tracing: Only calculate stats of tracepoint benchmarks for 2^32 times

When calculating the average and standard deviation, it is required that
the count be less than UINT_MAX, otherwise the do_div() will get
undefined results. After 2^32 counts of data, the average and standard
deviation should pretty much be set anyway.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 30 insertions, 7 deletions

diff --git a/kernel/trace/trace_benchmark.c b/kernel/trace/trace_benchmark.c
index 8bd3365a65b2..40a14cbcf8e0 100644
--- a/kernel/trace/trace_benchmark.c
+++ b/kernel/trace/trace_benchmark.c
@@ -16,7 +16,10 @@ static u64 bm_last;
 static u64 bm_max;
 static u64 bm_min;
 static u64 bm_first;
-static s64 bm_cnt;
+static u64 bm_cnt;
+static u64 bm_stddev;
+static unsigned int bm_avg;
+static unsigned int bm_std;
 
 /*
  * This gets called in a loop recording the time it took to write
@@ -66,22 +69,35 @@ static void trace_do_benchmark(void)
 
         bm_last = delta;
 
-        bm_total += delta;
-        bm_totalsq += delta * delta;
-
         if (delta > bm_max)
                 bm_max = delta;
         if (!bm_min || delta < bm_min)
                 bm_min = delta;
 
+        /*
+         * When bm_cnt is greater than UINT_MAX, it breaks the statistics
+         * accounting. Freeze the statistics when that happens.
+         * We should have enough data for the avg and stddev anyway.
+         */
+        if (bm_cnt > UINT_MAX) {
+                scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
+                    "last=%llu first=%llu max=%llu min=%llu ** avg=%u std=%d std^2=%lld",
+                          bm_last, bm_first, bm_max, bm_min, bm_avg, bm_std, bm_stddev);
+                return;
+        }
+
+        bm_total += delta;
+        bm_totalsq += delta * delta;
+
+
         if (bm_cnt > 1) {
                 /*
                  * Apply Welford's method to calculate standard deviation:
                  * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
                  */
                 stddev = (u64)bm_cnt * bm_totalsq - bm_total * bm_total;
-                do_div(stddev, bm_cnt);
-                do_div(stddev, bm_cnt - 1);
+                do_div(stddev, (u32)bm_cnt);
+                do_div(stddev, (u32)bm_cnt - 1);
         } else
                 stddev = 0;
 
@@ -119,6 +135,10 @@ static void trace_do_benchmark(void)
         scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
                   "last=%llu first=%llu max=%llu min=%llu avg=%u std=%d std^2=%lld",
                   bm_last, bm_first, bm_max, bm_min, avg, std, stddev);
+
+        bm_std = std;
+        bm_avg = avg;
+        bm_stddev = stddev;
 }
 
 static int benchmark_event_kthread(void *arg)
@@ -170,6 +190,9 @@ void trace_benchmark_unreg(void)
         bm_max = 0;
         bm_min = 0;
         bm_cnt = 0;
-        /* bm_first doesn't need to be reset but reset it anyway */
+        /* These don't need to be reset but reset them anyway */
         bm_first = 0;
+        bm_std = 0;
+        bm_avg = 0;
+        bm_stddev = 0;
 }