[Intel-gfx] [PATCH 1/3] drm/i915/perf: Reduce cpu overhead for blocking perf OA reads

[Umesh Nerlige Ramappa]

A condition in wait_event_interruptible seems to be checked twice before
waiting on the event to occur. These checks are redundant when hrtimer
events will call oa_buffer_check_unlocked to update the oa_buffer tail
pointers. The redundant checks add cpu overhead. Simplify the check
to reduce cpu overhead when using blocking io to read oa buffer reports.

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
---
 drivers/gpu/drm/i915/i915_perf.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c
index 5cde3e4e7be6..e28a3ab83fde 100644
--- a/drivers/gpu/drm/i915/i915_perf.c
+++ b/drivers/gpu/drm/i915/i915_perf.c
@@ -541,6 +541,32 @@ static bool oa_buffer_check_unlocked(struct i915_perf_stream *stream)
 	return pollin;
 }
 
+/**
+ * oa_buffer_check_reports - quick check if reports are available
+ * @stream: i915 stream instance
+ *
+ * The return from this function is used as a condition for
+ * wait_event_interruptible in blocking read. This is used to detect
+ * available reports.
+ *
+ * A condition in wait_event_interruptible seems to be checked twice before
+ * waiting on an event to occur. These checks are redundant when hrtimer events
+ * will call oa_buffer_check_unlocked to update the oa_buffer tail pointers. The
+ * redundant checks add cpu overhead. We simplify the check to reduce cpu
+ * overhead.
+ */
+static bool oa_buffer_check_reports(struct i915_perf_stream *stream)
+{
+	unsigned long flags;
+	bool available;
+
+	spin_lock_irqsave(&stream->oa_buffer.ptr_lock, flags);
+	available = stream->oa_buffer.tail != stream->oa_buffer.head;
+	spin_unlock_irqrestore(&stream->oa_buffer.ptr_lock, flags);
+
+	return available;
+}
+
 /**
  * append_oa_status - Appends a status record to a userspace read() buffer.
  * @stream: An i915-perf stream opened for OA metrics
@@ -1150,7 +1176,7 @@ static int i915_oa_wait_unlocked(struct i915_perf_stream *stream)
 		return -EIO;
 
 	return wait_event_interruptible(stream->poll_wq,
-					oa_buffer_check_unlocked(stream));
+					oa_buffer_check_reports(stream));
 }
 
 /**
-- 
2.20.1