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

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Message-ID: <20241011132356.28c48902@gandalf.local.home>
Date: Fri, 11 Oct 2024 13:23:56 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: tglozar@...hat.com
Cc: linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org,
 jkacur@...hat.com, lgoncalv@...hat.com, Attila Fazekas
 <afazekas@...hat.com>
Subject: Re: [PATCH 1/2] rtla/timerlat: Make timerlat_top_cpu->*_count
 unsigned long long

On Fri, 11 Oct 2024 14:10:14 +0200
tglozar@...hat.com wrote:

> 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>

Thanks, I'm applying these, but could you or someone else create a test
directory in rtla and even rv that tests this code. I just examine it and
run some basic operations, but I have no idea if it is really working or not.

Having a utest directory or something would be really beneficial. That way,
I can at least run that test before I push it up to my tree.

-- Steve