Re: Migrated CFS task getting an unfair advantage

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 09, 2016 at 02:52:57PM +0530, Pavan Kondeti wrote:

> When a CFS task is enqueued during migration (load balance or change in
> affinity), its vruntime is normalized before updating the current and
> cfs_rq->min_vruntime.

static void
enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
{
	/*
	 * Update the normalized vruntime before updating min_vruntime
	 * through calling update_curr().
	 */
	if (!(flags & ENQUEUE_WAKEUP) || (flags & ENQUEUE_WAKING))
		se->vruntime += cfs_rq->min_vruntime;

	update_curr(cfs_rq);

This, right? Some idiot wrote a comment but forgot to explain why.

> If the current entity is a low priority task or belongs to a cgroup
> that has lower cpu.shares and it is the only entity queued, there is a
> possibility of big update to the cfs_rq->min_vruntime.

> As the migrated task is normalized before this update, it gets an
> unfair advantage over tasks queued after this point. If the migrated
> task is a CPU hogger, the other CFS tasks queued on this CPU gets
> starved.

Because it takes a whole while for the newly placed task to gain on the
previous task, right?

> If we add the migrated task to destination CPU cfs_rq's rb tree before
> updating the current in enqueue_entity(), the cfs_rq->min_vruntime
> does not go beyond the newly migrated task. Is this an acceptable
> solution?

Hurm.. so I'm not sure how that would solve anything. The existing task
would still be shot far into the future.

What you want is to normalize after update_curr()... but we cannot do
that in the case cfs_rq->curr == se (which I suppose is what that
comment is on about).

Does something like the below work?

---
 kernel/sched/fair.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 33130529e9b5..3c114d971d84 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3157,17 +3157,25 @@ static inline void check_schedstat_required(void)
 static void
 enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 {
+	bool renorm = !(flags & ENQUEUE_WAKEUP) || (flags & ENQUEUE_WAKING);
+	bool curr = cfs_rq->curr == se;
+
 	/*
-	 * Update the normalized vruntime before updating min_vruntime
-	 * through calling update_curr().
+	 * If we're the current task, we must renormalise before calling
+	 * update_curr().
 	 */
-	if (!(flags & ENQUEUE_WAKEUP) || (flags & ENQUEUE_WAKING))
+	if (renorm && curr)
 		se->vruntime += cfs_rq->min_vruntime;
 
+	update_curr(cfs_rq);
+
 	/*
-	 * Update run-time statistics of the 'current'.
+	 * Otherwise, renormalise after, such that we're placed at the current
+	 * moment in time, instead of some random moment in the past.
 	 */
-	update_curr(cfs_rq);
+	if (renorm && !curr)
+		se->vruntime += cfs_rq->min_vruntime;
+
 	enqueue_entity_load_avg(cfs_rq, se);
 	account_entity_enqueue(cfs_rq, se);
 	update_cfs_shares(cfs_rq);
@@ -3183,7 +3191,7 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 		update_stats_enqueue(cfs_rq, se);
 		check_spread(cfs_rq, se);
 	}
-	if (se != cfs_rq->curr)
+	if (!curr)
 		__enqueue_entity(cfs_rq, se);
 	se->on_rq = 1;