Re: [RFC PATCH v3 15/21] drm/exec: Add a snapshot capability

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Wed, 2024-05-22 at 13:27 +0200, Christian König wrote:
> Am 21.05.24 um 09:16 schrieb Thomas Hellström:
> > When validating a buffer object for submission, we might need to
> > lock
> > a number of object for eviction to make room for the validation.
> > 
> > This makes it pretty likely that validation will eventually
> > succeed,
> > since eventually the validating process will hold most dma_resv
> > locks
> > of the buffer objects residing in the memory type being validated
> > for.
> > 
> > However, once validation of a single object has succeeded it might
> > not
> > be beneficial to hold on to those locks anymore, and the validator
> > would want to drop the locks of all objects taken during
> > validation.
> 
> Exactly avoiding that was one of the goals of developing the drm_exec
> object.
> 
> When objects are unlocked after evicting them it just gives
> concurrent 
> operations an opportunity to lock them and re-validate them into the 
> contended domain.
> 
> So why should that approach here be beneficial at all?

It's a matter of being nice to the rest of the system while *still
guaranteeing progress*. For each object we're trying to validate, we
keep on evicting other objects until we make progress even if we lock
all the objects in the domain.

If we were unlocking after each eviction, we can't really guarantee
progress.

OTOH, a concurrent locker of the object may well be one with higher
priority (lower ticket number) just wanting to perform a pagefault

So it's a tradeoff between locking just locking other processes out to
allow us to make one step of progress and to in addition hit them with
the big sledgehammer.

