sched: Improve latencies under load by decreasing minimum scheduling granularity

Mathieu reported bad latencies with make -j10 kind of kbuild
workloads - which is mostly caused by us scheduling with a
too coarse granularity.

Reduce the minimum granularity some more, to make sure we
can meet the latency target.

I got the following results (make -j10 kbuild load, average of 3
runs):

 vanilla:

  maximum latency: 38278.9 µs
  average latency:  7730.1 µs

 patched:

  maximum latency: 22702.1 µs
  average latency:  6684.8 µs

Mathieu also measured it:

|
| * wakeup-latency.c (SIGEV_THREAD) with make -j10
|
| - Mainline 2.6.35.2 kernel
|
| maximum latency: 45762.1 µs
| average latency: 7348.6 µs
|
| - With only Peter's smaller min_gran (shown below):
|
| maximum latency: 29100.6 µs
| average latency: 6684.1 µs
|
Reported-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reported-by: NLinus Torvalds <torvalds@linux-foundation.org>
Acked-by: NMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Suggested-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <AANLkTi=8m4g01wZPacySoF7U0PevTNVgJoZZrHiUD-pN@mail.gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -54,13 +54,13 @@ enum sched_tunable_scaling sysctl_sched_tunable_scaling
  * Minimal preemption granularity for CPU-bound tasks:
  * (default: 2 msec * (1 + ilog(ncpus)), units: nanoseconds)
  */
-unsigned int sysctl_sched_min_granularity = 2000000ULL;
-unsigned int normalized_sysctl_sched_min_granularity = 2000000ULL;
+unsigned int sysctl_sched_min_granularity = 750000ULL;
+unsigned int normalized_sysctl_sched_min_granularity = 750000ULL;
 
 /*
  * is kept at sysctl_sched_latency / sysctl_sched_min_granularity
  */
-static unsigned int sched_nr_latency = 3;
+static unsigned int sched_nr_latency = 8;
 
 /*
  * After fork, child runs first. If set to 0 (default) then