[PATCH V3 RESEND RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task

Raghavendra K T
2013-01-22T07:39:13Z

From: Peter Zijlstra <peterz@infradead.org>

In case of undercomitted scenarios, especially in large guests
yield_to overhead is significantly high. when run queue length of
source and target is one, take an opportunity to bail out and return
-ESRCH. This return condition can be further exploited to quickly come
out of PLE handler.

(History: Raghavendra initially worked on break out of kvm ple handler upon
seeing source runqueue length = 1, but it had to export rq length).
Peter came up with the elegant idea of return -ESRCH in scheduler core.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Acked-by: Andrew Jones <drjones@redhat.com>
Tested-by: Chegu Vinod <chegu_vinod@hp.com>
---

kernel/sched/core.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2d8927f..fc219a5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4289,7 +4289,10 @@ EXPORT_SYMBOL(yield);
* It's the caller's job to ensure that the target task struct
* can't go away on us before we can do any checks.
*
- * Returns true if we indeed boosted the target task.
+ * Returns:
+ * true (>0) if we indeed boosted the target task.
+ * false (0) if we failed to boost the target.
+ * -ESRCH if there's no task to yield to.
*/
bool __sched yield_to(struct task_struct *p, bool preempt)
{
@@ -4303,6 +4306,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt)

again:
p_rq = task_rq(p);
+ /*
+ * If we're the only runnable task on the rq and target rq also
+ * has only one task, there's absolutely no point in yielding.
+ */
+ if (rq->nr_running == 1 && p_rq->nr_running == 1) {
+ yielded = -ESRCH;
+ goto out_irq;
+ }
+
double_rq_lock(rq, p_rq);
while (task_rq(p) != p_rq) {
double_rq_unlock(rq, p_rq);
@@ -4310,13 +4322,13 @@ again:
}

if (!curr->sched_class->yield_to_task)
- goto out;
+ goto out_unlock;

if (curr->sched_class != p->sched_class)
- goto out;
+ goto out_unlock;

if (task_running(p_rq, p) || p->state)
- goto out;
+ goto out_unlock;

yielded = curr->sched_class->yield_to_task(rq, p, preempt);
if (yielded) {
@@ -4329,11 +4341,12 @@ again:
resched_task(p_rq->curr);
}

-out:
+out_unlock:
double_rq_unlock(rq, p_rq);
+out_irq:
local_irq_restore(flags);

- if (yielded)
+ if (yielded > 0)
schedule();

return yielded;