[PATCH 4/7] drm/i915/guc: Don't hog IRQs when destroying contexts

[Matthew Brost]

On Fri, Dec 10, 2021 at 05:07:12PM -0800, John Harrison wrote:
> On 12/10/2021 16:56, Matthew Brost wrote:
> > From: John Harrison <John.C.Harrison at Intel.com>
> > 
> > While attempting to debug a CT deadlock issue in various CI failures
> > (most easily reproduced with gem_ctx_create/basic-files), I was seeing
> > CPU deadlock errors being reported. This were because the context
> > destroy loop was blocking waiting on H2G space from inside an IRQ
> > spinlock. There was deadlock as such, it's just that the H2G queue was
> There was *no* deadlock as such
> 

Let's fix this up when applying the series.

With that:
Reviewed-by: Matthew Brost <matthew.brost at intel.com>

> John.
> 
> > full of context destroy commands and GuC was taking a long time to
> > process them. However, the kernel was seeing the large amount of time
> > spent inside the IRQ lock as a dead CPU. Various Bad Things(tm) would
> > then happen (heartbeat failures, CT deadlock errors, outstanding H2G
> > WARNs, etc.).
> > 
> > Re-working the loop to only acquire the spinlock around the list
> > management (which is all it is meant to protect) rather than the
> > entire destroy operation seems to fix all the above issues.
> > 
> > Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> > ---
> >   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 45 ++++++++++++-------
> >   1 file changed, 28 insertions(+), 17 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > index 36c2965db49b..96fcf869e3ff 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > @@ -2644,7 +2644,6 @@ static inline void guc_lrc_desc_unpin(struct intel_context *ce)
> >   	unsigned long flags;
> >   	bool disabled;
> > -	lockdep_assert_held(&guc->submission_state.lock);
> >   	GEM_BUG_ON(!intel_gt_pm_is_awake(gt));
> >   	GEM_BUG_ON(!lrc_desc_registered(guc, ce->guc_id.id));
> >   	GEM_BUG_ON(ce != __get_context(guc, ce->guc_id.id));
> > @@ -2660,7 +2659,7 @@ static inline void guc_lrc_desc_unpin(struct intel_context *ce)
> >   	}
> >   	spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> >   	if (unlikely(disabled)) {
> > -		__release_guc_id(guc, ce);
> > +		release_guc_id(guc, ce);
> >   		__guc_context_destroy(ce);
> >   		return;
> >   	}
> > @@ -2694,36 +2693,48 @@ static void __guc_context_destroy(struct intel_context *ce)
> >   static void guc_flush_destroyed_contexts(struct intel_guc *guc)
> >   {
> > -	struct intel_context *ce, *cn;
> > +	struct intel_context *ce;
> >   	unsigned long flags;
> >   	GEM_BUG_ON(!submission_disabled(guc) &&
> >   		   guc_submission_initialized(guc));
> > -	spin_lock_irqsave(&guc->submission_state.lock, flags);
> > -	list_for_each_entry_safe(ce, cn,
> > -				 &guc->submission_state.destroyed_contexts,
> > -				 destroyed_link) {
> > -		list_del_init(&ce->destroyed_link);
> > -		__release_guc_id(guc, ce);
> > +	while (!list_empty(&guc->submission_state.destroyed_contexts)) {
> > +		spin_lock_irqsave(&guc->submission_state.lock, flags);
> > +		ce = list_first_entry_or_null(&guc->submission_state.destroyed_contexts,
> > +					      struct intel_context,
> > +					      destroyed_link);
> > +		if (ce)
> > +			list_del_init(&ce->destroyed_link);
> > +		spin_unlock_irqrestore(&guc->submission_state.lock, flags);
> > +
> > +		if (!ce)
> > +			break;
> > +
> > +		release_guc_id(guc, ce);
> >   		__guc_context_destroy(ce);
> >   	}
> > -	spin_unlock_irqrestore(&guc->submission_state.lock, flags);
> >   }
> >   static void deregister_destroyed_contexts(struct intel_guc *guc)
> >   {
> > -	struct intel_context *ce, *cn;
> > +	struct intel_context *ce;
> >   	unsigned long flags;
> > -	spin_lock_irqsave(&guc->submission_state.lock, flags);
> > -	list_for_each_entry_safe(ce, cn,
> > -				 &guc->submission_state.destroyed_contexts,
> > -				 destroyed_link) {
> > -		list_del_init(&ce->destroyed_link);
> > +	while (!list_empty(&guc->submission_state.destroyed_contexts)) {
> > +		spin_lock_irqsave(&guc->submission_state.lock, flags);
> > +		ce = list_first_entry_or_null(&guc->submission_state.destroyed_contexts,
> > +					      struct intel_context,
> > +					      destroyed_link);
> > +		if (ce)
> > +			list_del_init(&ce->destroyed_link);
> > +		spin_unlock_irqrestore(&guc->submission_state.lock, flags);
> > +
> > +		if (!ce)
> > +			break;
> > +
> >   		guc_lrc_desc_unpin(ce);
> >   	}
> > -	spin_unlock_irqrestore(&guc->submission_state.lock, flags);
> >   }
> >   static void destroyed_worker_func(struct work_struct *w)
>