On Fri, 2015-01-16 at 09:46 -0800, Tim Chen wrote:
> On Thu, 2015-01-15 at 20:58 -0500, Steven Rostedt wrote:
>
> >
> > 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.
> > */
> >
> Okay. Updated in patch below.
>
> > > - 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.
>
> Agree. Updated in updated patch below.
>
> Thanks.
>
> Tim
>
Steven and Peter, are you okay with the updated patch?
Thanks.
Tim
> ---->8------
>
> From 5f676f7a351e85eb5cc64f1971dd03eca43b5271 Mon Sep 17 00:00:00 2001
> From: Tim Chen <tim.c.c...@linux.intel.com>
> Date: Fri, 12 Dec 2014 15:38:12 -0800
> Subject: [PATCH] sched-rt: Reduce rq lock contention by eliminating
> locking of
> non-feasible target
> To: Peter Zijlstra <pet...@infradead.org>
> Cc: Andi Kleen <a...@firstfloor.org>, Ingo Molnar <mi...@elte.hu>,
> Shawn Bohrer <sboh...@rgmadvisors.com>, Steven Rostedt
> <rost...@goodmis.org>, Suruchi Kadu <suruchi.a.k...@intel.com>, Doug
> Nelson <doug.nel...@intel.com>, linux-kernel@vger.kernel.org
>
> 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 | 17 ++++++++++++++++-
> 1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ee15f5a..46ebcb1 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1337,7 +1337,12 @@ 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();
> @@ -1614,6 +1619,16 @@ 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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