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Date: Sat, 24 Aug 2013 03:15:57 -0700
From: Paul Turner <pjt@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
LKML <linux-kernel@...r.kernel.org>,
Mike Galbraith <efault@....de>, Alex Shi <alex.shi@...el.com>,
Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Morten Rasmussen <morten.rasmussen@....com>,
Namhyung Kim <namhyung@...nel.org>,
Lei Wen <leiwen@...vell.com>, Rik van Riel <riel@...riel.com>,
Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH 04/10] sched, fair: Shrink sg_lb_stats and play memset games
On Mon, Aug 19, 2013 at 9:01 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> We can shrink sg_lb_stats because rq::nr_running is an 'unsigned int'
> and cpu numbers are 'int'
>
> Before:
> sgs: /* size: 72, cachelines: 2, members: 10 */
> sds: /* size: 184, cachelines: 3, members: 7 */
>
> After:
> sgs: /* size: 56, cachelines: 1, members: 10 */
> sds: /* size: 152, cachelines: 3, members: 7 */
>
> Further we can avoid clearing all of sds since we do a total
> clear/assignment of sg_stats in update_sg_lb_stats() with exception of
> busiest_stat.avg_load which is referenced in update_sd_pick_busiest().
>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
> kernel/sched/fair.c | 42 +++++++++++++++++++++++++++++++++++-------
> 1 file changed, 35 insertions(+), 7 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4282,12 +4282,12 @@ static unsigned long task_h_load(struct
> struct sg_lb_stats {
> unsigned long avg_load; /*Avg load across the CPUs of the group */
> unsigned long group_load; /* Total load over the CPUs of the group */
> - unsigned long sum_nr_running; /* Nr tasks running in the group */
> unsigned long sum_weighted_load; /* Weighted load of group's tasks */
> unsigned long load_per_task;
> - unsigned long group_capacity;
> - unsigned long idle_cpus;
> - unsigned long group_weight;
> + unsigned int sum_nr_running; /* Nr tasks running in the group */
> + unsigned int group_capacity;
> + unsigned int idle_cpus;
> + unsigned int group_weight;
> int group_imb; /* Is there an imbalance in the group ? */
> int group_has_capacity; /* Is there extra capacity in the group? */
> };
> @@ -4303,10 +4303,38 @@ struct sd_lb_stats {
> unsigned long total_pwr; /* Total power of all groups in sd */
> unsigned long avg_load; /* Average load across all groups in sd */
>
> - struct sg_lb_stats this_stat; /* Statistics of this group */
> struct sg_lb_stats busiest_stat;/* Statistics of the busiest group */
> + struct sg_lb_stats this_stat; /* Statistics of this group */
> };
>
> +static inline void init_sd_lb_stats(struct sd_lb_stats *sds)
> +{
> + /*
> + * struct sd_lb_stats {
> + * struct sched_group * busiest; // 0 8
> + * struct sched_group * this; // 8 8
> + * long unsigned int total_load; // 16 8
> + * long unsigned int total_pwr; // 24 8
> + * long unsigned int avg_load; // 32 8
> + * struct sg_lb_stats {
> + * long unsigned int avg_load; // 40 8
> + * long unsigned int group_load; // 48 8
> + * ...
> + * } busiest_stat; // 40 56
> + * struct sg_lb_stats this_stat; // 96 56
> + *
> + * // size: 152, cachelines: 3, members: 7
> + * // last cacheline: 24 bytes
> + * };
> + *
> + * Skimp on the clearing to avoid duplicate work. We can avoid clearing
> + * this_stat because update_sg_lb_stats() does a full clear/assignment.
> + * We must however clear busiest_stat::avg_load because
> + * update_sd_pick_busiest() reads this before assignment.
> + */
Does gcc seriously not get this right if we just write:
sds->busiest = NULL;
sds->local = NULL;
....
etc.?
> + memset(sds, 0, offsetof(struct sd_lb_stats, busiest_stat.group_load));
> +}
> +
> /**
> * get_sd_load_idx - Obtain the load index for a given sched domain.
> * @sd: The sched_domain whose load_idx is to be obtained.
> @@ -4665,7 +4693,7 @@ static inline void update_sd_lb_stats(st
> */
> if (prefer_sibling && !local_group &&
> sds->this && sds->this_stat.group_has_capacity)
> - sgs->group_capacity = min(sgs->group_capacity, 1UL);
> + sgs->group_capacity = min(sgs->group_capacity, 1U);
>
> /* Now, start updating sd_lb_stats */
> sds->total_load += sgs->group_load;
> @@ -4896,7 +4924,7 @@ static struct sched_group *find_busiest_
> struct sg_lb_stats *this, *busiest;
> struct sd_lb_stats sds;
>
> - memset(&sds, 0, sizeof(sds));
> + init_sd_lb_stats(&sds);
>
> /*
> * Compute the various statistics relavent for load balancing at
>
>
--
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