Re: [PATCH] ring-buffer: Show what clock function is used on timestamp errors

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 23 Mar 2026 20:22:12 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> The testing for tracing was triggering a timestamp count issue that was
> always off by one. This has been happening for some time but has never
> been reported by anyone else. It was finally discovered to be an issue
> with the "uptime" (jiffies) clock that happened to be traced and the
> internal recursion caused the discrepancy. This would have been much
> easier to solve if the clock function being used was displayed when the
> error was detected.
> 
> Add the clock function to the error output.

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/ring_buffer.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 170170bd83bd..a99d1c7d180b 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -4435,18 +4435,20 @@ static void check_buffer(struct ring_buffer_per_cpu *cpu_buffer,
>  	ret = rb_read_data_buffer(bpage, tail, cpu_buffer->cpu, &ts, &delta);
>  	if (ret < 0) {
>  		if (delta < ts) {
> -			buffer_warn_return("[CPU: %d]ABSOLUTE TIME WENT BACKWARDS: last ts: %lld absolute ts: %lld\n",
> -					   cpu_buffer->cpu, ts, delta);
> +			buffer_warn_return("[CPU: %d]ABSOLUTE TIME WENT BACKWARDS: last ts: %lld absolute ts: %lld clock:%pS\n",
> +					   cpu_buffer->cpu, ts, delta,
> +					   cpu_buffer->buffer->clock);
>  			goto out;
>  		}
>  	}
>  	if ((full && ts > info->ts) ||
>  	    (!full && ts + info->delta != info->ts)) {
> -		buffer_warn_return("[CPU: %d]TIME DOES NOT MATCH expected:%lld actual:%lld delta:%lld before:%lld after:%lld%s context:%s\n",
> +		buffer_warn_return("[CPU: %d]TIME DOES NOT MATCH expected:%lld actual:%lld delta:%lld before:%lld after:%lld%s context:%s\ntrace clock:%pS",
>  				   cpu_buffer->cpu,
>  				   ts + info->delta, info->ts, info->delta,
>  				   info->before, info->after,
> -				   full ? " (full)" : "", show_interrupt_level());
> +				   full ? " (full)" : "", show_interrupt_level(),
> +				   cpu_buffer->buffer->clock);
>  	}
>  out:
>  	atomic_dec(this_cpu_ptr(&checking));
> -- 
> 2.51.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>