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Date: Sun, 30 Jul 2023 10:43:21 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2] tracing: Add a debug_trace_printk() function
On Tue, 13 Jun 2023 09:55:37 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:
> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
>
> While doing some tracing and kernel debugging, I found that some of my
> trace_printk()s were being lost in the noise of the other code that was
> being traced. Having a way to write trace_printk() not in the top level
> trace buffer would have been useful.
>
> There was also a time I needed to debug ftrace itself, where
> trace_printk() did not hit the paths that were being debugged. But because
> the trace that was being debugged, was going into the top level ring
> buffer, it was causing issues for seeing what is to be traced.
>
> To solve both of the above, add a debug_trace_printk() that can be used
> just like trace_printk() except that it goes into a "debug" instance
> buffer instead. This can be used at boot up as well.
>
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
Sorry, I missed this patch.
As far as it depends CONFIG_FTRACE_DEBUG_PRINT, this looks good to me.
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
BTW, what about adding generic trace_loglvl_printk(loglvl, fmt, ...)?
This is a macro that will be expanded to 'trace_loglvl_<N>_printk(fmt, ...)'
where N is 0-9. Then we can filter the trace_loglvl_N_printk event to a
specific instance.
Thank you,
> ---
> Changes since v1: https://lore.kernel.org/linux-trace-kernel/20230612193337.0fb0d3ca@gandalf.local.home/
>
> - I had added a prompt to the kconfig but never committed it.
> It's now part of the patch.
>
> include/linux/kernel.h | 14 ++++++++++++++
> kernel/trace/Kconfig | 20 ++++++++++++++++++++
> kernel/trace/trace.c | 29 +++++++++++++++++++++++++++++
> 3 files changed, 63 insertions(+)
>
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 0d91e0af0125..594c9ba17fd4 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -432,6 +432,20 @@ __ftrace_vbprintk(unsigned long ip, const char *fmt, va_list ap);
> extern __printf(2, 0) int
> __ftrace_vprintk(unsigned long ip, const char *fmt, va_list ap);
>
> +#ifdef CONFIG_FTRACE_DEBUG_PRINT
> +extern __printf(2,0) void do_debug_trace_printk(unsigned long ip, const char *fmt, ...);
> +#define debug_trace_printk(fmt, ...) \
> +do { \
> + do_debug_trace_printk(_THIS_IP_, fmt, ##__VA_ARGS__); \
> +} while (0)
> +
> +extern void debug_tracing_stop(void);
> +#else
> +#define debug_trace_printk(fmt, ...) do { } while (0)
> +static inline void debug_tracing_stop(void) { }
> +#endif
> +
> +
> extern void ftrace_dump(enum ftrace_dump_mode oops_dump_mode);
> #else
> static inline void tracing_start(void) { }
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index abe5c583bd59..e07bca39dec9 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -974,6 +974,26 @@ config GCOV_PROFILE_FTRACE
> Note that on a kernel compiled with this config, ftrace will
> run significantly slower.
>
> +config FTRACE_DEBUG_PRINT
> + bool "Enable debug_trace_printk()"
> + depends on TRACING
> + help
> + This option enables the use of debug_trace_printk() instead of
> + using trace_printk(). The difference between the two is that
> + debug_trace_printk() traces are visible in the "debug" instance
> + found in:
> +
> + /sys/kernel/tracing/instances/debug
> +
> + This is useful when the trace printks should not interfere with
> + the normal top level tracing. It is also useful if the top level
> + tracing is very noisy and critical trace printks are dropped.
> + By using debug_trace_printk() the traces goes into as separate
> + ring buffer that will not be overridden by other trace events.
> +
> + If unsure say N (In fact, only say Y if you are debugging a
> + kernel and require this)
> +
> config FTRACE_SELFTEST
> bool
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 64a4dde073ef..c21a93cf5fd8 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -490,6 +490,10 @@ static struct trace_array global_trace = {
> .trace_flags = TRACE_DEFAULT_FLAGS,
> };
>
> +#ifdef CONFIG_FTRACE_DEBUG_PRINT
> +static struct trace_array *debug_trace;
> +#endif
> +
> LIST_HEAD(ftrace_trace_arrays);
>
> int trace_array_get(struct trace_array *this_tr)
> @@ -10455,8 +10459,33 @@ void __init early_trace_init(void)
> tracer_alloc_buffers();
>
> init_events();
> +
> +#ifdef CONFIG_FTRACE_DEBUG_PRINT
> + debug_trace = trace_array_get_by_name("debug");
> + if (WARN_ON(!debug_trace))
> + return;
> + trace_array_init_printk(debug_trace);
> +#endif
> }
>
> +#ifdef CONFIG_FTRACE_DEBUG_PRINT
> +__printf(2, 0)
> +void do_debug_trace_printk(unsigned long ip, const char *fmt, ...)
> +{
> + va_list ap;
> +
> + va_start(ap, fmt);
> + trace_array_vprintk(debug_trace, ip, fmt, ap);
> + va_end(ap);
> +}
> +
> +void debug_tracing_stop(void)
> +{
> + debug_trace_printk("Stopping debug tracing\n");
> + tracing_stop_tr(debug_trace);
> +}
> +#endif
> +
> void __init trace_init(void)
> {
> trace_event_init();
> --
> 2.39.2
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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