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>

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>
Steven Rostedt (Red Hat) · Steven Rostedt
1 parent 72e2fe38ea
Showing 1 changed file with 30 additions and 7 deletions Side-by-side Diff
kernel/trace/trace_benchmark.c
@@ -16,7 +16,10 @@
 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 @@
  
 	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 @@
 	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)
  
@@ -170,7 +190,10 @@
 	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;
 }
...	...	@@ -16,7 +16,10 @@
16	16	static u64 bm_max;
17	17	static u64 bm_min;
18	18	static u64 bm_first;
19		-static s64 bm_cnt;
	19	+static u64 bm_cnt;
	20	+static u64 bm_stddev;
	21	+static unsigned int bm_avg;
	22	+static unsigned int bm_std;
20	23
21	24	/*
22	25	* This gets called in a loop recording the time it took to write
23	26
24	27
...	...	@@ -66,22 +69,35 @@
66	69
67	70	bm_last = delta;
68	71
69		- bm_total += delta;
70		- bm_totalsq += delta * delta;
71		-
72	72	if (delta > bm_max)
73	73	bm_max = delta;
74	74	if (!bm_min \|\| delta < bm_min)
75	75	bm_min = delta;
76	76
	77	+ /*
	78	+ * When bm_cnt is greater than UINT_MAX, it breaks the statistics
	79	+ * accounting. Freeze the statistics when that happens.
	80	+ * We should have enough data for the avg and stddev anyway.
	81	+ */
	82	+ if (bm_cnt > UINT_MAX) {
	83	+ scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
	84	+ "last=%llu first=%llu max=%llu min=%llu ** avg=%u std=%d std^2=%lld",
	85	+ bm_last, bm_first, bm_max, bm_min, bm_avg, bm_std, bm_stddev);
	86	+ return;
	87	+ }
	88	+
	89	+ bm_total += delta;
	90	+ bm_totalsq += delta * delta;
	91	+
	92	+
77	93	if (bm_cnt > 1) {
78	94	/*
79	95	* Apply Welford's method to calculate standard deviation:
80	96	* s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
81	97	*/
82	98	stddev = (u64)bm_cnt * bm_totalsq - bm_total * bm_total;
83		- do_div(stddev, bm_cnt);
84		- do_div(stddev, bm_cnt - 1);
	99	+ do_div(stddev, (u32)bm_cnt);
	100	+ do_div(stddev, (u32)bm_cnt - 1);
85	101	} else
86	102	stddev = 0;
87	103
...	...	@@ -119,6 +135,10 @@
119	135	scnprintf(bm_str, BENCHMARK_EVENT_STRLEN,
120	136	"last=%llu first=%llu max=%llu min=%llu avg=%u std=%d std^2=%lld",
121	137	bm_last, bm_first, bm_max, bm_min, avg, std, stddev);
	138	+
	139	+ bm_std = std;
	140	+ bm_avg = avg;
	141	+ bm_stddev = stddev;
122	142	}
123	143
124	144	static int benchmark_event_kthread(void *arg)
125	145
...	...	@@ -170,7 +190,10 @@
170	190	bm_max = 0;
171	191	bm_min = 0;
172	192	bm_cnt = 0;
173		- /* bm_first doesn't need to be reset but reset it anyway */
	193	+ /* These don't need to be reset but reset them anyway */
174	194	bm_first = 0;
	195	+ bm_std = 0;
	196	+ bm_avg = 0;
	197	+ bm_stddev = 0;
175	198	}