[PATCH v10 07/10] drm/i915: Gate engine stats collection with a static key

Tvrtko Ursulin
2017-10-05T13:05:34Z

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

We add a new i915 ordered workqueue to be used only for tasks
not needing struct mutex.

v2: Rebase and some comments.
v3: Rebase.
v4: Checkpatch fixes.
v5: Rebase.
v6: Use system_long_wq to avoid being blocked by struct_mutex
users.
v7: Fix bad conflict resolution from last rebase. (Dmitry Rogozhkin)
v8: Rebase.
v9:
* Fix race between unordered enable followed by disable.
(Chris Wilson)
* Prettify order of local variable declarations. (Chris Wilson)
v10:
Chris Wilson:
* Fix workqueue allocation failure handling.
* Expand comment about critical workqueue ordering.
* Add some GEM_BUG_ONs to document impossible conditions.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
---
drivers/gpu/drm/i915/i915_drv.c | 13 +++-
drivers/gpu/drm/i915/i915_drv.h | 5 ++
drivers/gpu/drm/i915/i915_pmu.c | 65 ++++++++++++++++++--
drivers/gpu/drm/i915/intel_engine_cs.c | 17 ++++++
drivers/gpu/drm/i915/intel_ringbuffer.h | 101 ++++++++++++++++++++------------
5 files changed, 158 insertions(+), 43 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index d373a840566a..3853038c6fdd 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -804,12 +804,22 @@ static int i915_workqueues_init(struct drm_i915_private *dev_priv)
if (dev_priv->wq == NULL)
goto out_err;

+ /*
+ * Ordered workqueue for tasks not needing the global mutex but
+ * which still need to be serialized.
+ */
+ dev_priv->wq_misc = alloc_ordered_workqueue("i915-misc", 0);
+ if (dev_priv->wq_misc == NULL)
+ goto out_free_wq;
+
dev_priv->hotplug.dp_wq = alloc_ordered_workqueue("i915-dp", 0);
if (dev_priv->hotplug.dp_wq == NULL)
- goto out_free_wq;
+ goto out_free_wq_misc;

return 0;

+out_free_wq_misc:
+ destroy_workqueue(dev_priv->wq_misc);
out_free_wq:
destroy_workqueue(dev_priv->wq);
out_err:
@@ -830,6 +840,7 @@ static void i915_engines_cleanup(struct drm_i915_private *i915)
static void i915_workqueues_cleanup(struct drm_i915_private *dev_priv)
{
destroy_workqueue(dev_priv->hotplug.dp_wq);
+ destroy_workqueue(dev_priv->wq_misc);
destroy_workqueue(dev_priv->wq);
}

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 2dabe671bb50..efa3a8423cca 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2349,6 +2349,11 @@ struct drm_i915_private {
*/
struct workqueue_struct *wq;

+ /**
+ * @wq_misc: Workqueue for serialized tasks not needing struct mutex.
+ */
+ struct workqueue_struct *wq_misc;
+
/* Display functions */
struct drm_i915_display_funcs display;

diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
index 2671e3640639..d552e415becd 100644
--- a/drivers/gpu/drm/i915/i915_pmu.c
+++ b/drivers/gpu/drm/i915/i915_pmu.c
@@ -458,11 +458,20 @@ 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 the delayed disable was pending, cancel it and in
+ * this case no need to queue a new enable.
+ */
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_misc,
+ &engine->pmu.enable_busy_stats);
}
}
}
@@ -505,7 +514,22 @@ 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.
+ *
+ * PMU enable and disable callbacks are
+ * serialized with the spinlock, and since we do
+ * them in decoupled fashion from workers, it is
+ * critical to use an ordered work queue to
+ * preserve this ordering between the two
+ * events.
+ */
+ queue_delayed_work(i915->wq_misc,
+ &engine->pmu.disable_busy_stats,
+ round_jiffies_up_relative(HZ));
}
}
}
@@ -689,8 +713,26 @@ 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)
{
+ struct intel_engine_cs *engine;
+ enum intel_engine_id id;
int ret;

if (INTEL_GEN(i915) <= 2) {
@@ -725,6 +767,12 @@ 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);
+ }
+
ret = perf_pmu_register(&i915->pmu.base, "i915", -1);
if (ret == 0)
return;
@@ -743,6 +791,9 @@ 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;

