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SubjectRe: [RFC v1] sched/fair: search a task from the tail of the queue
On Fri, Aug 25, 2017 at 12:11:31AM +0200, Uladzislau Rezki (Sony) wrote:
> From: Uladzislau Rezki <urezki@gmail.com>
>
> 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.
>
> Thefore, 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: 0.644 avg
> patched: 0.637 avg
>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> ---
> kernel/sched/fair.c | 19 ++++++++++++++-----
> 1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c77e4b1d51c0..cda281c6bb29 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6357,7 +6357,7 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> if (hrtick_enabled(rq))
> hrtick_start_fair(rq, p);
>
> - return p;
> + goto done;
> simple:
> cfs_rq = &rq->cfs;
> #endif
> @@ -6378,6 +6378,14 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> if (hrtick_enabled(rq))
> hrtick_start_fair(rq, p);
>
> +done: __maybe_unused
> + /*
> + * Move the next running task to the front of
> + * the list, so our cfs_tasks list becomes MRU
> + * one.
> + */
> + list_move(&se->group_node, &rq->cfs_tasks);
> +
> return p;
>
> idle:

Could you also run something like:

$ taskset 1 perf bench sched pipe

to make sure the added list_move() doesn't hurt, I'm not sure group_node
and cfs_tasks are in cachelines we already touch for that operation.

And if you can see that list_move() hurt in "perf annotate", try moving
those members around to lines that we already need anyway.

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