[PATCH 5/5] perf top: check the latency of perf_top__mmap_read

[kan.liang@intel.com]

From: Kan Liang <kan.liang@intel.com>

The latency of perf_top__mmap_read should be lower than refresh time.
If not, give some hints to reduce the latency.

Signed-off-by: Kan Liang <kan.liang@intel.com>
---
 tools/perf/builtin-top.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index 721d786..f36936c 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -872,13 +872,23 @@ static void perf_top__mmap_read_idx(struct perf_top *top, int idx)
 
 static void perf_top__mmap_read(struct perf_top *top)
 {
+	unsigned long long start, end;
 	int i;
 
+	start = rdclock();
 	perf_evlist__toggle_bkw_mmap(top->evlist, BKW_MMAP_DATA_PENDING);
 	for (i = 0; i < top->evlist->nr_mmaps; i++)
 		perf_top__mmap_read_idx(top, i);
 	perf_evlist__toggle_bkw_mmap(top->evlist, BKW_MMAP_EMPTY);
 	perf_evlist__toggle_bkw_mmap(top->evlist, BKW_MMAP_RUNNING);
+	end = rdclock();
+
+	if ((end - start) > (unsigned long long)top->delay_secs * NSEC_PER_SEC)
+		ui__warning("Too slow to read ring buffer.\n"
+			    "Please try increasing the period (-c) or\n"
+			    "decreasing the freq (-F) or\n"
+			    "limiting the number of CPUs (-C)\n");
+
 }
 
 static int perf_top__start_counters(struct perf_top *top)
-- 
2.5.5