Re: [PATCH v3] tracing: Record trace_clock and recover when reboot

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 13 May 2025 09:31:27 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Record trace_clock information in the trace_scratch area and recover
> the trace_clock when boot, so that reader can docode the timestamp
> correctly.
> Note that since most trace_clocks records the timestamp in nano-
> seconds, this is not a bug. But some trace_clock, like counter and
> tsc will record the counter value. Only for those trace_clock user
> needs this information.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  Changes in v3:
>    - Save clock_id instead of its name.
>  Changes in v2:
>    - instead of exposing it via last_boot_info, set the current
>      trace_clock as the same clock we used in the last boot.
> ---
>  kernel/trace/trace.c |   16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index cf51c30b137f..2a060c62d686 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6066,6 +6066,7 @@ struct trace_mod_entry {
>  };
>  
>  struct trace_scratch {
> +	int			clock_id;

Should be "unsigned int"

>  	unsigned long		text_addr;
>  	unsigned long		nr_entries;
>  	struct trace_mod_entry	entries[];
> @@ -6181,6 +6182,7 @@ static void update_last_data(struct trace_array *tr)
>  	if (tr->scratch) {
>  		struct trace_scratch *tscratch = tr->scratch;
>  
> +		tscratch->clock_id = tr->clock_id;
>  		memset(tscratch->entries, 0,
>  		       flex_array_size(tscratch, entries, tscratch->nr_entries));
>  		tscratch->nr_entries = 0;
> @@ -7403,6 +7405,12 @@ int tracing_set_clock(struct trace_array *tr, const char *clockstr)
>  	tracing_reset_online_cpus(&tr->max_buffer);
>  #endif
>  
> +	if (tr->scratch && !(tr->flags & TRACE_ARRAY_FL_LAST_BOOT)) {
> +		struct trace_scratch *tscratch = tr->scratch;
> +
> +		tscratch->clock_id = i;
> +	}
> +
>  	mutex_unlock(&trace_types_lock);
>  
>  	return 0;
> @@ -9628,6 +9636,14 @@ static void setup_trace_scratch(struct trace_array *tr,
>  
>  	/* Scan modules to make text delta for modules. */
>  	module_for_each_mod(make_mod_delta, tr);
> +
> +	/* Set trace_clock as the same of the previous boot. */
> +	if (tscratch->clock_id != tr->clock_id) {
> +		if (tracing_set_clock(tr, trace_clocks[tscratch->clock_id].name) < 0) {

In case the tscratch->clock_id gets corrupted, we better make sure it
doesn't overflow:

		if (tscratch->clock_id >= ARRAY_SIZE(trace_clocks) ||
		    tracing_set_clock(tr, trace_clocks[tscratch->clock_id].name) < 0) {

-- Steve


> +			pr_info("the previous trace_clock info is not valid.");
> +			goto reset;
> +		}
> +	}
>  	return;
>   reset:
>  	/* Invalid trace modules */