Arjan van de Ven reported that on his test machine that he was seeing
stalls of greater than 1 frame greatly impacting the user experience. He
tracked this down to being the locked flush during a pagefault as being
the culprit hogging the struct_mutex and so blocking any other user from
proceeding. Stalling on a pagefault is bad behaviour on userspace's
part, for one it means that they are ignoring the coherency rules on
pointer access through the GTT, but fortunately we can apply the same
trick as the set-to-domain ioctl to do a lightweight, nonblocking flush
of outstanding rendering first.

"Prior to the patch it looks like this
(this one testrun does not show the 20ms+ I've seen occasionally)

  4.99 ms     2.36 ms    31360  __wait_seqno i915_wait_seqno i915_gem_object_wait_rendering i915_gem_object_set_to_gtt_domain i915_gem_fault __do_fault handle_
+pte_fault handle_mm_fault __do_page_fault do_page_fault page_fault
   4.99 ms     2.75 ms   107751  __wait_seqno i915_gem_wait_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   4.99 ms     1.63 ms     1666  i915_mutex_lock_interruptible i915_gem_fault __do_fault handle_pte_fault handle_mm_fault __do_page_fault do_page_fault page_fa
+ult
   4.93 ms     2.45 ms      980  i915_mutex_lock_interruptible intel_crtc_page_flip drm_mode_page_flip_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_
+sysret
   4.89 ms     2.20 ms     3283  i915_mutex_lock_interruptible i915_gem_wait_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   4.34 ms     1.66 ms     1715  i915_mutex_lock_interruptible i915_gem_pwrite_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   3.73 ms     3.73 ms       49  i915_mutex_lock_interruptible i915_gem_set_domain_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   3.17 ms     0.33 ms      931  i915_mutex_lock_interruptible i915_gem_madvise_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   2.97 ms     0.43 ms     1029  i915_mutex_lock_interruptible i915_gem_busy_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   2.55 ms     0.51 ms      735  i915_gem_get_tiling drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret

After the patch it looks like this:

   4.99 ms     2.14 ms    22212  __wait_seqno i915_gem_wait_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   4.86 ms     0.99 ms    14170  __wait_seqno i915_gem_object_wait_rendering__nonblocking i915_gem_fault __do_fault handle_pte_fault handle_mm_fault __do_page_
+fault do_page_fault page_fault
   3.59 ms     1.31 ms      325  i915_gem_get_tiling drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   3.37 ms     3.37 ms       65  i915_mutex_lock_interruptible i915_gem_wait_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   2.58 ms     2.58 ms       65  i915_mutex_lock_interruptible i915_gem_do_execbuffer.isra.23 i915_gem_execbuffer2 drm_ioctl i915_compat_ioctl compat_sys_ioctl
+ia32_sysret
   2.19 ms     2.19 ms       65  i915_mutex_lock_interruptible intel_crtc_page_flip drm_mode_page_flip_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_
+sysret
   2.18 ms     2.18 ms       65  i915_mutex_lock_interruptible i915_gem_busy_ioctl drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret
   1.66 ms     1.66 ms       65  i915_gem_set_tiling drm_ioctl i915_compat_ioctl compat_sys_ioctl ia32_sysret

It may not look like it, but this is quite a large difference, and I've
been unable to reproduce > 5 msec delays at all, while before they do
happen (just not in the trace above)."

gem_gtt_hog on an old Pineview (GMA3150),
before: 4969.119ms
after:  4122.749ms
Reported-by: Arjan van de Ven <arjan.van.de.ven@intel.com>
Testcase: igt/gem_gtt_hog
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>