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

[김준수 <iamjoonsoo.kim@lge.com>]

> -----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.

Thanks.

> check to which group the env->dst_cpu belongs to. Only for that group
> should you do the above check and set of balance_cpu and the below check
> of balance_cpu != env->dst_cpu.
> 
> > +
> > +	if (balance_cpu == -1)
> > +		balance_cpu = group_balance_cpu(sg);
> > +
> > +	/*
> > +	 * First idle cpu or the first cpu(busiest) in this sched group
> > +	 * is eligible for doing load balancing at this and above domains.
> > +	 */
> > +	return balance_cpu != env->dst_cpu;
> 
> Regards
> Preeti U Murthy