On Tue, 06 Jan 2015 11:01:51 -0800
Tim Chen <tim.c.c...@linux.intel.com> wrote:
> Didn't get any response for this patch probably due to the holidays.
> Reposting it as we will like to get it merged to help our database
> workload.
>
> This patch added checks that prevent futile attempts to move rt tasks
> to cpu with active tasks of equal or higher priority. This reduces
> run queue lock contention and improves the performance of a well
> known OLTP benchmark by 0.7%.
>
> Signed-off-by: Tim Chen <tim.c.c...@linux.intel.com>
> ---
> kernel/sched/rt.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ee15f5a..0e4382e 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1337,7 +1337,8 @@ select_task_rq_rt(struct task_struct *p, int
> cpu, int sd_flag, int flags) curr->prio <= p->prio)) {
> int target = find_lowest_rq(p);
>
Please add a comment here that says something like:
/*
* Don't bother moving it if the destination CPU is
* not running a lower priority task.
*/
> - if (target != -1)
> + if (target != -1 &&
> + p->prio < cpu_rq(target)->rt.highest_prio.curr)
> cpu = target;
> }
> rcu_read_unlock();
> @@ -1613,6 +1614,12 @@ static struct rq *find_lock_lowest_rq(struct
> task_struct *task, struct rq *rq) break;
>
> lowest_rq = cpu_rq(cpu);
> +
> + if (lowest_rq->rt.highest_prio.curr <= task->prio) {
> + /* target rq has tasks of equal or higher priority,
> try again */
> + lowest_rq = NULL;
> + continue;
This should just break out and not try again. The reason for the other
try again is because of the double_lock which can release the locks
which can cause a process waiting for the lock to sneak in and
change the priorities. But this case, a try again is highly unlikely to
do anything differently (no locks are released) and just waste cycles.
-- Steve
> + }
>
> /* if the prio of this runqueue changed, try again */
> if (double_lock_balance(rq, lowest_rq)) {
--
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