sched: smart wake-affine

wake-affine stuff is always trying to pull wakee close to waker, by theory,
this will bring benefit if waker's cpu cached hot data for wakee, or the
extreme ping-pong case.

And testing show it could benefit hackbench 15% at most.

However, the whole stuff is somewhat blindly and time-consuming, some
workload therefore suffer.

And testing show it could damage pgbench 50% at most.

Thus, wake-affine stuff should be smarter, and realise when to stop
it's thankless effort.

This patch introduced per task 'nr_wakee_switch', which will be increased
each time the task switch it's wakee.

So a high 'nr_wakee_switch' means the task has more than one wakee, and
less the wakee number, higher the wakeup frequency.

Now when making the decision on whether to pull or not, pay attention on
the wakee with a high 'nr_wakee_switch', pull such task may benefit wakee,
but that imply waker will face cruel competition later, it could be very
crule or very fast depends on the story behind 'nr_wakee_switch', whatever,
waker therefore suffer.

Furthermore, if waker also has a high 'nr_wakee_switch', that imply multiple
tasks rely on it, waker's higher latency will damage all those tasks, pull
wakee in such cases seems to be a bad deal.

Thus, when 'waker->nr_wakee_switch / wakee->nr_wakee_switch' become higher
and higher, the deal seems to be worse and worse.

This patch therefore help wake-affine stuff to stop it's work when:

	wakee->nr_wakee_switch > factor &&
	waker->nr_wakee_switch > (factor * wakee->nr_wakee_switch)
The factor here is the online cpu number, so more cpu will lead to more pull
since the trial become more severe.

After applied the patch, pgbench show 42% improvement at most.

Test:
	Test with 12 cpu X86 server and tip 3.10.0-rc1.

				base	smart

	| db_size | clients |  tps  | |  tps  |
	+---------+---------+-------+ +-------+
	| 21 MB   |       1 | 10749 | | 10337 |
	| 21 MB   |       2 | 21382 | | 21391 |
	| 21 MB   |       4 | 41570 | | 41808 |
	| 21 MB   |       8 | 52828 | | 58792 |
	| 21 MB   |      12 | 48447 | | 54553 |
	| 21 MB   |      16 | 46246 | | 56726 |	+22.66%
	| 21 MB   |      24 | 43850 | | 56853 |	+29.65%
	| 21 MB   |      32 | 43455 | | 55846 |	+28.51%
	| 7483 MB |       1 |  9290 | |  8848 |
	| 7483 MB |       2 | 19347 | | 19351 |
	| 7483 MB |       4 | 37135 | | 37511 |
	| 7483 MB |       8 | 47310 | | 50210 |
	| 7483 MB |      12 | 42721 | | 49396 |
	| 7483 MB |      16 | 41016 | | 51826 |	+26.36%
	| 7483 MB |      24 | 37540 | | 52579 |	+40.06%
	| 7483 MB |      32 | 36756 | | 51332 |	+39.66%
	| 15 GB   |       1 |  8758 | |  8670 |
	| 15 GB   |       2 | 19204 | | 19249 |
	| 15 GB   |       4 | 36997 | | 37199 |
	| 15 GB   |       8 | 46578 | | 50681 |
	| 15 GB   |      12 | 42141 | | 48671 |
	| 15 GB   |      16 | 40518 | | 51280 |	+26.56%
	| 15 GB   |      24 | 36788 | | 52329 |	+42.24%
	| 15 GB   |      32 | 36056 | | 50350 | +39.64%

CC: Ingo Molnar <mingo@kernel.org>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Mike Galbraith <efault@gmx.de>
Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
Signed-off-by: Paul Reioux <reioux@gmail.com>
CPU hotplug, writeback: Don't call writeback_set_ratelimit() too often during hotplug

The CPU hotplug callback related to writeback calls writeback_set_ratelimit()
during every state change in the hotplug sequence. This is unnecessary
since num_online_cpus() changes only once during the entire hotplug operation.

So invoke the function only once per hotplug, thereby avoiding the
unnecessary repetition of those costly calculations.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
intellidemand: re-enable performance lock option

with low freq enabled kernels, minimum performance lock will help music
playback to be smooth and glitch free

Signed-off-by: Paul Reioux <reioux@gmail.com>