[RFC v3 11/11] drm/i915: Gate engine stats collection with a static key

Tvrtko Ursulin
2017-09-13T12:18:27Z

From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

This reduces the cost of the software engine busyness tracking
to a single no-op instruction when there are no listeners.

v2: Rebase and some comments.
v3: Rebase.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
---
drivers/gpu/drm/i915/i915_pmu.c | 54 +++++++++++++++++++++++--
drivers/gpu/drm/i915/intel_engine_cs.c | 17 ++++++++
drivers/gpu/drm/i915/intel_ringbuffer.h | 70 ++++++++++++++++++++++-----------
3 files changed, 113 insertions(+), 28 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index 3c0c5d0549b3..d734879e67ee 100644
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -460,11 +460,17 @@ static void i915_pmu_enable(struct perf_event *event)
GEM_BUG_ON(sample >= I915_PMU_SAMPLE_BITS);
GEM_BUG_ON(engine->pmu.enable_count[sample] == ~0);
if (engine->pmu.enable_count[sample]++ == 0) {
+ /*
+ * Enable engine busy stats tracking if needed or
+ * alternatively cancel the scheduled disabling of the
+ * same.
+ */
if (engine_needs_busy_stats(engine) &&
!engine->pmu.busy_stats) {
- engine->pmu.busy_stats =
- intel_enable_engine_stats(engine) == 0;
- WARN_ON_ONCE(!engine->pmu.busy_stats);
+ engine->pmu.busy_stats = true;
+ if (!cancel_delayed_work(&engine->pmu.disable_busy_stats))
+ queue_work(i915->wq,
+ &engine->pmu.enable_busy_stats);
}
}
}
@@ -507,7 +513,15 @@ static void i915_pmu_disable(struct perf_event *event)
if (!engine_needs_busy_stats(engine) &&
engine->pmu.busy_stats) {
engine->pmu.busy_stats = false;
- intel_disable_engine_stats(engine);
+ /*
+ * We request a delayed disable to handle the
+ * rapid on/off cycles on events which can
+ * happen when tools like perf stat start in a
+ * nicer way.
+ */
+ queue_delayed_work(i915->wq,
+ &engine->pmu.disable_busy_stats,
+ round_jiffies_up_relative(HZ));
}
}
}
@@ -715,9 +729,27 @@ static int i915_pmu_cpu_offline(unsigned int cpu, struct hlist_node *node)
return 0;
}

+static void __enable_busy_stats(struct work_struct *work)
+{
+ struct intel_engine_cs *engine =
+ container_of(work, typeof(*engine), pmu.enable_busy_stats);
+
+ WARN_ON_ONCE(intel_enable_engine_stats(engine));
+}
+
+static void __disable_busy_stats(struct work_struct *work)
+{
+ struct intel_engine_cs *engine =
+ container_of(work, typeof(*engine), pmu.disable_busy_stats.work);
+
+ intel_disable_engine_stats(engine);
+}
+
void i915_pmu_register(struct drm_i915_private *i915)
{
int ret;
+ struct intel_engine_cs *engine;
+ enum intel_engine_id id;

if (INTEL_GEN(i915) <= 2) {
ret = -ENOTSUPP;
@@ -751,6 +783,12 @@ void i915_pmu_register(struct drm_i915_private *i915)
i915->pmu.timer.function = i915_sample;
i915->pmu.enable = 0;

+ for_each_engine(engine, i915, id) {
+ INIT_WORK(&engine->pmu.enable_busy_stats, __enable_busy_stats);
+ INIT_DELAYED_WORK(&engine->pmu.disable_busy_stats,
+ __disable_busy_stats);
+ }
+
if (perf_pmu_register(&i915->pmu.base, "i915", -1))
i915->pmu.base.event_init = NULL;

@@ -761,6 +799,9 @@ void i915_pmu_register(struct drm_i915_private *i915)

void i915_pmu_unregister(struct drm_i915_private *i915)
{
+ struct intel_engine_cs *engine;
+ enum intel_engine_id id;
+
if (!i915->pmu.base.event_init)
return;

@@ -772,6 +813,11 @@ void i915_pmu_unregister(struct drm_i915_private *i915)

hrtimer_cancel(&i915->pmu.timer);

+ for_each_engine(engine, i915, id) {
+ flush_work(&engine->pmu.enable_busy_stats);
+ flush_delayed_work(&engine->pmu.disable_busy_stats);
+ }
+
perf_pmu_unregister(&i915->pmu.base);
i915->pmu.base.event_init = NULL;
}
diff --git a/drivers/gpu/drm/i915/intel_engine_cs.c b/drivers/gpu/drm/i915/intel_engine_cs.c
index e2152dd21b4a..e4a8eb83a018 100644
--- a/drivers/gpu/drm/i915/intel_engine_cs.c
+++ b/drivers/gpu/drm/i915/intel_engine_cs.c
@@ -21,6 +21,7 @@
* IN THE SOFTWARE.
*
*/
+#include <linux/static_key.h>

#include "i915_drv.h"
#include "intel_ringbuffer.h"
@@ -1419,6 +1420,10 @@ bool intel_engine_can_store_dword(struct intel_engine_cs *engine)
}
}

