Re: [RFC/PATCH] ftrace: Reduce size of function graph entries

[Steven Rostedt]

On Mon, 23 May 2016 00:26:15 +0900
Namhyung Kim <namhyung@xxxxxxxxxx> wrote:

> Currently ftrace_graph_ent{,_entry} and ftrace_graph_ret{,_entry} struct
> can have padding bytes at the end due to alignment in 64-bit data type.
> As these data are recorded so frequently, those paddings waste
> non-negligible space.  As some archs can have efficient unaligned
> accesses, reducing the alignment can save ~10% of data size:
> 
>   ftrace_graph_ent_entry:  24 -> 20
>   ftrace_graph_ret_entry:  48 -> 44
> 
> Also I moved the 'overrun' field in struct ftrace_graph_ret to minimize
> the padding.  Tested on x86_64 only.

I'd like to see this tested on other archs too.

[ Added linux-arch so maybe other arch maintainers may know about this ]

> 
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
>  include/linux/ftrace.h       | 16 ++++++++++++----
>  kernel/trace/trace.h         | 11 +++++++++++
>  kernel/trace/trace_entries.h |  4 ++--
>  3 files changed, 25 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index dea12a6e413b..35c523ba5c59 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -751,25 +751,33 @@ extern void ftrace_init(void);
>  static inline void ftrace_init(void) { }
>  #endif
>  
> +#ifndef CONFIG_HAVE_64BIT_ALIGNED_ACCESS
> +# define FTRACE_ALIGNMENT	4
> +#else
> +# define FTRACE_ALIGNMENT	8
> +#endif

Swap the above. Having the #ifndef is more confusing to understand than
to have a #ifdef.

> +
> +#define FTRACE_ALIGN_DATA	__attribute__((packed, aligned(FTRACE_ALIGNMENT)))

Do we really need to pack it? I mean, just get rid of the hole (like
you did with the movement of the overrun) and shouldn't the array be
aligned normally without holes, if the arch can support it? Doesn't gcc
take care of that?

-- Steve

> +
>  /*
>   * Structure that defines an entry function trace.
>   */
>  struct ftrace_graph_ent {
>  	unsigned long func; /* Current function */
>  	int depth;
> -};
> +} FTRACE_ALIGN_DATA;
>  
>  /*
>   * Structure that defines a return function trace.
>   */
>  struct ftrace_graph_ret {
>  	unsigned long func; /* Current function */
> -	unsigned long long calltime;
> -	unsigned long long rettime;
>  	/* Number of functions that overran the depth limit for current task */
>  	unsigned long overrun;
> +	unsigned long long calltime;
> +	unsigned long long rettime;
>  	int depth;
> -};
> +} FTRACE_ALIGN_DATA;
>  
>  /* Type of the callback handlers for tracing function graph*/
>  typedef void (*trace_func_graph_ret_t)(struct ftrace_graph_ret *); /* return */
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 5167c366d6b7..d2dd49ca55ee 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -80,6 +80,12 @@ enum trace_type {
>  	FTRACE_ENTRY(name, struct_name, id, PARAMS(tstruct), PARAMS(print), \
>  		     filter)
>  
> +#undef FTRACE_ENTRY_PACKED
> +#define FTRACE_ENTRY_PACKED(name, struct_name, id, tstruct, print,	\
> +			    filter)					\
> +	FTRACE_ENTRY(name, struct_name, id, PARAMS(tstruct), PARAMS(print), \
> +		     filter) FTRACE_ALIGN_DATA
> +
>  #include "trace_entries.h"
>  
>  /*
> @@ -1600,6 +1606,11 @@ int set_tracer_flag(struct trace_array *tr, unsigned int mask, int enabled);
>  #define FTRACE_ENTRY_DUP(call, struct_name, id, tstruct, print, filter)	\
>  	FTRACE_ENTRY(call, struct_name, id, PARAMS(tstruct), PARAMS(print), \
>  		     filter)
> +#undef FTRACE_ENTRY_PACKED
> +#define FTRACE_ENTRY_PACKED(call, struct_name, id, tstruct, print, filter) \
> +	FTRACE_ENTRY(call, struct_name, id, PARAMS(tstruct), PARAMS(print), \
> +		     filter)
> +
>  #include "trace_entries.h"
>  
>  #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_FUNCTION_TRACER)
> diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
> index ee7b94a4810a..5c30efcda5e6 100644
> --- a/kernel/trace/trace_entries.h
> +++ b/kernel/trace/trace_entries.h
> @@ -72,7 +72,7 @@ FTRACE_ENTRY_REG(function, ftrace_entry,
>  );
>  
>  /* Function call entry */
> -FTRACE_ENTRY(funcgraph_entry, ftrace_graph_ent_entry,
> +FTRACE_ENTRY_PACKED(funcgraph_entry, ftrace_graph_ent_entry,
>  
>  	TRACE_GRAPH_ENT,
>  
> @@ -88,7 +88,7 @@ FTRACE_ENTRY(funcgraph_entry, ftrace_graph_ent_entry,
>  );
>  
>  /* Function return entry */
> -FTRACE_ENTRY(funcgraph_exit, ftrace_graph_ret_entry,
> +FTRACE_ENTRY_PACKED(funcgraph_exit, ftrace_graph_ret_entry,
>  
>  	TRACE_GRAPH_RET,
>