Asking the GPU to busywait on a memory address, perhaps not unexpectedly
in hindsight for a shared system, leads to bus contention that affects
CPU programs trying to concurrently access memory. This can manifest as
a drop in transcode throughput on highly over-saturated workloads.

The only clue offered by perf, is that the bus-cycles (perf stat -e
bus-cycles) jumped by 50% when enabling semaphores. This corresponds
with extra CPU active cycles being attributed to intel_idle's mwait.

This patch introduces a heuristic to try and detect when more than one
client is submitting to the GPU pushing it into an oversaturated state.
As we already keep track of when the semaphores are signaled, we can
inspect their state on submitting the busywait batch and if we planned
to use a semaphore but were too late, conclude that the GPU is
overloaded and not try to use semaphores in future requests. In
practice, this means we optimistically try to use semaphores for the
first frame of a transcode job split over multiple engines, and fail if
there are multiple clients active and continue not to use semaphores for
the subsequent frames in the sequence. Periodically, we try to
optimistically switch semaphores back on whenever the client waits to
catch up with the transcode results.

With 1 client, on Broxton J3455, with the relative fps normalized by %cpu:

x no semaphores
+ drm-tip
* patched
+------------------------------------------------------------------------+
|                                                    *                   |
|                                                    *+                  |
|                                                    **+                 |
|                                                    **+  x              |
|                                x               *  +**+  x              |
|                                x  x       *    *  +***x xx             |
|                                x  x       *    * *+***x *x             |
|                                x  x*   +  *    * *****x *x x           |
|                         +    x xx+x*   + ***   * ********* x   *       |
|                         +    x xx+x*   * *** +** ********* xx  *       |
|    *   +         ++++*  +    x*x****+*+* ***+*************+x*  *       |
|*+ +** *+ + +* + *++****** *xxx**********x***+*****************+*++    *|
|                                   |__________A_____M_____|             |
|                           |_______________A____M_________|             |
|                                 |____________A___M________|            |
+------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x 120       2.60475       3.50941       3.31123     3.2143953    0.21117399
+ 120        2.3826       3.57077       3.25101     3.1414161    0.28146407
Difference at 95.0% confidence
	-0.0729792 +/- 0.0629585
	-2.27039% +/- 1.95864%
	(Student's t, pooled s = 0.248814)
* 120       2.35536       3.66713        3.2849     3.2059917    0.24618565
No difference proven at 95.0% confidence

With 10 clients over-saturating the pipeline:

x no semaphores
+ drm-tip
* patched
+------------------------------------------------------------------------+
|                     ++                                        **       |
|                     ++                                        **       |
|                     ++                                        **       |
|                     ++                                        **       |
|                     ++                                    xx ***       |
|                     ++                                    xx ***       |
|                     ++                                    xxx***       |
|                     ++                                    xxx***       |
|                    +++                                    xxx***       |
|                    +++                                    xx****       |
|                    +++                                    xx****       |
|                    +++                                    xx****       |
|                    +++                                    xx****       |
|                    ++++                                   xx****       |
|                   +++++                                   xx****       |
|                   +++++                                 x x******      |
|                  ++++++                                 xxx*******     |
|                  ++++++                                 xxx*******     |
|                  ++++++                                 xxx*******     |
|                  ++++++                                 xx********     |
|                  ++++++                               xxxx********     |
|                  ++++++                               xxxx********     |
|                ++++++++                             xxxxx*********     |
|+ +  +        + ++++++++                           xxx*xx**********x*  *|
|                                                         |__A__|        |
|                 |__AM__|                                               |
|                                                            |__A_|      |
+------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x 120       2.47855        2.8972       2.72376     2.7193402   0.074604933
+ 120       1.17367       1.77459       1.71977     1.6966782   0.085850697
Difference at 95.0% confidence
	-1.02266 +/- 0.0203502
	-37.607% +/- 0.748352%
	(Student's t, pooled s = 0.0804246)
* 120       2.57868       3.00821       2.80142     2.7923878   0.058646477
Difference at 95.0% confidence
	0.0730476 +/- 0.0169791
	2.68622% +/- 0.624383%
	(Student's t, pooled s = 0.0671018)

Indicating that we've recovered the regression from enabling semaphores
on this saturated setup, with a hint towards an overall improvement.

Very similar, but of smaller magnitude, results are observed on both
Skylake(gt2) and Kabylake(gt4). This may be due to the reduced impact of
bus-cycles, where we see a 50% hit on Broxton, it is only 10% on the big
core, in this particular test.

One observation to make here is that for a greedy client trying to
maximise its own throughput, using semaphores is the right choice. It is
only the holistic system-wide view that semaphores of one client
impacts another and reduces the overall throughput where we would choose
to disable semaphores.

The most noticeable negactive impact this has is on the no-op
microbenchmarks, which are also very notable for having no cpu bus load.
In particular, this increases the runtime and energy consumption of
gem_exec_whisper.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Cc: Dmitry Ermilov <dmitry.ermilov@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190504070707.30902-1-chris@chris-wilson.co.uk