Patch "sched/fair: Fix effective_load() to consistently use smoothed load" has been added to the 4.6-stable tree

This is a note to let you know that I've just added the patch titled

sched/fair: Fix effective_load() to consistently use smoothed load

to the 4.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sched-fair-fix-effective_load-to-consistently-use-smoothed-load.patch
and it can be found in the queue-4.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Peter Zijlstra <[email protected]>
Date: Fri, 24 Jun 2016 15:53:54 +0200
Subject: sched/fair: Fix effective_load() to consistently use smoothed load

From: Peter Zijlstra <[email protected]>

commit 7dd4912594daf769a46744848b05bd5bc6d62469 upstream.

Starting with the following commit:

fde7d22e01aa ("sched/fair: Fix overly small weight for interactive group entities")

calc_tg_weight() doesn't compute the right value as expected by effective_load().

The difference is in the 'correction' term. In order to ensure \Sum rw_j >= rw_i we cannot use tg->load_avg directly, since that might be lagging a correction on the current cfs_rq->avg.load_avg value. Therefore we use tg->load_avg - cfs_rq->tg_load_avg_contrib + cfs_rq->avg.load_avg.

Now, per the referenced commit, calc_tg_weight() doesn't use cfs_rq->avg.load_avg, as is later used in @w, but uses cfs_rq->load.weight instead.

So stop using calc_tg_weight() and do it explicitly.

The effects of this bug are wake_affine() making randomly poor choices in cgroup-intense workloads.

---
 kernel/sched/fair.c |   15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

@@ -687,8 +687,6 @@ void init_entity_runnable_average(struct
 	/* when this task enqueue'ed, it will contribute to its cfs_rq's load_avg */
 }
 
-static inline unsigned long cfs_rq_runnable_load_avg(struct cfs_rq *cfs_rq);
-static inline unsigned long cfs_rq_load_avg(struct cfs_rq *cfs_rq);
 #else
 void init_entity_runnable_average(struct sched_entity *se)
 {

@@ -4822,19 +4820,24 @@ static long effective_load(struct task_g
 		return wl;
 
 	for_each_sched_entity(se) {
-		long w, W;
+		struct cfs_rq *cfs_rq = se->my_q;
+		long W, w = cfs_rq_load_avg(cfs_rq);
 
-		tg = se->my_q->tg;
+		tg = cfs_rq->tg;
 
 		/*
 		 * W = @wg + \Sum rw_j
 		 */
-		W = wg + calc_tg_weight(tg, se->my_q);
+		W = wg + atomic_long_read(&tg->load_avg);
+
+		/* Ensure \Sum rw_j >= rw_i */
+		W -= cfs_rq->tg_load_avg_contrib;
+		W += w;
 
 		/*
 		 * w = rw_i + @wl
 		 */
-		w = cfs_rq_load_avg(se->my_q) + wl;
+		w += wl;
 
 		/*
 		 * wl = S * s'_i; see (2)

Patches currently in stable-queue which might be from [email protected] are

queue-4.6/x86-quirks-apply-nvidia_bugs-quirk-only-on-root-bus.patch queue-4.6/x86-quirks-add-early-quirk-to-reset-apple-airport-card.patch queue-4.6/sched-fair-fix-effective_load-to-consistently-use-smoothed-load.patch queue-4.6/sched-debug-fix-deadlock-when-enabling-sched-events.patch queue-4.6/x86-quirks-reintroduce-scanning-of-secondary-buses.patch queue-4.6/kernel-sysrq-watchdog-sched-core-reset-watchdog-on-all-cpus-while-processing-sysrq-w.patch