As wake_qs gain users, we can teach rwsems about them such that
waiters can be awoken without the wait_lock. This is for both
readers and writer, the former being the most ideal candidate
as we can batch the wakeups shortening the critical region that
much more -- ie writer task blocking a bunch of tasks waiting to
service page-faults (mmap_sem readers).

In general applying wake_qs to rwsem (xadd) is not difficult as
the wait_lock is intended to be released soon _anyways_, with
the exception of when a writer slowpath will proactively wakeup
any queued readers if it sees that the lock is owned by a reader,
in which we simply do the wakeups with the lock held (see comment
in __rwsem_down_write_failed_common()).

Similar to other locking primitives, delaying the waiter being
awoken does allow, at least in theory, the lock to be stolen in
the case of writers, however no harm was seen in this (in fact
lock stealing tends to be a _good_ thing in most workloads), and
this is a tiny window anyways.

Some page-fault (pft) and mmap_sem intensive benchmarks show some
pretty constant reduction in systime (by up to ~8 and ~10%) on a
2-socket, 12 core AMD box. In addition, on an 8-core Westmere doing
page allocations (page_test)

aim9:
	 4.6-rc6				4.6-rc6
						rwsemv2
Min      page_test   378167.89 (  0.00%)   382613.33 (  1.18%)
Min      exec_test      499.00 (  0.00%)      502.67 (  0.74%)
Min      fork_test     3395.47 (  0.00%)     3537.64 (  4.19%)
Hmean    page_test   395433.06 (  0.00%)   414693.68 (  4.87%)
Hmean    exec_test      499.67 (  0.00%)      505.30 (  1.13%)
Hmean    fork_test     3504.22 (  0.00%)     3594.95 (  2.59%)
Stddev   page_test    17426.57 (  0.00%)    26649.92 (-52.93%)
Stddev   exec_test        0.47 (  0.00%)        1.41 (-199.05%)
Stddev   fork_test       63.74 (  0.00%)       32.59 ( 48.86%)
Max      page_test   429873.33 (  0.00%)   456960.00 (  6.30%)
Max      exec_test      500.33 (  0.00%)      507.66 (  1.47%)
Max      fork_test     3653.33 (  0.00%)     3650.90 ( -0.07%)

	     4.6-rc6     4.6-rc6
			 rwsemv2
User            1.12        0.04
System          0.23        0.04
Elapsed       727.27      721.98
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman.Long@hpe.com
Cc: dave@stgolabs.net
Cc: jason.low2@hp.com
Cc: peter@hurleysoftware.com
Link: http://lkml.kernel.org/r/1463165787-25937-2-git-send-email-dave@stgolabs.netSigned-off-by: Ingo Molnar <mingo@kernel.org>