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

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Aug 09, 2021 at 04:40:11PM +0200, Daniel Vetter wrote:
> 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".

Ok, int it is.

Matt

> -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