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

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Message-ID: <f67e508d-5d40-596e-9d98-55b9ce2c8f8e@redhat.com>
Date: Fri, 11 Oct 2024 11:09:39 -0400 (EDT)
From: John Kacur <jkacur@...hat.com>
To: Tomas Glozar <tglozar@...hat.com>
cc: Steven Rostedt <rostedt@...dmis.org>, linux-trace-kernel@...r.kernel.org, 
    LKML <linux-kernel@...r.kernel.org>, 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, 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>
> ---
>  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
> 
> 
> 
Reviewed by: John Kacur <jkacur@...hat.com>
Tested by: John Kacur <jkacur@...hat.com>