RFC kvm: Improving undercommit scenarios

In some special scenarios like #vcpu <= #pcpu, PLE handler may
prove very costly, because there is no need to iterate over vcpus and do unsuccessful yield_to burning CPU.

The first patch optimizes all the yield_to by bailing out when there is no need to continue in yield_to (i.e., when there is only one task in source and target rq).

Second patch uses that in PLE handler. Further when a yield_to fails we do not immediately go out of PLE handler instead we try thrice to have better statistical possibility of false return. Otherwise that would affect moderate overcommit cases.

Result on 3.7.0-rc6 kernel shows around 140% improvement for ebizzy 1x and around 51% for dbench 1x with 32 core PLE machine with 32 vcpu guest.

base = 3.7.0-rc6
machine: 32 core mx3850 x5 PLE mc

--+-----------+-----------+-----------+------------+-----------+
ebizzy (rec/sec higher is beter)
--+-----------+-----------+-----------+------------+-----------+
base stdev patched stdev %improve
--+-----------+-----------+-----------+------------+-----------+

1x   2511.3000    21.5409    6051.8000   170.2592   140.98276   
2x   2679.4000   332.4482    2692.3000   251.4005     0.48145
3x   2253.5000   266.4243    2192.1667   178.9753    -2.72169

--+-----------+-----------+-----------+------------+-----------+

--+-----------+-----------+-----------+------------+-----------+
dbench (throughput in MB/sec. higher is better)
--+-----------+-----------+-----------+------------+-----------+
base stdev patched stdev %improve
--+-----------+-----------+-----------+------------+-----------+

1x  6677.4080   638.5048    10098.0060   3449.7026     51.22643
2x  2012.6760    64.7642    2019.0440     62.6702       0.31639
3x  1302.0783    40.8336    1292.7517     27.0515      -0.71629

--+-----------+-----------+-----------+------------+-----------+

Here is the refernce of no ple result.
ebizzy-1x_nople 7592.6000 rec/sec
dbench_1x_nople 7853.6960 MB/sec

The result says we can still improve by 60% for ebizzy, but overall we are getting impressive performance with the patches.

Changes Since V2:
- Dropped global measures usage patch (Peter Zilstra)
- Do not bail out on first failure (Avi Kivity)
- Try thrice for the failure of yield_to to get statistically more correct behaviour.

Changes since V1:
- Discard the idea of exporting nrrunning and optimize in core scheduler (Peter)
- Use yield() instead of schedule in overcommit scenarios (Rik)
- Use loadavg knowledge to detect undercommit/overcommit

Peter Zijlstra (1):
Bail out of yield_to when source and target runqueue has one task

Raghavendra K T (1):
Handle yield_to failure return for potential undercommit case

Please let me know your comments and suggestions.

Link for the discussion of V3 original:
https://lkml.org/lkml/2012/11/26/166

Link for V2:
https://lkml.org/lkml/2012/10/29/287

Link for V1:
https://lkml.org/lkml/2012/9/21/168

 kernel/sched/core.c | 25 +++++++++++++++++++------
 virt/kvm/kvm_main.c | 26 ++++++++++++++++----------
 2 files changed, 35 insertions(+), 16 deletions(-)