[Mesa-dev] [PATCH] i965: oa-counters: keep on reading reports until delimiting timestamp

[Robert Bragg]

On Mar 30, 2017 16:16, "Lionel Landwerlin" <lionel.g.landwerlin at intel.com>
wrote:

While exercising reading report with moderate load, we might have to
wait for all the reports to land in the OA buffer, otherwise we might
miss some reports. That means we need to keep on reading the OA stream
until the last report we read has a timestamp older that the timestamp
recorded by the MI_REPORT_PERF_COUNT at the end of the performance
query.


The expectation is that since we have received the second
MI_REPORT_PERF_COUNT report that implies any periodic/automatic reports
that the could possibly relate to a query must have been written to the
OABUFFER. Since we read() as much as is available from i915 perf when we
come to finish processing a query then we shouldn't miss anything.

We shouldn't synchronously wait in a busy loop until we've found a report
that has a timestamp >= our end MI_RPC because that could be a really
significant amount of time (e.g. 50+ milliseconds on HSW). NB. the periodic
samples are just to account for counter overflow. On Gen8+ the loop would
likely block until the next context switch happens.

Was this proposal based on a code inspection or did you maybe hit a problem
correctly measuring something?

Maybe if based on inspection we can find somewhere to put a comment to
clarify the assumptions above?

Br,
- Robert


Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Cc: Robert Bragg <robert at sixbynine.org>
---
 src/mesa/drivers/dri/i965/brw_performance_query.c | 51
++++++++++++++---------
 1 file changed, 32 insertions(+), 19 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/brw_performance_query.c
b/src/mesa/drivers/dri/i965/brw_performance_query.c
index 4a94e4b3cc..076c59e633 100644
--- a/src/mesa/drivers/dri/i965/brw_performance_query.c
+++ b/src/mesa/drivers/dri/i965/brw_performance_query.c
@@ -647,38 +647,50 @@ discard_all_queries(struct brw_context *brw)
 }

 static bool
-read_oa_samples(struct brw_context *brw)
+read_oa_samples_until(struct brw_context *brw,
+                      uint32_t start_timestamp,
+                      uint32_t end_timestamp)
 {
-   while (1) {
+   uint32_t last_timestamp = start_timestamp;
+
+   while (last_timestamp < end_timestamp) {
       struct brw_oa_sample_buf *buf = get_free_sample_buf(brw);
+      uint32_t offset;
       int len;

       while ((len = read(brw->perfquery.oa_stream_fd, buf->buf,
-                         sizeof(buf->buf))) < 0 && errno == EINTR)
+                         sizeof(buf->buf))) < 0 &&
+             (errno == EINTR || errno == EAGAIN))
          ;

       if (len <= 0) {
          exec_list_push_tail(&brw->perfquery.free_sample_buffers,
&buf->link);

-         if (len < 0) {
-            if (errno == EAGAIN)
-               return true;
-            else {
-               DBG("Error reading i915 perf samples: %m\n");
-               return false;
-            }
-         } else {
+         if (len < 0)
+            DBG("Error reading i915 perf samples: %m\n");
+         else
             DBG("Spurious EOF reading i915 perf samples\n");
-            return false;
-         }
+
+         return false;
       }

       buf->len = len;
       exec_list_push_tail(&brw->perfquery.sample_buffers, &buf->link);
+
+      /* Go through the reports and update the last timestamp. */
+      while (offset < buf->len) {
+         const struct drm_i915_perf_record_header *header =
+            (const struct drm_i915_perf_record_header *) &buf->buf[offset];
+         uint32_t *report = (uint32_t *) (header + 1);
+
+         if (header->type == DRM_I915_PERF_RECORD_SAMPLE)
+            last_timestamp = report[1];
+
+         offset += header->size;
+      }
    }

-   unreachable("not reached");
-   return false;
+   return true;
 }

 /**
@@ -709,10 +721,6 @@ accumulate_oa_reports(struct brw_context *brw,

    assert(o->Ready);

-   /* Collect the latest periodic OA reports from i915 perf */
-   if (!read_oa_samples(brw))
-      goto error;
-
    drm_intel_bo_map(obj->oa.bo, false);
    query_buffer = obj->oa.bo->virtual;

@@ -728,6 +736,11 @@ accumulate_oa_reports(struct brw_context *brw,
       goto error;
    }

+   /* Collect the latest periodic OA reports from i915 perf */
+   if (!read_oa_samples_until(brw, start[1], end[1]))
+      goto error;
+
+
    /* See if we have any periodic reports to accumulate too... */

    /* N.B. The oa.samples_head was set when the query began and
--
2.11.0
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