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Date: Fri, 2 Aug 2013 18:05:51 +0900
From: ±èÁؼö <iamjoonsoo.kim@....com>
To: "'Preeti U Murthy'" <preeti@...ux.vnet.ibm.com>
Cc: "'Ingo Molnar'" <mingo@...hat.com>,
"'Peter Zijlstra'" <peterz@...radead.org>,
<linux-kernel@...r.kernel.org>, "'Mike Galbraith'" <efault@....de>,
"'Paul Turner'" <pjt@...gle.com>,
"'Alex Shi'" <alex.shi@...el.com>,
"'Vincent Guittot'" <vincent.guittot@...aro.org>,
"'Morten Rasmussen'" <morten.rasmussen@....com>,
"'Namhyung Kim'" <namhyung@...nel.org>,
"'Joonsoo Kim'" <js1304@...il.com>
Subject: RE: [PATCH v2 2/3] sched: factor out code to should_we_balance()
> -----Original Message-----
> From: Preeti U Murthy [mailto:preeti@...ux.vnet.ibm.com]
> Sent: Friday, August 02, 2013 1:23 PM
> To: Joonsoo Kim
> Cc: Ingo Molnar; Peter Zijlstra; linux-kernel@...r.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@....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
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