Re: [tip:sched/core] sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target

From: Steven Rostedt
Date: Tue Feb 24 2015 - 11:12:58 EST


On Sun, 1 Feb 2015 09:52:24 -0800
tip-bot for Tim Chen <tipbot@xxxxxxxxx> wrote:

> Commit-ID: 80e3d87b2c5582db0ab5e39610ce3707d97ba409
> Gitweb: http://git.kernel.org/tip/80e3d87b2c5582db0ab5e39610ce3707d97ba409
> Author: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> AuthorDate: Fri, 12 Dec 2014 15:38:12 -0800
> Committer: Ingo Molnar <mingo@xxxxxxxxxx>
> CommitDate: Fri, 30 Jan 2015 19:38:49 +0100
>
> sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target
>
> This patch adds checks that prevens futile attempts to move rt tasks
> to a 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.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: Shawn Bohrer <sbohrer@xxxxxxxxxxxxxxx>
> Cc: Suruchi Kadu <suruchi.a.kadu@xxxxxxxxx>
> Cc: Doug Nelson<doug.nelson@xxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/1421430374.2399.27.camel@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>

I see this has been added to mainline already. I think this should go
to stable as well. As far back as it applies. I'll even add this to the
stable-rt if need be. But I rather pull it into stable-rt via the
stable branches.

Thanks,

-- Steve

> ---
> kernel/sched/rt.c | 17 ++++++++++++++++-
> 1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 6725e3c..f4d4b07 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1340,7 +1340,12 @@ 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);
>
> - if (target != -1)
> + /*
> + * Don't bother moving it if the destination CPU is
> + * not running a lower priority task.
> + */
> + if (target != -1 &&
> + p->prio < cpu_rq(target)->rt.highest_prio.curr)
> cpu = target;
> }
> rcu_read_unlock();
> @@ -1617,6 +1622,16 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
>
> lowest_rq = cpu_rq(cpu);
>
> + if (lowest_rq->rt.highest_prio.curr <= task->prio) {
> + /*
> + * Target rq has tasks of equal or higher priority,
> + * retrying does not release any lock and is unlikely
> + * to yield a different result.
> + */
> + lowest_rq = NULL;
> + break;
> + }
> +
> /* if the prio of this runqueue changed, try again */
> if (double_lock_balance(rq, lowest_rq)) {
> /*

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