perf top: Check the latency of perf_top__mmap_read()

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>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1516310792-208685-18-git-send-email-kan.liang@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-top.c

@@ -889,8 +889,10 @@ static void perf_top__mmap_read(struct perf_top *top)
 {
 	bool overwrite = top->record_opts.overwrite;
 	struct perf_evlist *evlist = top->evlist;
+	unsigned long long start, end;
 	int i;
 
+	start = rdclock();
 	if (overwrite)
 		perf_evlist__toggle_bkw_mmap(evlist, BKW_MMAP_DATA_PENDING);
 
@@ -901,6 +903,13 @@ static void perf_top__mmap_read(struct perf_top *top)
 		perf_evlist__toggle_bkw_mmap(evlist, BKW_MMAP_EMPTY);
 		perf_evlist__toggle_bkw_mmap(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");
 }
 
 /*