[Intel-gfx] [PATCH] drm/i915: Ignore stuck requests when considering hangs

[Chris Wilson]

On Mon, Aug 22, 2016 at 02:39:30PM +0300, Mika Kuoppala wrote:
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > 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 at chris-wilson.co.uk>
> > Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_irq.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
> > index ebb83d5a448b..7610eca4f687 100644
> > --- a/drivers/gpu/drm/i915/i915_irq.c
> > +++ b/drivers/gpu/drm/i915/i915_irq.c
> > @@ -3079,6 +3079,7 @@ static void i915_hangcheck_elapsed(struct work_struct *work)
> >  		bool busy = intel_engine_has_waiter(engine);
> >  		u64 acthd;
> >  		u32 seqno;
> > +		u32 submit;
> >  
> >  		semaphore_clear_deadlocks(dev_priv);
> >  
> > @@ -3094,9 +3095,10 @@ static void i915_hangcheck_elapsed(struct work_struct *work)
> >  
> >  		acthd = intel_engine_get_active_head(engine);
> >  		seqno = intel_engine_get_seqno(engine);
> > +		submit = READ_ONCE(engine->last_submitted_seqno);
> >  
> >  		if (engine->hangcheck.seqno == seqno) {
> > -			if (!intel_engine_is_active(engine)) {
> > +			if (i915_seqno_passed(seqno, submit)) {
> 
> Setting of busy could be moved in the in scope.

busy/busy_count have lost their meanings. I really did consider renaming
them to something like rearm instead. I hesistated in doing exactly the
same as you suggested here just in case the rearming is sensitive to the
ordering. (I don't think so, it just required some clear thought towards
the ordering of enabling hangcheck from the waiter vs disabling here.)
 
> Also the check could be seqno == submit and warning if we see
> seqno on engine past submit.

That's true, but I'm not concerned about such an error - it should
result in no breakages, and don't plan on fixing such a warning if it
were ever to be reported ;)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre