Missing Events

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I'm capturing events on my x64 Dell XPS 13 (i7-6560U) laptop w/ Debian
Test and kernel 4.13.4. For the capture, I'm doing:

trace-cmd start -b 2000 -D -i -p function -l "*vblank*" -e i915:i915_pipe_update_vblank_evaded
  -e sched:sched_switch -e sched:sched_process_fork -e sched:sched_process_exec
  -e sched:sched_process_exit -e drm:drm_vblank_event -e drm:drm_vblank_event_queued
  -e drm:drm_vblank_event_delivered -e *fence:*fence_signaled -e i915:i915_flip_request
  -e i915:i915_flip_complete -e i915:intel_gpu_freq_change -e i915:i915_gem_request_add
  -e i915:i915_gem_request_submit -e i915:i915_gem_request_in -e i915:i915_gem_request_out
  -e i915:intel_engine_notify -e i915:i915_gem_request_wait_begin -e i915:i915_gem_request_wait_end

<... run app ...>

trace-cmd snapshot -s
trace-cm extract -s -k -o trace.dat

When I do a capture running just "glxgears -fullscreen", every
drm_vblank_event exists where expected, and has the expected events
ahead of it. I can grep sequence numbers in the trace and get this every
time:

trace-cmd report unloaded_system.dat | grep "seq=50856"
    Xorg-666   [001]   850.248191: drm_vblank_event_queued: file=0xffff916525fcec00, crtc=0, seq=50856
    Xorg-666   [001]   850.263652: drm_vblank_event_queued: file=0xffff916525fcec00, crtc=0, seq=50856
  <idle>-0     [001]   850.264745: drm_vblank_event_delivered: file=0xffff916525fcec00, crtc=0, seq=50856
  <idle>-0     [001]   850.264753: drm_vblank_event_delivered: file=0xffff916525fcec00, crtc=0, seq=50856
  <idle>-0     [001]   850.264756: drm_vblank_event_delivered: file=0xffff916525fcec00, crtc=0, seq=50856
  <idle>-0     [001]   850.264758: drm_vblank_event:     crtc=0, seq=50856

On a loaded system (running Unigine Heaven demo + glxgears -fullscreen),
the above happens most of the time, but there are several missing
drm_vblank_events and several drm_vblank_events that don't have any
matching drm_vblank_event_queued or delivered events. Example:

trace-cmd report loaded_system.dat | grep "seq=93977"
  <idle>-0     [001]  5161.664754: drm_vblank_event:     crtc=0, seq=93977

This looks to me like we are missing events in the capture. Is this
possible? Is there any way to mitigate if so?

Thanks much.
 -Mike
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