linux-kernel - [PATCH 1/2] rtla/timerlat: Make timerlat_top_cpu->*

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Message-ID: <20241011121015.2868751-1-tglozar@redhat.com>
Date: Fri, 11 Oct 2024 14:10:14 +0200
From: tglozar@...hat.com
To: rostedt@...dmis.org
Cc: linux-trace-kernel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	jkacur@...hat.com,
	lgoncalv@...hat.com,
	Tomas Glozar <tglozar@...hat.com>,
	Attila Fazekas <afazekas@...hat.com>
Subject: [PATCH 1/2] rtla/timerlat: Make timerlat_top_cpu->*_count unsigned long long

From: Tomas Glozar <tglozar@...hat.com>

Most fields of struct timerlat_top_cpu are unsigned long long, but the
fields {irq,thread,user}_count are int (32-bit signed).

This leads to overflow when tracing on a large number of CPUs for a long
enough time:
$ rtla timerlat top -a20 -c 1-127 -d 12h
...
  0 12:00:00   |          IRQ Timer Latency (us)        |         Thread Timer Latency (us)
CPU COUNT      |      cur       min       avg       max |      cur       min       avg       max
 1 #43200096  |        0         0         1         2 |        3         2         6        12
...
127 #43200096  |        0         0         1         2 |        3         2         5        11
ALL #119144 e4 |                  0         5         4 |                  2        28        16

The average latency should be 0-1 for IRQ and 5-6 for thread, but is
reported as 5 and 28, about 4 to 5 times more, due to the count
overflowing when summed over all CPUs: 43200096 * 127 = 5486412192,
however, 1191444898 (= 5486412192 mod MAX_INT) is reported instead, as
seen on the last line of the output, and the averages are thus ~4.6
times higher than they should be (5486412192 / 1191444898 = ~4.6).

Fix the issue by changing {irq,thread,user}_count fields to unsigned
long long, similarly to other fields in struct timerlat_top_cpu and to
the count variable in timerlat_top_print_sum.

Reported-by: Attila Fazekas <afazekas@...hat.com>
Signed-off-by: Tomas Glozar <tglozar@...hat.com>
---
 tools/tracing/rtla/src/timerlat_top.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/tracing/rtla/src/timerlat_top.c b/tools/tracing/rtla/src/timerlat_top.c
index 210b0f533534..ee7c291fc9bb 100644
--- a/tools/tracing/rtla/src/timerlat_top.c
+++ b/tools/tracing/rtla/src/timerlat_top.c
@@ -54,9 +54,9 @@ struct timerlat_top_params {
 };
 
 struct timerlat_top_cpu {
-	int			irq_count;
-	int			thread_count;
-	int			user_count;
+	unsigned long long	irq_count;
+	unsigned long long	thread_count;
+	unsigned long long	user_count;
 
 	unsigned long long	cur_irq;
 	unsigned long long	min_irq;
@@ -280,7 +280,7 @@ static void timerlat_top_print(struct osnoise_tool *top, int cpu)
 	/*
 	 * Unless trace is being lost, IRQ counter is always the max.
 	 */
-	trace_seq_printf(s, "%3d #%-9d |", cpu, cpu_data->irq_count);
+	trace_seq_printf(s, "%3d #%-9llu |", cpu, cpu_data->irq_count);
 
 	if (!cpu_data->irq_count) {
 		trace_seq_printf(s, "%s %s %s %s |", no_value, no_value, no_value, no_value);
-- 
2.47.0