[tip:sched/urgent] sched/autogroup: Fix 64-bit kernel nice level adjustment

Commit-ID:  83929cce95251cc77e5659bf493bd424ae0e7a67
Gitweb:     http://git.kernel.org/tip/83929cce95251cc77e5659bf493bd424ae0e7a67
Author:     Mike Galbraith [off-list ref]
AuthorDate: Wed, 23 Nov 2016 11:33:37 +0100
Committer:  Ingo Molnar [off-list ref]
CommitDate: Thu, 24 Nov 2016 05:45:02 +0100

sched/autogroup: Fix 64-bit kernel nice level adjustment

Michael Kerrisk reported:

Regarding the previous paragraph...  My tests indicate
that writing *any* value to the autogroup [nice priority level]
file causes the task group to get a lower priority.

Because autogroup didn't call the then meaningless scale_load()...

Autogroup nice level adjustment has been broken ever since load
resolution was increased for 64-bit kernels.  Use scale_load() to
scale group weight.

Michael Kerrisk tested this patch to fix the problem:

Applied and tested against 4.9-rc6 on an Intel u7 (4 cores).
Test setup:

Terminal window 1: running 40 CPU burner jobs
Terminal window 2: running 40 CPU burner jobs
Terminal window 1: running  1 CPU burner job

Demonstrated that:
* Writing "0" to the autogroup file for TW1 now causes no change
  to the rate at which the process on the terminal consume CPU.
* Writing -20 to the autogroup file for TW1 caused those processes
  to get the lion's share of CPU while TW2 TW3 get a tiny amount.
* Writing -20 to the autogroup files for TW1 and TW3 allowed the
  process on TW3 to get as much CPU as it was getting as when
  the autogroup nice values for both terminals were 0.

Reported-by: Michael Kerrisk <redacted>
Tested-by: Michael Kerrisk <redacted>
Signed-off-by: Mike Galbraith <redacted>
Cc: Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Cc: Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>
Cc: Peter Zijlstra <redacted>
Cc: Thomas Gleixner <redacted>
Cc: linux-man <redacted>
Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
Link: http://lkml.kernel.org/r/1479897217.4306.6.camel-Mmb7MZpHnFY@public.gmane.org
Signed-off-by: Ingo Molnar <redacted>
---
 kernel/sched/auto_group.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/auto_group.c b/kernel/sched/auto_group.c
index f1c8fd5..da39489 100644
--- a/kernel/sched/auto_group.c
+++ b/kernel/sched/auto_group.c

@@ -212,6 +212,7 @@ int proc_sched_autogroup_set_nice(struct task_struct *p, int nice)
 {
 	static unsigned long next = INITIAL_JIFFIES;
 	struct autogroup *ag;
+	unsigned long shares;
 	int err;
 
 	if (nice < MIN_NICE || nice > MAX_NICE)

@@ -230,9 +231,10 @@ int proc_sched_autogroup_set_nice(struct task_struct *p, int nice)
 
 	next = HZ / 10 + jiffies;
 	ag = autogroup_task_get(p);
+	shares = scale_load(sched_prio_to_weight[nice + 20]);
 
 	down_write(&ag->lock);
-	err = sched_group_set_shares(ag->tg, sched_prio_to_weight[nice + 20]);
+	err = sched_group_set_shares(ag->tg, shares);
 	if (!err)
 		ag->nice = nice;
 	up_write(&ag->lock);
--

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