+DEFINE_STATIC_KEY_FALSE(i915_engine_stats_key);
+static DEFINE_MUTEX(i915_engine_stats_mutex);
+static int i915_engine_stats_ref;
+
/**
* intel_enable_engine_stats() - Enable engine busy tracking on engine
* @engine: engine to enable stats collection
@@ -1434,16 +1439,24 @@ int intel_enable_engine_stats(struct intel_engine_cs *engine)
if (!i915.enable_execlists)
return -ENODEV;

+ mutex_lock(&i915_engine_stats_mutex);
+
spin_lock_irqsave(&engine->stats.lock, flags);
if (engine->stats.enabled == ~0)
goto busy;
engine->stats.enabled++;
spin_unlock_irqrestore(&engine->stats.lock, flags);

+ if (i915_engine_stats_ref++ == 0)
+ static_branch_enable(&i915_engine_stats_key);
+
+ mutex_unlock(&i915_engine_stats_mutex);
+
return 0;

busy:
spin_unlock_irqrestore(&engine->stats.lock, flags);
+ mutex_unlock(&i915_engine_stats_mutex);

return -EBUSY;
}
@@ -1461,6 +1474,7 @@ void intel_disable_engine_stats(struct intel_engine_cs *engine)
if (!i915.enable_execlists)
return;

+ mutex_lock(&i915_engine_stats_mutex);
spin_lock_irqsave(&engine->stats.lock, flags);
WARN_ON_ONCE(engine->stats.enabled == 0);
if (--engine->stats.enabled == 0) {
@@ -1468,6 +1482,9 @@ void intel_disable_engine_stats(struct intel_engine_cs *engine)
engine->stats.start = engine->stats.total = 0;
}
spin_unlock_irqrestore(&engine->stats.lock, flags);
+ if (--i915_engine_stats_ref == 0)
+ static_branch_disable(&i915_engine_stats_key);
+ mutex_unlock(&i915_engine_stats_mutex);
}

/**
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index fe554fc76867..65dea686fc7c 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -269,6 +269,22 @@ struct intel_engine_cs {
* @busy_stats: Has enablement of engine stats tracking been requested.
*/
bool busy_stats;
+ /**
+ * @enable_busy_stats: Work item for engine busy stats enabling.
+ *
+ * Since the action can sleep it needs to be decoupled from the
+ * perf API callback.
+ */
+ struct work_struct enable_busy_stats;
+ /**
+ * @disable_busy_stats: Work item for busy stats disabling.
+ *
+ * Same as with @enable_busy_stats action, with the difference
+ * that we delay it in case there are rapid enable-disable
+ * actions, which can happen during tool startup (like perf
+ * stat).
+ */
+ struct delayed_work disable_busy_stats;
} pmu;

/*
@@ -794,48 +810,54 @@ bool intel_engine_can_store_dword(struct intel_engine_cs *engine);
struct intel_engine_cs *
intel_engine_lookup_user(struct drm_i915_private *i915, u8 class, u8 instance);

+DECLARE_STATIC_KEY_FALSE(i915_engine_stats_key);
+
static inline void intel_engine_context_in(struct intel_engine_cs *engine)
{
unsigned long flags;

- if (READ_ONCE(engine->stats.enabled) == 0)
- return;
+ if (static_branch_unlikely(&i915_engine_stats_key)) {
+ if (READ_ONCE(engine->stats.enabled) == 0)
+ return;

- spin_lock_irqsave(&engine->stats.lock, flags);
+ spin_lock_irqsave(&engine->stats.lock, flags);

- if (engine->stats.enabled > 0) {
- if (engine->stats.ref++ == 0)
- engine->stats.start = ktime_get();
- GEM_BUG_ON(engine->stats.ref == 0);
- }
+ if (engine->stats.enabled > 0) {
+ if (engine->stats.ref++ == 0)
+ engine->stats.start = ktime_get();
+ GEM_BUG_ON(engine->stats.ref == 0);
+ }

- spin_unlock_irqrestore(&engine->stats.lock, flags);
+ spin_unlock_irqrestore(&engine->stats.lock, flags);
+ }
}

static inline void intel_engine_context_out(struct intel_engine_cs *engine)
{
unsigned long flags;

- if (READ_ONCE(engine->stats.enabled) == 0)
- return;
+ if (static_branch_unlikely(&i915_engine_stats_key)) {
+ if (READ_ONCE(engine->stats.enabled) == 0)
+ return;

- spin_lock_irqsave(&engine->stats.lock, flags);
+ spin_lock_irqsave(&engine->stats.lock, flags);

- if (engine->stats.enabled > 0) {
- /*
- * After turning on engine stats, context out might be the
- * first event which then needs to be ignored (ref == 0).
- */
- if (engine->stats.ref && --engine->stats.ref == 0) {
- ktime_t last = ktime_sub(ktime_get(),
- engine->stats.start);
+ if (engine->stats.enabled > 0) {
+ /*
+ * After turning on engine stats, context out might be
+ * the first event which then needs to be ignored.
+ */
+ if (engine->stats.ref && --engine->stats.ref == 0) {
+ ktime_t last = ktime_sub(ktime_get(),
+ engine->stats.start);

- engine->stats.total = ktime_add(engine->stats.total,
- last);
+ engine->stats.total =
+ ktime_add(engine->stats.total, last);
+ }
}
- }

- spin_unlock_irqrestore(&engine->stats.lock, flags);
+ spin_unlock_irqrestore(&engine->stats.lock, flags);
+ }
}

int intel_enable_engine_stats(struct intel_engine_cs *engine);
--
2.9.5