drm/i915: Ignore stuck requests when considering hangs
authorChris Wilson <chris@chris-wilson.co.uk>
Sat, 20 Aug 2016 14:54:08 +0000 (15:54 +0100)
committerChris Wilson <chris@chris-wilson.co.uk>
Mon, 22 Aug 2016 12:09:11 +0000 (13:09 +0100)
commit34730fed9fd54e893ec0177c97a509de3ee0a5a4
treee925a3bfc0d5c20570e0aeef59be83460df38505
parentbb8f9cffad4469e3daa4d3759d5b6cc72939ab45
drm/i915: Ignore stuck requests when considering hangs

If the engine isn't being retired (worker starvation?) then it is
possible for us to repeatedly observe that between consecutive
hangchecks the seqno on the ring to be the same and there remain
unretired requests. Ignore these completely and only regard the engine
as busy for the purpose of hang detection (not stall detection) if there
are outstanding breadcrumbs.

In recent history we have looked at using both the request and seqno as
indication of activity on the engine, but that was reduced to just
inspecting seqno in commit cffa781e5907 ("drm/i915: Simplify check for
idleness in hangcheck"). However, in commit dcff85c8443e ("drm/i915:
Enable i915_gem_wait_for_idle() without holding struct_mutex"), I made
the decision to use the new common lockless function, under the
assumption that request retirement was more frequent than hangcheck and
so we would not have a stuck busy check. The flaw there was in
forgetting that we accumulate the hang score, and so successive checks
seeing a stuck request, albeit with the GPU advancing elsewhere and so
not necessary the same stuck request, would eventually trigger the hang.

Fixes: dcff85c8443e ("drm/i915: Enable i915_gem_wait_for_idle()...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160820145408.32180-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
drivers/gpu/drm/i915/i915_irq.c