drm/i915/execlists: Minimalistic timeslicing

If we have multiple contexts of equal priority pending execution,
activate a timer to demote the currently executing context in favour of
the next in the queue when that timeslice expires. This enforces
fairness between contexts (so long as they allow preemption -- forced
preemption, in the future, will kick those who do not obey) and allows
us to avoid userspace blocking forward progress with e.g. unbounded
MI_SEMAPHORE_WAIT.

For the starting point here, we use the jiffie as our timeslice so that
we should be reasonably efficient wrt frequent CPU wakeups.

Testcase: igt/gem_exec_scheduler/semaphore-resolve
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190620142052.19311-2-chris@chris-wilson.co.uk

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
index b4f7b81a3c3e..9940bad37812 100644
--- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
@@ -12,6 +12,7 @@
 #include <linux/kref.h>
 #include <linux/list.h>
 #include <linux/llist.h>
+#include <linux/timer.h>
 #include <linux/types.h>
 
 #include "i915_gem.h"
@@ -150,6 +151,11 @@ struct intel_engine_execlists {
 	struct tasklet_struct tasklet;
 
 	/**
+	 * @timer: kick the current context if its timeslice expires
+	 */
+	struct timer_list timer;
+
+	/**
 	 * @default_priolist: priority list for I915_PRIORITY_NORMAL
 	 */
 	struct i915_priolist default_priolist;