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

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On 11/27/2012 2:30 AM, Raghavendra K T wrote:
On 11/26/2012 07:05 PM, Andrew Jones wrote:
On Mon, Nov 26, 2012 at 05:37:54PM +0530, Raghavendra K T wrote:
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

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@xxxxxxxxxxxxx>
Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
---

  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;


Acked-by: Andrew Jones <drjones@xxxxxxxxxx>


Thank you Drew.

Marcelo Gleb.. Please let me know if you have comments / concerns on the patches..

Andrew, Vinod, IMO, the patch set looks good for undercommit scenarios
especially for large guests where we do have overhead of vcpu iteration
of ple handler..

.

Thanks Raghu. Will try to get this latest patch set evaluated and get back to you.

Vinod

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