@@ -754,6 +805,12 @@ void i915_pmu_unregister(struct drm_i915_private *i915)

hrtimer_cancel(&i915->pmu.timer);

+ for_each_engine(engine, i915, id) {
+ GEM_BUG_ON(engine->pmu.busy_stats);
+ GEM_BUG_ON(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 53d3b8f3c222..cbfde1149822 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"
@@ -1653,6 +1654,10 @@ intel_engine_lookup_user(struct drm_i915_private *i915, u8 class, u8 instance)
return i915->engine_class[class][instance];
}

+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
@@ -1668,6 +1673,8 @@ int intel_enable_engine_stats(struct intel_engine_cs *engine)
if (!i915_modparams.enable_execlists)
return -ENODEV;

+ mutex_lock(&i915_engine_stats_mutex);
+
spin_lock_irqsave(&engine->stats.lock, flags);
if (engine->stats.enabled == ~0)
goto busy;
@@ -1675,10 +1682,16 @@ int intel_enable_engine_stats(struct intel_engine_cs *engine)
engine->stats.enabled_at = ktime_get();
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;
}
@@ -1696,6 +1709,7 @@ void intel_disable_engine_stats(struct intel_engine_cs *engine)
if (!i915_modparams.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) {
@@ -1705,6 +1719,9 @@ void intel_disable_engine_stats(struct intel_engine_cs *engine)
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 e61ab4f3dc90..adcc79d2e414 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -361,6 +361,22 @@ struct intel_engine_cs {
* 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;

/*
@@ -902,59 +918,68 @@ 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.active++ == 0)
- engine->stats.start = ktime_get();
- GEM_BUG_ON(engine->stats.active == 0);
- }
+ if (engine->stats.enabled > 0) {
+ if (engine->stats.active++ == 0)
+ engine->stats.start = ktime_get();
+ GEM_BUG_ON(engine->stats.active == 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;
-
- spin_lock_irqsave(&engine->stats.lock, flags);
-
- if (engine->stats.enabled > 0) {
- ktime_t last, now = ktime_get();
-
- if (engine->stats.active && --engine->stats.active == 0) {
- /*
- * Decrement the active context count and in case GPU
- * is now idle add up to the running total.
- */
- last = ktime_sub(now, engine->stats.start);
-
- engine->stats.total = ktime_add(engine->stats.total,
- last);
- } else if (engine->stats.active == 0) {
- /*
- * After turning on engine stats, context out might be
- * the first event in which case we account from the
- * time stats gathering was turned on.
- */
- last = ktime_sub(now, engine->stats.enabled_at);
-
- engine->stats.total = ktime_add(engine->stats.total,
- last);
+ if (static_branch_unlikely(&i915_engine_stats_key)) {
+ if (READ_ONCE(engine->stats.enabled) == 0)
+ return;
+
+ spin_lock_irqsave(&engine->stats.lock, flags);
+
+ if (engine->stats.enabled > 0) {
+ ktime_t last, now = ktime_get();
+
+ if (engine->stats.active &&
+ --engine->stats.active == 0) {
+ /*
+ * Decrement the active context count and in
+ * case GPU is now idle add up to the running
+ * total.
+ */
+ last = ktime_sub(now, engine->stats.start);
+
+ engine->stats.total =
+ ktime_add(engine->stats.total, last);
+ } else if (engine->stats.active == 0) {
+ /*
+ * After turning on engine stats, context out
+ * might be the first event in which case we
+ * account from the time stats gathering was
+ * turned on.
+ */
+ last = ktime_sub(now, engine->stats.enabled_at);
+
+ 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);