Date:	Thu, 22 Aug 2013 10:06:23 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [for-next][PATCH 1/5] tracing: Add __tracepoint_string() to
 export string pointers

On Thu, Aug 22, 2013 at 12:32:26PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> 
> There are several tracepoints (mostly in RCU), that reference a string
> pointer and uses the print format of "%s" to display the string that
> exists in the kernel, instead of copying the actual string to the
> ring buffer (saves time and ring buffer space).
> 
> But this has an issue with userspace tools that read the binary buffers
> that has the address of the string but has no access to what the string
> itself is. The end result is just output that looks like:
> 
>  rcu_dyntick:          ffffffff818adeaa 1 0
>  rcu_dyntick:          ffffffff818adeb5 0 140000000000000
>  rcu_dyntick:          ffffffff818adeb5 0 140000000000000
>  rcu_utilization:      ffffffff8184333b
>  rcu_utilization:      ffffffff8184333b
> 
> The above is pretty useless when read by the userspace tools. Ideally
> we would want something that looks like this:
> 
>  rcu_dyntick:          Start 1 0
>  rcu_dyntick:          End 0 140000000000000
>  rcu_dyntick:          Start 140000000000000 0
>  rcu_callback:         rcu_preempt rhp=0xffff880037aff710 func=put_cred_rcu 0/4
>  rcu_callback:         rcu_preempt rhp=0xffff880078961980 func=file_free_rcu 0/5
>  rcu_dyntick:          End 0 1
> 
> The trace_printk() which also only stores the address of the string
> format instead of recording the string into the buffer itself, exports
> the mapping of kernel addresses to format strings via the printk_format
> file in the debugfs tracing directory.
> 
> The tracepoint strings can use this same method and output the format
> to the same file and the userspace tools will be able to decipher
> the address without any modification.
> 
> The tracepoint strings need its own section to save the strings because
> the trace_printk section will cause the trace_printk() buffers to be
> allocated if anything exists within the section. trace_printk() is only
> used for debugging and should never exist in the kernel, we can not use
> the trace_printk sections.
> 
> Add a new tracepoint_str section that will also be examined by the output
> of the printk_format file.
> 
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  include/asm-generic/vmlinux.lds.h |    7 ++++++-
>  include/linux/ftrace_event.h      |   34 ++++++++++++++++++++++++++++++++++
>  kernel/trace/trace.h              |    3 +++
>  kernel/trace/trace_printk.c       |   19 +++++++++++++++++++
>  4 files changed, 62 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 69732d2..83e2c31 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -122,8 +122,12 @@
>  #define TRACE_PRINTKS() VMLINUX_SYMBOL(__start___trace_bprintk_fmt) = .;      \
>  			 *(__trace_printk_fmt) /* Trace_printk fmt' pointer */ \
>  			 VMLINUX_SYMBOL(__stop___trace_bprintk_fmt) = .;
> +#define TRACEPOINT_STR() VMLINUX_SYMBOL(__start___tracepoint_str) = .;	\
> +			 *(__tracepoint_str) /* Trace_printk fmt' pointer */ \
> +			 VMLINUX_SYMBOL(__stop___tracepoint_str) = .;
>  #else
>  #define TRACE_PRINTKS()
> +#define TRACEPOINT_STR()
>  #endif
> 
>  #ifdef CONFIG_FTRACE_SYSCALLS
> @@ -190,7 +194,8 @@
>  	VMLINUX_SYMBOL(__stop___verbose) = .;				\
>  	LIKELY_PROFILE()		       				\
>  	BRANCH_PROFILE()						\
> -	TRACE_PRINTKS()
> +	TRACE_PRINTKS()							\
> +	TRACEPOINT_STR()
> 
>  /*
>   * Data section helpers
> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
> index 4372658..81af18a 100644
> --- a/include/linux/ftrace_event.h
> +++ b/include/linux/ftrace_event.h
> @@ -357,6 +357,40 @@ do {									\
>  		__trace_printk(ip, fmt, ##args);			\
>  } while (0)
> 
> +/**
> + * tracepoint_string - register constant persistent string to trace system
> + * @str - a constant persistent string that will be referenced in tracepoints
> + *
> + * If constant strings are being used in tracepoints, it is faster and
> + * more efficient to just save the pointer to the string and reference
> + * that with a printf "%s" instead of saving the string in the ring buffer
> + * and wasting space and time.
> + *
> + * The problem with the above approach is that userspace tools that read
> + * the binary output of the trace buffers do not have access to the string.
> + * Instead they just show the address of the string which is not very
> + * useful to users.
> + *
> + * With tracepoint_string(), the string will be registered to the tracing
> + * system and exported to userspace via the debugfs/tracing/printk_formats
> + * file that maps the string address to the string text. This way userspace
> + * tools that read the binary buffers have a way to map the pointers to
> + * the ASCII strings they represent.
> + *
> + * The @str used must be a constant string and persistent as it would not
> + * make sense to show a string that no longer exists. But it is still fine
> + * to be used with modules, because when modules are unloaded, if they
> + * had tracepoints, the ring buffers are cleared too. As long as the string
> + * does not change during the life of the module, it is fine to use
> + * tracepoint_string() within a module.
> + */
> +#define tracepoint_string(str)						\
> +	({								\
> +		static const char *___tp_str __tracepoint_string = str; \
> +		___tp_str;						\
> +	})
> +#define __tracepoint_string	__attribute__((section("__tracepoint_str")))
> +
>  #ifdef CONFIG_PERF_EVENTS
>  struct perf_event;
> 
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 4a4f6e1..ba321f1 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -1022,6 +1022,9 @@ extern struct list_head ftrace_events;
>  extern const char *__start___trace_bprintk_fmt[];
>  extern const char *__stop___trace_bprintk_fmt[];
> 
> +extern const char *__start___tracepoint_str[];
> +extern const char *__stop___tracepoint_str[];
> +
>  void trace_printk_init_buffers(void);
>  void trace_printk_start_comm(void);
>  int trace_keep_overwrite(struct tracer *tracer, u32 mask, int set);
> diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
> index a9077c1..2900817 100644
> --- a/kernel/trace/trace_printk.c
> +++ b/kernel/trace/trace_printk.c
> @@ -244,12 +244,31 @@ static const char **find_next(void *v, loff_t *pos)
>  {
>  	const char **fmt = v;
>  	int start_index;
> +	int last_index;
> 
>  	start_index = __stop___trace_bprintk_fmt - __start___trace_bprintk_fmt;
> 
>  	if (*pos < start_index)
>  		return __start___trace_bprintk_fmt + *pos;
> 
> +	/*
> +	 * The __tracepoint_str section is treated the same as the
> +	 * __trace_printk_fmt section. The difference is that the
> +	 * __trace_printk_fmt section should only be used by trace_printk()
> +	 * in a debugging environment, as if anything exists in that section
> +	 * the trace_prink() helper buffers are allocated, which would just
> +	 * waste space in a production environment.
> +	 *
> +	 * The __tracepoint_str sections on the other hand are used by
> +	 * tracepoints which need to map pointers to their strings to
> +	 * the ASCII text for userspace.
> +	 */
> +	last_index = start_index;
> +	start_index = __stop___tracepoint_str - __start___tracepoint_str;
> +
> +	if (*pos < last_index + start_index)
> +		return __start___tracepoint_str + (*pos - last_index);
> +
>  	return find_next_mod_format(start_index, v, fmt, pos);
>  }
> 
> -- 
> 1.7.10.4
> 
> 

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