Re: [Intel-gfx] [PATCH 7/8] drm/i915/gt: Drop mutex serialisation between context pin/unpin

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 06-01-2020 om 11:22 schreef Chris Wilson:
> The last remaining reason for serialising the pin/unpin of the
> intel_context is to ensure that our preallocated wakerefs are not
> consumed too early (i.e. the unpin of the previous phase does not emit
> the idle barriers for this phase before we even submit). All of the
> other operations within the context pin/unpin are supposed to be
> atomic...  Therefore, we can reduce the serialisation to being just on
> the i915_active.preallocated_barriers itself and drop the nested
> pin_mutex from intel_context_unpin().
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_context.c | 18 +++++-------------
>  drivers/gpu/drm/i915/i915_active.c      | 19 +++++++++++++++----
>  2 files changed, 20 insertions(+), 17 deletions(-)

For whole series, except 5 and 6:

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

For 5 and 6, I think they look sane but I'm not the right person to review. :)

> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> index 4d0bc1478ccd..34ec958d400e 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.c
> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> @@ -86,22 +86,14 @@ int __intel_context_do_pin(struct intel_context *ce)
>  
>  void intel_context_unpin(struct intel_context *ce)
>  {
> -	if (likely(atomic_add_unless(&ce->pin_count, -1, 1)))
> +	if (!atomic_dec_and_test(&ce->pin_count))
>  		return;
>  
> -	/* We may be called from inside intel_context_pin() to evict another */
> -	intel_context_get(ce);
> -	mutex_lock_nested(&ce->pin_mutex, SINGLE_DEPTH_NESTING);
> -
> -	if (likely(atomic_dec_and_test(&ce->pin_count))) {
> -		CE_TRACE(ce, "retire\n");
> +	CE_TRACE(ce, "unpin\n");
> +	ce->ops->unpin(ce);
>  
> -		ce->ops->unpin(ce);
> -
> -		intel_context_active_release(ce);
> -	}
> -
> -	mutex_unlock(&ce->pin_mutex);
> +	intel_context_get(ce);
> +	intel_context_active_release(ce);
>  	intel_context_put(ce);
>  }
>  
Might want to put a comment here why intel_context_get is needed?
> diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
> index cfe09964622b..f3da5c06f331 100644
> --- a/drivers/gpu/drm/i915/i915_active.c
> +++ b/drivers/gpu/drm/i915/i915_active.c
> @@ -605,12 +605,15 @@ int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
>  					    struct intel_engine_cs *engine)
>  {
>  	intel_engine_mask_t tmp, mask = engine->mask;
> +	struct llist_node *pos = NULL, *next;
>  	struct intel_gt *gt = engine->gt;
> -	struct llist_node *pos, *next;
>  	int err;
>  
>  	GEM_BUG_ON(i915_active_is_idle(ref));
> -	GEM_BUG_ON(!llist_empty(&ref->preallocated_barriers));
> +
> +	/* Wait until the previous preallocation is completed */
> +	while (!llist_empty(&ref->preallocated_barriers))
> +		cond_resched();
>  
>  	/*
>  	 * Preallocate a node for each physical engine supporting the target
> @@ -653,16 +656,24 @@ int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
>  		GEM_BUG_ON(rcu_access_pointer(node->base.fence) != ERR_PTR(-EAGAIN));
>  
>  		GEM_BUG_ON(barrier_to_engine(node) != engine);
> -		llist_add(barrier_to_ll(node), &ref->preallocated_barriers);
> +		next = barrier_to_ll(node);
> +		next->next = pos;
> +		if (!pos)
> +			pos = next;
>  		intel_engine_pm_get(engine);
>  	}
>  
> +	GEM_BUG_ON(!llist_empty(&ref->preallocated_barriers));
> +	llist_add_batch(next, pos, &ref->preallocated_barriers);
> +
>  	return 0;
>  
>  unwind:
> -	llist_for_each_safe(pos, next, take_preallocated_barriers(ref)) {
> +	while (pos) {
>  		struct active_node *node = barrier_from_ll(pos);
>  
> +		pos = pos->next;
> +
>  		atomic_dec(&ref->count);
>  		intel_engine_pm_put(barrier_to_engine(node));
>  


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