@@ -35,9 +35,9 @@ config DRM_I915_PREEMPT_TIMEOUT
May be 0 to disable the timeout.
-config DRM_I915_SPIN_REQUEST
- int "Busywait for request completion (us)"
- default 5 # microseconds
+config DRM_I915_MAX_REQUEST_BUSYWAIT
+ int "Busywait for request completion limit (ns)"
+ default 8000 # nanoseconds
help
Before sleeping waiting for a request (GPU operation) to complete,
we may spend some time polling for its completion. As the IRQ may
@@ -45,6 +45,9 @@ config DRM_I915_SPIN_REQUEST
check if the request will complete in the time it would have taken
us to enable the interrupt.
+ This is adjustable via
+ /sys/class/drm/card?/engine/*/max_busywait_duration_ns
+
May be 0 to disable the initial spin. In practice, we estimate
the cost of enabling the interrupt (if currently disabled) to be
a few microseconds.
@@ -313,6 +313,8 @@ static int intel_engine_setup(struct intel_gt *gt, enum intel_engine_id id)
engine->props.heartbeat_interval_ms =
CONFIG_DRM_I915_HEARTBEAT_INTERVAL;
+ engine->props.max_busywait_duration_ns =
+ CONFIG_DRM_I915_MAX_REQUEST_BUSYWAIT;
engine->props.preempt_timeout_ms =
CONFIG_DRM_I915_PREEMPT_TIMEOUT;
engine->props.stop_timeout_ms =
@@ -537,6 +537,7 @@ struct intel_engine_cs {
struct {
unsigned long heartbeat_interval_ms;
+ unsigned long max_busywait_duration_ns;
unsigned long preempt_timeout_ms;
unsigned long stop_timeout_ms;
unsigned long timeslice_duration_ms;
@@ -142,6 +142,54 @@ all_caps_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
static struct kobj_attribute all_caps_attr =
__ATTR(known_capabilities, 0444, all_caps_show, NULL);
+static ssize_t
+max_spin_store(struct kobject *kobj, struct kobj_attribute *attr,
+ const char *buf, size_t count)
+{
+ struct intel_engine_cs *engine = kobj_to_engine(kobj);
+ unsigned long long duration;
+ int err;
+
+ /*
+ * When waiting for a request, if is it currently being executed
+ * on the GPU, we busywait for a short while before sleeping. The
+ * premise is that most requests are short, and if it is already
+ * executing then there is a good chance that it will complete
+ * before we can setup the interrupt handler and go to sleep.
+ * We try to offset the cost of going to sleep, by first spinning
+ * on the request -- if it completed in less time than it would take
+ * to go sleep, process the interrupt and return back to the client,
+ * then we have saved the client some latency, albeit at the cost
+ * of spinning on an expensive CPU core.
+ *
+ * While we try to avoid waiting at all for a request that is unlikely
+ * to complete, deciding how long it is worth spinning is for is an
+ * arbitrary decision: trading off power vs latency.
+ */
+
+ err = kstrtoull(buf, 0, &duration);
+ if (err)
+ return err;
+
+ if (duration > jiffies_to_nsecs(2))
+ return -EINVAL;
+
+ WRITE_ONCE(engine->props.max_busywait_duration_ns, duration);
+
+ return count;
+}
+
+static ssize_t
+max_spin_show(struct kobject *kobj, struct kobj_attribute *attr, char *buf)
+{
+ struct intel_engine_cs *engine = kobj_to_engine(kobj);
+
+ return sprintf(buf, "%lu\n", engine->props.max_busywait_duration_ns);
+}
+
+static struct kobj_attribute max_spin_attr =
+__ATTR(max_busywait_duration_ns, 0644, max_spin_show, max_spin_store);
+
static ssize_t
timeslice_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t count)
@@ -224,6 +272,7 @@ void intel_engines_add_sysfs(struct drm_i915_private *i915)
&mmio_attr.attr,
&caps_attr.attr,
&all_caps_attr.attr,
+ &max_spin_attr.attr,
NULL
};
@@ -1371,7 +1371,7 @@ void i915_request_add(struct i915_request *rq)
mutex_unlock(&tl->mutex);
}
-static unsigned long local_clock_us(unsigned int *cpu)
+static unsigned long local_clock_ns(unsigned int *cpu)
{
unsigned long t;
@@ -1388,7 +1388,7 @@ static unsigned long local_clock_us(unsigned int *cpu)
* stop busywaiting, see busywait_stop().
*/
*cpu = get_cpu();
- t = local_clock() >> 10;
+ t = local_clock();
put_cpu();
return t;
@@ -1398,15 +1398,15 @@ static bool busywait_stop(unsigned long timeout, unsigned int cpu)
{
unsigned int this_cpu;
- if (time_after(local_clock_us(&this_cpu), timeout))
+ if (time_after(local_clock_ns(&this_cpu), timeout))
return true;
return this_cpu != cpu;
}
-static bool __i915_spin_request(const struct i915_request * const rq,
- int state, unsigned long timeout_us)
+static bool __i915_spin_request(const struct i915_request * const rq, int state)
{
+ unsigned long timeout_ns;
unsigned int cpu;
/*
@@ -1434,7 +1434,8 @@ static bool __i915_spin_request(const struct i915_request * const rq,
* takes to sleep on a request, on the order of a microsecond.
*/
- timeout_us += local_clock_us(&cpu);
+ timeout_ns = READ_ONCE(rq->engine->props.max_busywait_duration_ns);
+ timeout_ns += local_clock_ns(&cpu);
do {
if (i915_request_completed(rq))
return true;
@@ -1442,7 +1443,7 @@ static bool __i915_spin_request(const struct i915_request * const rq,
if (signal_pending_state(state, current))
break;
- if (busywait_stop(timeout_us, cpu))
+ if (busywait_stop(timeout_ns, cpu))
break;
cpu_relax();
@@ -1528,8 +1529,8 @@ long i915_request_wait(struct i915_request *rq,
* completion. That requires having a good predictor for the request
* duration, which we currently lack.
*/
- if (IS_ACTIVE(CONFIG_DRM_I915_SPIN_REQUEST) &&
- __i915_spin_request(rq, state, CONFIG_DRM_I915_SPIN_REQUEST)) {
+ if (IS_ACTIVE(CONFIG_DRM_I915_MAX_REQUEST_BUSYWAIT) &&
+ __i915_spin_request(rq, state)) {
dma_fence_signal(&rq->fence);
goto out;
}