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

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

[Steven Rostedt]

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.

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

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

[Masami Hiramatsu]

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>