[patch v2 1/2] sched: check for prev_cpu == this_cpu before calling wake_affine()

[Suresh Siddha]

On a single cpu system with SMT, in the scenario of one SMT thread being
idle with another SMT thread running a task and doing a non sync wakeup of
another task, we see (from the traces) that the woken up task ends up running
on the busy thread instead of the idle thread. Idle balancing that comes
in little bit later is fixing the scernaio.

But fixing this wake balance and running the woken up task directly on the
idle SMT thread improved the performance (phoronix 7zip compression workload)
by ~9% on an atom platform.

During the process wakeup, select_task_rq_fair() and wake_affine() makes
the decision to wakeup the task either on the previous cpu that the task
ran or the cpu that the task is currently woken up.

select_task_rq_fair() also goes through to see if there are any idle siblings
for the cpu that the task is woken up on. This is to ensure that we select
any idle sibling rather than choose a busy cpu.

In the above load scenario, it so happens that the prev_cpu (that the
task ran before) and this_cpu (where it is woken up currently) are the same. And
in this case, it looks like wake_affine() returns 0 and ultimately not selecting
the idle sibling chosen by select_idle_sibling() in select_task_rq_fair().
Further down the path of select_task_rq_fair(), we ultimately select
the currently running cpu (busy SMT thread instead of the idle SMT thread).

Check for prev_cpu == this_cpu before calling wake_affine() and no need to do
any fancy stuff(and ultimately taking wrong decisions) in this case.

Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
---
Changes from v1:
   Move the "this_cpu == prev_cpu" check before calling wake_affine()
---
 kernel/sched_fair.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

Index: tip/kernel/sched_fair.c
===================================================================
--- tip.orig/kernel/sched_fair.c
+++ tip/kernel/sched_fair.c
@@ -1454,6 +1454,7 @@ static int select_task_rq_fair(struct ta
 	int want_affine = 0;
 	int want_sd = 1;
 	int sync = wake_flags & WF_SYNC;
+	int this_cpu = cpu;
 
 	if (sd_flag & SD_BALANCE_WAKE) {
 		if (sched_feat(AFFINE_WAKEUPS) &&
@@ -1545,8 +1546,10 @@ static int select_task_rq_fair(struct ta
 			update_shares(tmp);
 	}
 
-	if (affine_sd && wake_affine(affine_sd, p, sync))
-		return cpu;
+	if (affine_sd) {
+		if (this_cpu == prev_cpu || wake_affine(affine_sd, p, sync))
+			return cpu;
+	}
 
 	while (sd) {
 		int load_idx = sd->forkexec_idx;


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