/Thomas

> 
> Regards,
> Christian.
> 
> > 
> > Introduce a drm_exec snapshot functionality that can be used to
> > record the locks held at a certain time, and a restore
> > functionality
> > that restores the drm_exec state to the snapshot by dropping all
> > locks.
> > 
> > Snapshots can be nested if needed.
> > 
> > Cc: Christian König <christian.koenig@amd.com>
> > Cc: Somalapuram Amaranath <Amaranath.Somalapuram@amd.com>
> > Cc: Matthew Brost <matthew.brost@intel.com>
> > Cc: <dri-devel@lists.freedesktop.org>
> > Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
> > ---
> >   drivers/gpu/drm/drm_exec.c | 55
> > +++++++++++++++++++++++++++++++++++++-
> >   include/drm/drm_exec.h     | 23 +++++++++++++++-
> >   2 files changed, 76 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_exec.c
> > b/drivers/gpu/drm/drm_exec.c
> > index 1383680ffa4a..9eea5d0d3a98 100644
> > --- a/drivers/gpu/drm/drm_exec.c
> > +++ b/drivers/gpu/drm/drm_exec.c
> > @@ -57,6 +57,7 @@ static void drm_exec_unlock_all(struct drm_exec
> > *exec)
> >   	struct drm_gem_object *obj;
> >   	unsigned long index;
> >   
> > +	WARN_ON(exec->snap);
> >   	drm_exec_for_each_locked_object_reverse(exec, index, obj)
> > {
> >   		dma_resv_unlock(obj->resv);
> >   		drm_gem_object_put(obj);
> > @@ -90,6 +91,7 @@ void drm_exec_init(struct drm_exec *exec, u32
> > flags, unsigned nr)
> >   	exec->num_objects = 0;
> >   	exec->contended = DRM_EXEC_DUMMY;
> >   	exec->prelocked = NULL;
> > +	exec->snap = NULL;
> >   }
> >   EXPORT_SYMBOL(drm_exec_init);
> >   
> > @@ -301,7 +303,6 @@ int drm_exec_lock_obj(struct drm_exec *exec,
> > struct drm_gem_object *obj)
> >   		goto error_unlock;
> >   
> >   	return 0;
> > -
> >   error_unlock:
> >   	dma_resv_unlock(obj->resv);
> >   	return ret;
> > @@ -395,5 +396,57 @@ int drm_exec_prepare_array(struct drm_exec
> > *exec,
> >   }
> >   EXPORT_SYMBOL(drm_exec_prepare_array);
> >   
> > +/**
> > + * drm_exec_restore() - Restore the drm_exec state to the point of
> > a snapshot.
> > + * @exec: The drm_exec object with the state.
> > + * @snap: The snapshot state.
> > + *
> > + * Restores the drm_exec object by means of unlocking and dropping
> > references
> > + * to objects locked after the snapshot.
> > + */
> > +void drm_exec_restore(struct drm_exec *exec, struct
> > drm_exec_snapshot *snap)
> > +{
> > +	struct drm_gem_object *obj;
> > +	unsigned int index;
> > +
> > +	exec->snap = snap->saved_snap;
> > +
> > +	drm_exec_for_each_locked_object_reverse(exec, index, obj)
> > {
> > +		if (index + 1 == snap->num_locked)
> > +			break;
> > +
> > +		dma_resv_unlock(obj->resv);
> > +		drm_gem_object_put(obj);
> > +		exec->objects[index] = NULL;
> > +	}
> > +
> > +	exec->num_objects = snap->num_locked;
> > +
> > +	if (!exec->prelocked)
> > +		exec->prelocked = snap->prelocked;
> > +	else
> > +		drm_gem_object_put(snap->prelocked);
> > +}
> > +EXPORT_SYMBOL(drm_exec_restore);
> > +
> > +/**
> > + * drm_exec_snapshot() - Take a snapshot of the drm_exec state
> > + * @exec: The drm_exec object with the state.
> > + * @snap: The snapshot state.
> > + *
> > + * Records the @exec state in @snap. The @snap object is typically
> > allocated
> > + * in the stack of the caller.
> > + */
> > +void drm_exec_snapshot(struct drm_exec *exec, struct
> > drm_exec_snapshot *snap)
> > +{
> > +	snap->num_locked = exec->num_objects;
> > +	snap->prelocked = exec->prelocked;
> > +	if (snap->prelocked)
> > +		drm_gem_object_get(snap->prelocked);
> > +	snap->saved_snap = exec->snap;
> > +	exec->snap = snap;
> > +}
> > +EXPORT_SYMBOL(drm_exec_snapshot);
> > +
> >   MODULE_DESCRIPTION("DRM execution context");
> >   MODULE_LICENSE("Dual MIT/GPL");
> > diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
> > index ea0f2117ee0c..0ce4d749511b 100644
> > --- a/include/drm/drm_exec.h
> > +++ b/include/drm/drm_exec.h
> > @@ -19,7 +19,6 @@ struct drm_exec {
> >   	 * @flags: Flags to control locking behavior
> >   	 */
> >   	u32                     flags;
> > -
> >   	/**
> >   	 * @ticket: WW ticket used for acquiring locks
> >   	 */
> > @@ -49,6 +48,25 @@ struct drm_exec {
> >   	 * @prelocked: already locked GEM object due to contention
> >   	 */
> >   	struct drm_gem_object *prelocked;
> > +
> > +	/**
> > +	 * @snap: Pointer to the last snapshot taken or NULL if
> > none.
> > +	 */
> > +	struct drm_exec_snapshot *snap;
> > +};
> > +
> > +/**
> > + * struct drm_exec_snapshot - drm_exec snapshot information
> > + */
> > +struct drm_exec_snapshot {
> > +	/** @saved_snap: Pointer to the previous snapshot or NULL.
> > */
> > +	struct drm_exec_snapshot *saved_snap;
> > +
> > +	/** @prelocked: Refcounted pointer to the prelocked object
> > at snapshot time. */
> > +	struct drm_gem_object *prelocked;
> > +
> > +	/** @num_locked: Number of locked objects at snapshot
> > time. */
> > +	unsigned long num_locked;
> >   };
> >   
> >   int drm_exec_handle_contended(struct drm_exec *exec);
> > @@ -160,5 +178,8 @@ int drm_exec_prepare_array(struct drm_exec
> > *exec,
> >   			   struct drm_gem_object **objects,
> >   			   unsigned int num_objects,
> >   			   unsigned int num_fences);
> > +void drm_exec_snapshot(struct drm_exec *exec, struct
> > drm_exec_snapshot *snap);
> > +void drm_exec_restore(struct drm_exec *exec, struct
> > drm_exec_snapshot *snap);
> > +
> >   
> >   #endif
>