Re: [Intel-gfx] [PATCH v5 01/12] drm/i915: Reference objects on the ww object list

On Thu, Jun 17, 2021 at 08:30:07AM +0200, Thomas Hellström wrote:

Since the ww transaction endpoint easily end up far out-of-scope of the objects on the ww object list, particularly for contending lock objects, make sure we reference objects on the list so they don't disappear under us.

This comes with a performance penalty so it's been debated whether this is really needed. But I think this is motivated by the fact that locking is typically difficult to get right, and whatever we can do to make it simpler for developers moving forward should be done, unless the performance impact is far too high.

Signed-off-by: Thomas Hellström thomas.hellstrom@linux.intel.com Reviewed-by: Matthew Auld matthew.auld@intel.com

Acked-by: Daniel Vetter daniel.vetter@ffwll.ch

I've looked the past 2-3 weeks in-depth at our execbuf code. That has definitely gone way too far into "very clevery" territory, and safe is so much better than clever.

If there's a fundamental performance issue, we need to fix this in a fundamental way. E.g. for this one here a possible solution could be VM_BIND, at least in the fastpath, where we don't need to look-up any objects, nor refcount them, nor anything else (at least that's the goal). Only some per vm/request book-keeping and done.

Also I think we can easily claw this back once we get to the cleanup part of this work: i915_vma_pin has a bunch of atomics (and lots of locks in slow-paths) of its own, which are largely redundant now that object state is protected by dma_resv_lock. Once that's cleaned up we can pay our atomic inc/dec here with the removed atomic ops from the vma side I think.

Anyway just figured I drop some thoughts and my ack on the direction you're pushing here. -Daniel

---

drivers/gpu/drm/i915/gem/i915_gem_object.h | 8 ++++++-- drivers/gpu/drm/i915/i915_gem.c | 4 ++++ 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h index d66aa00d023a..241666931945 100644 --- a/drivers/gpu/drm/i915/gem/i915_gem_object.h +++ b/drivers/gpu/drm/i915/gem/i915_gem_object.h @@ -169,13 +169,17 @@ static inline int __i915_gem_object_lock(struct drm_i915_gem_object *obj, else ret = dma_resv_lock(obj->base.resv, ww ? &ww->ctx : NULL);

• if (!ret && ww)

• if (!ret && ww) {

• i915_gem_object_get(obj);
  

  list_add_tail(&obj->obj_link, &ww->obj_list);
• } if (ret == -EALREADY) ret = 0;

• if (ret == -EDEADLK)

• if (ret == -EDEADLK) {

• i915_gem_object_get(obj);
  

  ww->contended = obj;

• }

  return ret;

} diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index 6a0a3f0e36e1..c62dcd0e341a 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -1222,6 +1222,7 @@ static void i915_gem_ww_ctx_unlock_all(struct i915_gem_ww_ctx *ww) while ((obj = list_first_entry_or_null(&ww->obj_list, struct drm_i915_gem_object, obj_link))) { list_del(&obj->obj_link); i915_gem_object_unlock(obj);

• i915_gem_object_put(obj);
  

  }

}

@@ -1229,6 +1230,7 @@ void i915_gem_ww_unlock_single(struct drm_i915_gem_object *obj) { list_del(&obj->obj_link); i915_gem_object_unlock(obj);

• i915_gem_object_put(obj);

}

void i915_gem_ww_ctx_fini(struct i915_gem_ww_ctx *ww) @@ -1253,6 +1255,8 @@ int __must_check i915_gem_ww_ctx_backoff(struct i915_gem_ww_ctx *ww)

if (!ret) list_add_tail(&ww->contended->obj_link, &ww->obj_list);

• else

• i915_gem_object_put(ww->contended);
  
  

  ww->contended = NULL;

-- 2.31.1

---

Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx