Re: [PATCH v2 2/3] sched: factor out code to should_we_balance()

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 08/02/2013 02:35 PM, 김준수 wrote:
> 
> 
>> -----Original Message-----
>> From: Preeti U Murthy [mailto:preeti@linux.vnet.ibm.com]
>> Sent: Friday, August 02, 2013 1:23 PM
>> To: Joonsoo Kim
>> Cc: Ingo Molnar; Peter Zijlstra; linux-kernel@vger.kernel.org; Mike
>> Galbraith; Paul Turner; Alex Shi; Vincent Guittot; Morten Rasmussen;
>> Namhyung Kim; Joonsoo Kim
>> Subject: Re: [PATCH v2 2/3] sched: factor out code to should_we_balance()
>>
>> Hi Joonsoo,
> 
> Hello, Preeti.
> 
>>
>> On 08/02/2013 07:20 AM, Joonsoo Kim wrote:
>>> Now checking whether this cpu is appropriate to balance or not is
>>> embedded into update_sg_lb_stats() and this checking has no direct
>>> relationship to this function. There is not enough reason to place
>>> this checking at update_sg_lb_stats(), except saving one iteration for
>>> sched_group_cpus.
>>>
>>> In this patch, I factor out this checking to should_we_balance()
>> function.
>>> And before doing actual work for load_balancing, check whether this
>>> cpu is appropriate to balance via should_we_balance(). If this cpu is
>>> not a candidate for balancing, it quit the work immediately.
>>>
>>> With this change, we can save two memset cost and can expect better
>>> compiler optimization.
>>>
>>> Below is result of this patch.
>>>
>>> * Vanilla *
>>>    text	   data	    bss	    dec	    hex	filename
>>>   34499	   1136	    116	  35751	   8ba7	kernel/sched/fair.o
>>>
>>> * Patched *
>>>    text	   data	    bss	    dec	    hex	filename
>>>   34243	   1136	    116	  35495	   8aa7	kernel/sched/fair.o
>>>
>>> In addition, rename @balance to @should_balance in order to represent
>>> its purpose more clearly.
>>>
>>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index
>>> eaae77e..7f51b8c 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -4426,22 +4426,17 @@ fix_small_capacity(struct sched_domain *sd,
>> struct sched_group *group)
>>>   * @group: sched_group whose statistics are to be updated.
>>>   * @load_idx: Load index of sched_domain of this_cpu for load calc.
>>>   * @local_group: Does group contain this_cpu.
>>> - * @balance: Should we balance.
>>>   * @sgs: variable to hold the statistics for this group.
>>>   */
>>>  static inline void update_sg_lb_stats(struct lb_env *env,
>>>  			struct sched_group *group, int load_idx,
>>> -			int local_group, int *balance, struct sg_lb_stats
> *sgs)
>>> +			int local_group, struct sg_lb_stats *sgs)
>>>  {
>>>  	unsigned long nr_running, max_nr_running, min_nr_running;
>>>  	unsigned long load, max_cpu_load, min_cpu_load;
>>> -	unsigned int balance_cpu = -1, first_idle_cpu = 0;
>>>  	unsigned long avg_load_per_task = 0;
>>>  	int i;
>>>
>>> -	if (local_group)
>>> -		balance_cpu = group_balance_cpu(group);
>>> -
>>>  	/* Tally up the load of all CPUs in the group */
>>>  	max_cpu_load = 0;
>>>  	min_cpu_load = ~0UL;
>>> @@ -4454,15 +4449,9 @@ 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) {
>>> -			if (idle_cpu(i) && !first_idle_cpu &&
>>> -					cpumask_test_cpu(i,
>> sched_group_mask(group))) {
>>> -				first_idle_cpu = 1;
>>> -				balance_cpu = i;
>>> -			}
>>> -
>>> +		if (local_group)
>>>  			load = target_load(i, load_idx);
>>> -		} else {
>>> +		else {
>>>  			load = source_load(i, load_idx);
>>>  			if (load > max_cpu_load)
>>>  				max_cpu_load = load;
>>> @@ -4482,22 +4471,9 @@ static inline void update_sg_lb_stats(struct
>> lb_env *env,
>>>  			sgs->idle_cpus++;
>>>  	}
>>>
>>> -	/*
>>> -	 * First idle cpu or the first cpu(busiest) in this sched group
>>> -	 * is eligible for doing load balancing at this and above
>>> -	 * domains. In the newly idle case, we will allow all the cpu's
>>> -	 * to do the newly idle load balance.
>>> -	 */
>>> -	if (local_group) {
>>> -		if (env->idle != CPU_NEWLY_IDLE) {
>>> -			if (balance_cpu != env->dst_cpu) {
>>> -				*balance = 0;
>>> -				return;
>>> -			}
>>> -			update_group_power(env->sd, env->dst_cpu);
>>> -		} else if (time_after_eq(jiffies, group->sgp->next_update))
>>> -			update_group_power(env->sd, env->dst_cpu);
>>> -	}
>>
>> Observe what is happening in the above code which checks if we should
>> balance on the env->dst_cpu.
>>
>> Only if the env->dst_cpu "belongs" to the group considered in
>> update_sg_lb_stats(), which means local_group = 1,should the above checks
>> be carried out.
>>
>> Meaning, if there is a better CPU in the same group to which
>> env->dst_cpu belongs, to carry out load balancing for the system (in the
>> above case, balance_cpu), cancel the current iteration of load balancing
>> on env->dst_cpu. Wait for the right cpu in this group to do the load
>> balancing.
>>
>> Keeping this in mind see the below comments around should_we_balance().
> 
> Okay.
> 
>>
>>> @@ -5001,13 +4964,47 @@ static int need_active_balance(struct lb_env
>>> *env)
>>>
>>>  static int active_load_balance_cpu_stop(void *data);
>>>
>>> +static int should_we_balance(struct lb_env *env) {
>>> +	struct sched_group *sg = env->sd->groups;
>>> +	struct cpumask *sg_cpus, *sg_mask;
>>> +	int cpu, balance_cpu = -1;
>>> +
>>> +	/*
>>> +	 * In the newly idle case, we will allow all the cpu's
>>> +	 * to do the newly idle load balance.
>>> +	 */
>>> +	if (env->idle == CPU_NEWLY_IDLE)
>>> +		return 1;
>>> +
>>> +	sg_cpus = sched_group_cpus(sg);
>>> +	sg_mask = sched_group_mask(sg);
>>> +	/* Try to find first idle cpu */
>>> +	for_each_cpu_and(cpu, sg_cpus, env->cpus) {
>>> +		if (!cpumask_test_cpu(cpu, sg_mask) || idle_cpu(cpu))
>>> +			continue;
>>> +
>>> +		balance_cpu = cpu;
>>> +		break;
>>> +	}
>>
>> You need to iterate over all the groups of the sched domain env->sd and
>> not just the first group of env->sd like you are doing above. This is to
> 
> I don't think so.
> IIRC, env->sd->groups always means local group,
> so we don't need to find our group by iterating over all the groups.

Take a look at update_sd_lb_stats(). That should clarify this. There is
an exclusive
local_group check there.

sd->groups points to the first group in the list of groups under this sd.

Regards
Preeti U Murthy