	19.96"					19.10"
>> 	21.76"					20.03"
>>
>> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxx>
>> ---
>>  kernel/sched/fair.c | 35 ++++++++++++++++-------------------
>>  1 file changed, 16 insertions(+), 19 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index bccdd89..3623ba4 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -978,7 +978,7 @@ static inline unsigned long group_weight(struct task_struct *p, int nid)
>>  
>>  static unsigned long weighted_cpuload(const int cpu);
>>  static unsigned long source_load(int cpu);
>> -static unsigned long target_load(int cpu);
>> +static unsigned long target_load(int cpu, int imbalance_pct);
>>  static unsigned long power_of(int cpu);
>>  static long effective_load(struct task_group *tg, int cpu, long wl, long wg);
>>  
>> @@ -3809,11 +3809,17 @@ static unsigned long source_load(int cpu)
>>   * Return a high guess at the load of a migration-target cpu weighted
>>   * according to the scheduling class and "nice" value.
>>   */
>> -static unsigned long target_load(int cpu)
>> +static unsigned long target_load(int cpu, int imbalance_pct)
>>  {
>>  	struct rq *rq = cpu_rq(cpu);
>>  	unsigned long total = weighted_cpuload(cpu);
>>  
>> +	/*
>> +	 * without cpu_load decay, in most of time cpu_load is same as total
>> +	 * so we need to make target a bit heavier to reduce task migration
>> +	 */
>> +	total = total * imbalance_pct / 100;
>> +
>>  	if (!sched_feat(LB_BIAS))
>>  		return total;
>>  
>> @@ -4033,7 +4039,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
>>  	this_cpu  = smp_processor_id();
>>  	prev_cpu  = task_cpu(p);
>>  	load	  = source_load(prev_cpu);
>> -	this_load = target_load(this_cpu);
>> +	this_load = target_load(this_cpu, 100);
>>  
>>  	/*
>>  	 * If sync wakeup then subtract the (maximum possible)
>> @@ -4089,7 +4095,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
>>  
>>  	if (balanced ||
>>  	    (this_load <= load &&
>> -	     this_load + target_load(prev_cpu) <= tl_per_task)) {
>> +	     this_load + target_load(prev_cpu, 100) <= tl_per_task)) {
>>  		/*
>>  		 * This domain has SD_WAKE_AFFINE and
>>  		 * p is cache cold in this domain, and
>> @@ -4112,7 +4118,6 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>>  {
>>  	struct sched_group *idlest = NULL, *group = sd->groups;
>>  	unsigned long min_load = ULONG_MAX, this_load = 0;
>> -	int imbalance = 100 + (sd->imbalance_pct-100)/2;
>>  
>>  	do {
>>  		unsigned long load, avg_load;
>> @@ -4132,10 +4137,10 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>>  
>>  		for_each_cpu(i, sched_group_cpus(group)) {
>>  			/* Bias balancing toward cpus of our domain */
>> -			if (local_group)
>> +			if (i == this_cpu)
> 
> What is the motivation for changing the local_group load calculation?
> Now the load contributions of all cpus in the local group, except
> this_cpu, will contribute more as their contribution (this_load) is
> determined using target_load() instead.

This part code 147cbb4bbe99, written in 2005 for x86, at that time, only
2 cores(guess no HT at that time) in cpu socket. With the cores number
increasing trend, the sched_group become large and large, to give whole
group this bias value is becoming non-sense. So it looks reasonable to
just bias this cpu only.
> 
> If I'm not mistaken, that will lead to more frequent load balancing as
> the local_group bias has been reduced. That is the opposite of your
> intentions based on your comment in target_load().
> 
>>  				load = source_load(i);
>>  			else
>> -				load = target_load(i);
>> +				load = target_load(i, sd->imbalance_pct);
> 
> You scale by sd->imbalance_pct instead of 100+(sd->imbalance_pct-100)/2
> that you removed above. sd->imbalance_pct may have been arbitrarily
> chosen in the past, but changing it may affect behavior. 

the first commit with this line:
int imbalance = 100 + (sd->imbalance_pct-100)/2;
is 147cbb4bbe99, that also has no explanation for this imbalance value.
AFAIK, at that time, maybe only 2 LCPU in one socket, so, guess it is
reason to use half of imbalance_pct.
but sched_domain is used widely for many kinds of platform/cpu type, the
value is arbitrary too.
Another reason to use it, I scaled any cpu load which is not this_cpu,
so a bit conservation is to use origin imbalance_pct, not half of it.
> 
>>  
>>  			avg_load += load;
>>  		}
>> @@ -4151,7 +4156,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
>>  		}
>>  	} while (group = group->next, group != sd->groups);
>>  
>> -	if (!idlest || 100*this_load < imbalance*min_load)
>> +	if (!idlest || this_load < min_load)
>>  		return NULL;
>>  	return idlest;
>>  }
>> @@ -5476,9 +5481,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
>>  
>>  		nr_running = rq->nr_running;
>>  
>> -		/* Bias balancing toward cpus of our domain */
>> -		if (local_group)
>> -			load = target_load(i);
>> +		/* Bias balancing toward dst cpu */
>> +		if (env->dst_cpu == i)
>> +			load = target_load(i, env->sd->imbalance_pct);
> 
> Here you do the same group load bias change as above.
> 
>>  		else
>>  			load = source_load(i);
>>  
>> @@ -5918,14 +5923,6 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>>  		if ((local->idle_cpus < busiest->idle_cpus) &&
>>  		    busiest->sum_nr_running <= busiest->group_weight)
>>  			goto out_balanced;
>> -	} else {
>> -		/*
>> -		 * In the CPU_NEWLY_IDLE, CPU_NOT_IDLE cases, use
>> -		 * imbalance_pct to be conservative.
>> -		 */
>> -		if (100 * busiest->avg_load <=
>> -				env->sd->imbalance_pct * local->avg_load)
>> -			goto out_balanced;
>>  	}
>>  
>>  force_balance:
> 
> As said my previous replies to this series, I think this problem should
> be solved by fixing the cause of the problem, that is the cpu_load
> calculation, instead of biasing the cpu_load where-ever it is used to
> hide the problem.
> 
> Doing a bit of git archaeology reveals that the cpu_load code goes back
> to 7897986bad8f6cd50d6149345aca7f6480f49464 and that in the original
> patch x86 was using *_idx > 0 for all sched_domain levels except SMT. In
> my opinion that made logical sense. If we are about to change to *_idx=0
> we are removing the main idea behind that code and there needs to be a
> new one. Otherwise, cpu_load doesn't make sense.

The commit is written in 2005. The CFS scheduler merged into kernel in
Oct 2007. it is a too old legacy for us ...
> 
> Morten
> 


-- 
Thanks
    Alex
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