Re: [Intel-gfx] [PATCH 15/46] drm/i915/guc: Introduce context parent-child relationship

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Aug 09, 2021 at 04:37:55PM +0200, Daniel Vetter wrote:
> On Tue, Aug 03, 2021 at 03:29:12PM -0700, Matthew Brost wrote:
> > Introduce context parent-child relationship. Once this relationship is
> > created all pinning / unpinning operations are directed to the parent
> > context. The parent context is responsible for pinning all of its'
> > children and itself.
> > 
> > This is a precursor to the full GuC multi-lrc implementation but aligns
> > to how GuC mutli-lrc interface is defined - a single H2G is used
> > register / deregister all of the contexts simultaneously.
> > 
> > Subsequent patches in the series will implement the pinning / unpinning
> > operations for parent / child contexts.
> > 
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_context.c       | 29 +++++++++++++++++++
> >  drivers/gpu/drm/i915/gt/intel_context.h       | 18 ++++++++++++
> >  drivers/gpu/drm/i915/gt/intel_context_types.h | 12 ++++++++
> >  3 files changed, 59 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> > index 745e84c72c90..8cb92b10b547 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> > @@ -395,6 +395,8 @@ intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
> >  	spin_lock_init(&ce->guc_state.lock);
> >  	INIT_LIST_HEAD(&ce->guc_state.fences);
> >  
> > +	INIT_LIST_HEAD(&ce->guc_child_list);
> > +
> >  	spin_lock_init(&ce->guc_active.lock);
> >  	INIT_LIST_HEAD(&ce->guc_active.requests);
> >  
> > @@ -414,10 +416,17 @@ intel_context_init(struct intel_context *ce, struct intel_engine_cs *engine)
> >  
> >  void intel_context_fini(struct intel_context *ce)
> >  {
> > +	struct intel_context *child, *next;
> > +
> >  	if (ce->timeline)
> >  		intel_timeline_put(ce->timeline);
> >  	i915_vm_put(ce->vm);
> >  
> > +	/* Need to put the creation ref for the children */
> > +	if (intel_context_is_parent(ce))
> > +		for_each_child_safe(ce, child, next)
> > +			intel_context_put(child);
> > +
> >  	mutex_destroy(&ce->pin_mutex);
> >  	i915_active_fini(&ce->active);
> >  }
> > @@ -533,6 +542,26 @@ struct i915_request *intel_context_find_active_request(struct intel_context *ce)
> >  	return active;
> >  }
> >  
> > +void intel_context_bind_parent_child(struct intel_context *parent,
> > +				     struct intel_context *child)
> > +{
> > +	/*
> > +	 * Callers responsibility to validate that this function is used
> > +	 * correctly but we use GEM_BUG_ON here ensure that they do.
> > +	 */
> > +	GEM_BUG_ON(!intel_engine_uses_guc(parent->engine));
> > +	GEM_BUG_ON(intel_context_is_pinned(parent));
> > +	GEM_BUG_ON(intel_context_is_child(parent));
> > +	GEM_BUG_ON(intel_context_is_pinned(child));
> > +	GEM_BUG_ON(intel_context_is_child(child));
> > +	GEM_BUG_ON(intel_context_is_parent(child));
> > +
> > +	parent->guc_number_children++;
> > +	list_add_tail(&child->guc_child_link,
> > +		      &parent->guc_child_list);
> > +	child->parent = parent;
> > +}
> > +
> >  #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
> >  #include "selftest_context.c"
> >  #endif
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context.h b/drivers/gpu/drm/i915/gt/intel_context.h
> > index c41098950746..ad6ce5ac4824 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_context.h
> > @@ -44,6 +44,24 @@ void intel_context_free(struct intel_context *ce);
> >  int intel_context_reconfigure_sseu(struct intel_context *ce,
> >  				   const struct intel_sseu sseu);
> >  
> > +static inline bool intel_context_is_child(struct intel_context *ce)
> > +{
> > +	return !!ce->parent;
> > +}
> > +
> > +static inline bool intel_context_is_parent(struct intel_context *ce)
> > +{
> > +	return !!ce->guc_number_children;
> > +}
> > +
> > +void intel_context_bind_parent_child(struct intel_context *parent,
> > +				     struct intel_context *child);
> > +
> > +#define for_each_child(parent, ce)\
> > +	list_for_each_entry(ce, &(parent)->guc_child_list, guc_child_link)
> > +#define for_each_child_safe(parent, ce, cn)\
> > +	list_for_each_entry_safe(ce, cn, &(parent)->guc_child_list, guc_child_link)
> > +
> >  /**
> >   * intel_context_lock_pinned - Stablises the 'pinned' status of the HW context
> >   * @ce - the context
> > diff --git a/drivers/gpu/drm/i915/gt/intel_context_types.h b/drivers/gpu/drm/i915/gt/intel_context_types.h
> > index 2df79ba39867..66b22b370a72 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_context_types.h
> > +++ b/drivers/gpu/drm/i915/gt/intel_context_types.h
> > @@ -202,6 +202,18 @@ struct intel_context {
> >  	/* GuC context blocked fence */
> >  	struct i915_sw_fence guc_blocked;
> >  
> > +	/* Head of children list or link in parent's children list */
> 
> Kerneldoc layout would be nice, plus explaining when exactly this is
> set or the list empty (e.g. guch_child_list is empty if and only if
> guc_number_children > 0 and parent == NULL).
> 
> Also mentionting that these are invariant over the lifetime of the object
> would be nice.
> 
> Finally some words on refcounting (like who holds a reference on whom and
> how we guarantee that use-after-free doesn't go boom since you have links
> both ways). It looks like parent holds a reference on the child, so how do
> you make sure the child looking at the parent doesn't go boom?
> -Daniel
> 
> > +	union {
> > +		struct list_head guc_child_list;	/* parent */
> > +		struct list_head guc_child_link;	/* child */
> > +	};
> > +
> > +	/* Pointer to parent */
> > +	struct intel_context *parent;
> > +
> > +	/* Number of children if parent */
> > +	u8 guc_number_children;

Another one: Can we really not afford a int here? The nasty thing about
unsigned is that wrap-around is well-defined, which is why gcc won't ever
complain about it. Which hides bugs. Same for next patch, which also
micro-optimizes a few fields to be tiny.

We generally don't have thousands of contexts hanging around, unless
there's a reason (which should be documented) this feels like it's
squarely on the wrong side of "don't prematurely optimize".
-Daniel

> > +
> >  	/*
> >  	 * GuC priority management
> >  	 */
> > -- 
> > 2.28.0
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch