[Intel-gfx] [PATCH 10/12] drm/i915/execlists: Reduce lock contention between schedule/submit_request

[Chris Wilson]

On Mon, May 15, 2017 at 11:51:52AM +0100, Tvrtko Ursulin wrote:
> 
> On 11/05/2017 20:59, Chris Wilson wrote:
> >If we do not require to perform priority bumping, and we haven't yet
> >submitted the request, we can update its priority in situ and skip
> >acquiring the engine locks -- thus avoiding any contention between us
> >and submit/execute.
> >
> >v2: Remove the stack element from the list if we can do the early
> >assignment.
> >
> >Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >---
> > drivers/gpu/drm/i915/intel_lrc.c | 13 +++++++++++++
> > 1 file changed, 13 insertions(+)
> >
> >diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> >index 7e41529bd074..3cfb4b145b75 100644
> >--- a/drivers/gpu/drm/i915/intel_lrc.c
> >+++ b/drivers/gpu/drm/i915/intel_lrc.c
> >@@ -767,6 +767,19 @@ static void execlists_schedule(struct drm_i915_gem_request *request, int prio)
> > 		list_safe_reset_next(dep, p, dfs_link);
> > 	}
> >
> >+	/* If we didn't need to bump any existing priorities, and we haven't
> >+	 * yet submitted this request (i.e. there is no potential race with
> >+	 * execlists_submit_request()), we can set our own priority and skip
> >+	 * acquiring the engine locks.
> >+	 */
> >+	if (request->priotree.priority == INT_MIN) {
> >+		GEM_BUG_ON(!list_empty(&request->priotree.link));
> >+		request->priotree.priority = prio;
> >+		if (stack.dfs_link.next == stack.dfs_link.prev)
> >+			return;
> >+		__list_del_entry(&stack.dfs_link);
> 
> Hm is early processing the out of queue request with no dependencies
> safe? Recently you fixed a race in this area, which AFAIK was about
> doing that outside the engine lock and then request becoming ready
> in parallel, racing with the FIFO order.

Yes. In this case we know that we have commited the request as we are in
i915_add_request (the only time we may schedule with prio == INT_MIN).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre