[RFT][PATCH] sched: scale sysctl_sched_shares_ratelimit with nr_cpus

[Peter Zijlstra <peterz@infradead.org>]

[And now with proper lkml address - so much for auto-add address entries]

Dhaval, Vatsa,

Could you guys give this patch a spin on the big iron and possibly tune
the default shares_ratelimit value to give satisfactory fairness on your
large machines while considering the overhead?


---
Subject: sched: scale sysctl_sched_shares_ratelimit with nr_cpus

David reported that his Niagra spend a little too much time in
tg_shares_up(), which considering he has a large cpu count makes sense.

So scale the ratelimit value with the number of cpus like we do for
other controls as well.

Reported-by: David Miller <davem@davemloft.net>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
 kernel/sched.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -809,9 +809,9 @@ const_debug unsigned int sysctl_sched_nr
 
 /*
  * ratelimit for updating the group shares.
- * default: 0.5ms
+ * default: 0.25ms
  */
-const_debug unsigned int sysctl_sched_shares_ratelimit = 500000;
+const_debug unsigned int sysctl_sched_shares_ratelimit = 250000;
 
 /*
  * period over which we measure -rt task cpu usage in us.
@@ -5732,6 +5732,8 @@ static inline void sched_init_granularit
 		sysctl_sched_latency = limit;
 
 	sysctl_sched_wakeup_granularity *= factor;
+
+	sysctl_sched_shares_ratelimit *= factor;
 }
 
 #ifdef CONFIG_SMP