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sched/fair: Search a task from the tail of the queue

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As a first step this patch makes cfs_tasks list as MRU one.
It means, that when a next task is picked to run on physical
CPU it is moved to the front of the list.

Therefore, the cfs_tasks list is more or less sorted (except
woken tasks) starting from recently given CPU time tasks toward
tasks with max wait time in a run-queue, i.e. MRU list.

Second, as part of the load balance operation, this approach
starts detach_tasks()/detach_one_task() from the tail of the
queue instead of the head, giving some advantages:

 - tends to pick a task with highest wait time;

 - tasks located in the tail are less likely cache-hot,
   therefore the can_migrate_task() decision is higher.

hackbench illustrates slightly better performance. For example
doing 1000 samples and 40 groups on i5-3320M CPU, it shows below
figures:

 default: 0.657 avg
 patched: 0.646 avg

Signed-off-by: Uladzislau Rezki (Sony) <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Kirill Tkhai <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Nicolas Pitre <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Oleksiy Avramchenko <[email protected]>
Cc: Paul Turner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tim Chen <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Uladzislau Rezki 2017-09-13 12:24:30 +02:00 • committed by Ingo Molnar
commit 93824900a2
1 changed files with 16 additions and 8 deletions
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24
kernel/sched/fair.c
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@ -6628,10 +6628,7 @@ again:
set_next_entity(cfs_rq, se);
}
if (hrtick_enabled(rq))
hrtick_start_fair(rq, p);
return p;
goto done;
simple:
#endif
@ -6645,6 +6642,16 @@ simple:
p = task_of(se);
done: __maybe_unused
#ifdef CONFIG_SMP
/*
* Move the next running task to the front of
* the list, so our cfs_tasks list becomes MRU
* one.
*/
list_move(&p->se.group_node, &rq->cfs_tasks);
#endif
if (hrtick_enabled(rq))
hrtick_start_fair(rq, p);
@ -7080,11 +7087,12 @@ static void detach_task(struct task_struct *p, struct lb_env *env)
*/
static struct task_struct *detach_one_task(struct lb_env *env)
{
struct task_struct *p, *n;
struct task_struct *p;
lockdep_assert_held(&env->src_rq->lock);
list_for_each_entry_safe(p, n, &env->src_rq->cfs_tasks, se.group_node) {
list_for_each_entry_reverse(p,
&env->src_rq->cfs_tasks, se.group_node) {
if (!can_migrate_task(p, env))
continue;
@ -7130,7 +7138,7 @@ static int detach_tasks(struct lb_env *env)
if (env->idle != CPU_NOT_IDLE && env->src_rq->nr_running <= 1)
break;
p = list_first_entry(tasks, struct task_struct, se.group_node);
p = list_last_entry(tasks, struct task_struct, se.group_node);
env->loop++;
/* We've more or less seen every task there is, call it quits */
@ -7180,7 +7188,7 @@ static int detach_tasks(struct lb_env *env)
continue;
next:
list_move_tail(&p->se.group_node, tasks);
list_move(&p->se.group_node, tasks);
}
/*