On 23/11/2021 19:52, Rodrigo Vivi wrote:
On Tue, Nov 23, 2021 at 09:39:25AM +0000, Tvrtko Ursulin wrote:
On 17/11/2021 22:49, Vinay Belgaumkar wrote:
From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Everytime we come to the end of a virtual engine's context, re-randomise
it's siblings[]. As we schedule the siblings' tasklets in the order they
are in the array, earlier entries are executed first (when idle) and so
will be preferred when scheduling the next virtual request. Currently,
we only update the array when switching onto a new idle engine, so we
prefer to stick on the last execute engine, keeping the work compact.
However, it can be beneficial to spread the work out across idle
engines, so choose another sibling as our preferred target at the end of
the context's execution.
This partially brings back, from a different angle, the more dynamic
scheduling behavior which has been lost since bugfix 90a987205c6c
("drm/i915/gt: Only swap to a random sibling once upon creation").
Shouldn't we use the Fixes tag here since this is targeting to fix one
of the performance regressions of this patch?
Probably not but hard to say. Note that it wasn't a performance
regression that was reported but power.
And to go back to what we said elsewhere in the thread, I am actually
with you in thinking that in the ideal world we need PnP testing across
a variety of workloads and platforms. And "in the ideal world" should
really be in the normal world. It is not professional to be reactive to
isolated bug reports from users, without being able to see the overall
picture.
One day we could experiment with using engine busyness as criteria (instead
of random). Back in the day busyness was kind of the best strategy, although
sampled at submit, not at the trailing edge like here, but it still may be
able to settle down to engine configuration better in some scenarios. Only
testing could say.
Still, from memory random also wasn't that bad so this should be okay for
now.
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Since you reviewed and it looks to be a middle ground point in terms
of when to balancing (always like in the initial implementation vs
only once like the in 90a987205c6c).
If this one is really fixing the regression by itself:
Acked-by: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
on this patch here.
But I still don't want to take the risk with touching the freq with
race to idle, until not convinced that it is absolutely needed and
that we are not breaking the world out there.
Yes agreed in principle, we have users with different priorities.
However the RPS patches in the series, definitely the 1st one which
looks at classes versus individual engines, sound plausible to me. Given
the absence of automated PnP testing mentioned above, in the past it was
usually Chris who was making the above and beyond effort to evaluate
changes like these on as many platforms as he could, and with different
workloads. Not sure who has the mandate and drive to fill that space but
something will need to happen.
Regards,
Tvrtko
Regards,
Tvrtko
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Vinay Belgaumkar <vinay.belgaumkar@xxxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
---
.../drm/i915/gt/intel_execlists_submission.c | 80 ++++++++++++-------
1 file changed, 52 insertions(+), 28 deletions(-)
diff --git a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
index ca03880fa7e4..b95bbc8fb91a 100644
--- a/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
+++ b/drivers/gpu/drm/i915/gt/intel_execlists_submission.c
@@ -539,6 +539,41 @@ static void execlists_schedule_in(struct i915_request *rq, int idx)
GEM_BUG_ON(intel_context_inflight(ce) != rq->engine);
}
+static void virtual_xfer_context(struct virtual_engine *ve,
+ struct intel_engine_cs *engine)
+{
+ unsigned int n;
+
+ if (likely(engine == ve->siblings[0]))
+ return;
+
+ if (!intel_engine_has_relative_mmio(engine))
+ lrc_update_offsets(&ve->context, engine);
+
+ /*
+ * Move the bound engine to the top of the list for
+ * future execution. We then kick this tasklet first
+ * before checking others, so that we preferentially
+ * reuse this set of bound registers.
+ */
+ for (n = 1; n < ve->num_siblings; n++) {
+ if (ve->siblings[n] == engine) {
+ swap(ve->siblings[n], ve->siblings[0]);
+ break;
+ }
+ }
+}
+
+static int ve_random_sibling(struct virtual_engine *ve)
+{
+ return prandom_u32_max(ve->num_siblings);
+}
+
+static int ve_random_other_sibling(struct virtual_engine *ve)
+{
+ return 1 + prandom_u32_max(ve->num_siblings - 1);
+}
+
static void
resubmit_virtual_request(struct i915_request *rq, struct virtual_engine *ve)
{
@@ -578,8 +613,23 @@ static void kick_siblings(struct i915_request *rq, struct intel_context *ce)
rq->execution_mask != engine->mask)
resubmit_virtual_request(rq, ve);
- if (READ_ONCE(ve->request))
+ /*
+ * Reschedule with a new "preferred" sibling.
+ *
+ * The tasklets are executed in the order of ve->siblings[], so
+ * siblings[0] receives preferrential treatment of greedily checking
+ * for execution of the virtual engine. At this point, the virtual
+ * engine is no longer in the current GPU cache due to idleness or
+ * contention, so it can be executed on any without penalty. We
+ * re-randomise at this point in order to spread light loads across
+ * the system, heavy overlapping loads will continue to be greedily
+ * executed by the first available engine.
+ */
+ if (READ_ONCE(ve->request)) {
+ virtual_xfer_context(ve,
+ ve->siblings[ve_random_other_sibling(ve)]);
tasklet_hi_schedule(&ve->base.sched_engine->tasklet);
+ }
}
static void __execlists_schedule_out(struct i915_request * const rq,
@@ -1030,32 +1080,6 @@ first_virtual_engine(struct intel_engine_cs *engine)
return NULL;
}
-static void virtual_xfer_context(struct virtual_engine *ve,
- struct intel_engine_cs *engine)
-{
- unsigned int n;
-
- if (likely(engine == ve->siblings[0]))
- return;
-
- GEM_BUG_ON(READ_ONCE(ve->context.inflight));
- if (!intel_engine_has_relative_mmio(engine))
- lrc_update_offsets(&ve->context, engine);
-
- /*
- * Move the bound engine to the top of the list for
- * future execution. We then kick this tasklet first
- * before checking others, so that we preferentially
- * reuse this set of bound registers.
- */
- for (n = 1; n < ve->num_siblings; n++) {
- if (ve->siblings[n] == engine) {
- swap(ve->siblings[n], ve->siblings[0]);
- break;
- }
- }
-}
-
static void defer_request(struct i915_request *rq, struct list_head * const pl)
{
LIST_HEAD(list);
@@ -3590,7 +3614,7 @@ static void virtual_engine_initial_hint(struct virtual_engine *ve)
* NB This does not force us to execute on this engine, it will just
* typically be the first we inspect for submission.
*/
- swp = prandom_u32_max(ve->num_siblings);
+ swp = ve_random_sibling(ve);
if (swp)
swap(ve->siblings[swp], ve->siblings[0]);
}
