drm/i915/execlists: Pull tasklet interrupt-bh local to direct submission

We dropped calling process_csb prior to handling direct submission in
order to avoid the nesting of spinlocks and lift process_csb() and the
majority of the tasklet out of irq-off. However, we do want to avoid
ksoftirqd latency in the fast path, so try and pull the interrupt-bh
local to direct submission if we can acquire the tasklet's lock.

v2: Document the read of pending[0] from outside the tasklet with
READ_ONCE.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200325120227.8044-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index f88d3b95c4e102900f79ab9f730d733ef1c2962d..d49baade09862ebfe9458acc8eaa040332def96d 100644 (file)
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -2891,6 +2891,13 @@ static void __submit_queue_imm(struct intel_engine_cs *engine)
        if (reset_in_progress(execlists))
                return; /* defer until we restart the engine following reset */
 
+       /* Hopefully we clear execlists->pending[] to let us through */
+       if (READ_ONCE(execlists->pending[0]) &&
+           tasklet_trylock(&execlists->tasklet)) {
+               process_csb(engine);
+               tasklet_unlock(&execlists->tasklet);
+       }
+
        __execlists_submission_tasklet(engine);
 }