Re: [PATCH v2] function_graph: Improve fgraph LRU data initialization

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 30 Sep 2024 07:10:31 +0000
Liao Chang <liaochang1@huawei.com> wrote:

> This patch uses [first ... last] = value to initialize fgraph_array[].
> And use fgraph_lru_next and fgraph_lru_last as the indicator of
> initialization.

The only thing this patch does is to allow the use of [first...last]
annotation for initialization. What actual benefit does that give us?

In other words, why would I want to apply this? Just so that we can use
[first...last] annotation with the added cost of having to manage setting
fgraph_lru_next and fgraph_lru_last to -1 and then comparing them?

Personally, I find the original code easier to maintain, as it's simple and
doesn't add extra management.

-- Steve


> 
> v2->v1:
> Fixup the build error reported by kernel test robot <lkp@intel.com>.
> Since some architectures use ftrace_graph_entry_stub() for the static
> ftrace scenario, then restore the definition without static keyword in
> the original patch [1]. And rebasing patch to next-20240927.
> 
> [1] https://lore.kernel.org/all/20240912111550.1752115-1-liaochang1@huawei.com
> 
> Signed-off-by: Liao Chang <liaochang1@huawei.com>
> ---
>  kernel/trace/fgraph.c | 54 +++++++++++++++++++++----------------------
>  1 file changed, 27 insertions(+), 27 deletions(-)
> 
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index d7d4fb403f6f..eb2fbc0338c7 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -172,20 +172,41 @@ enum {
>  DEFINE_STATIC_KEY_FALSE(kill_ftrace_graph);
>  int ftrace_graph_active;
>  
> -static struct fgraph_ops *fgraph_array[FGRAPH_ARRAY_SIZE];
> +int ftrace_graph_entry_stub(struct ftrace_graph_ent *trace,
> +			    struct fgraph_ops *gops)
> +{
> +	return 0;
> +}
> +
> +static void ftrace_graph_ret_stub(struct ftrace_graph_ret *trace,
> +				  struct fgraph_ops *gops)
> +{
> +}
> +
> +static struct fgraph_ops fgraph_stub = {
> +	.entryfunc = ftrace_graph_entry_stub,
> +	.retfunc = ftrace_graph_ret_stub,
> +};
> +
> +static struct fgraph_ops *fgraph_array[FGRAPH_ARRAY_SIZE] = {
> +	[0 ... FGRAPH_ARRAY_SIZE - 1] = &fgraph_stub,
> +};
>  static unsigned long fgraph_array_bitmask;
>  
>  /* LRU index table for fgraph_array */
>  static int fgraph_lru_table[FGRAPH_ARRAY_SIZE];
> -static int fgraph_lru_next;
> -static int fgraph_lru_last;
> +static int fgraph_lru_next = -1;
> +static int fgraph_lru_last = -1;
>  
>  /* Initialize fgraph_lru_table with unused index */
>  static void fgraph_lru_init(void)
>  {
> -	int i;
> +	if ((fgraph_lru_next >= 0) && (fgraph_lru_last >= 0))
> +		return;
>  
> -	for (i = 0; i < FGRAPH_ARRAY_SIZE; i++)
> +	fgraph_lru_next = fgraph_lru_last = 0;
> +
> +	for (int i = 0; i < FGRAPH_ARRAY_SIZE; i++)
>  		fgraph_lru_table[i] = i;
>  }
>  
> @@ -483,22 +504,6 @@ int __weak ftrace_disable_ftrace_graph_caller(void)
>  }
>  #endif
>  
> -int ftrace_graph_entry_stub(struct ftrace_graph_ent *trace,
> -			    struct fgraph_ops *gops)
> -{
> -	return 0;
> -}
> -
> -static void ftrace_graph_ret_stub(struct ftrace_graph_ret *trace,
> -				  struct fgraph_ops *gops)
> -{
> -}
> -
> -static struct fgraph_ops fgraph_stub = {
> -	.entryfunc = ftrace_graph_entry_stub,
> -	.retfunc = ftrace_graph_ret_stub,
> -};
> -
>  static struct fgraph_ops *fgraph_direct_gops = &fgraph_stub;
>  DEFINE_STATIC_CALL(fgraph_func, ftrace_graph_entry_stub);
>  DEFINE_STATIC_CALL(fgraph_retfunc, ftrace_graph_ret_stub);
> @@ -1250,12 +1255,7 @@ int register_ftrace_graph(struct fgraph_ops *gops)
>  
>  	mutex_lock(&ftrace_lock);
>  
> -	if (!fgraph_array[0]) {
> -		/* The array must always have real data on it */
> -		for (i = 0; i < FGRAPH_ARRAY_SIZE; i++)
> -			fgraph_array[i] = &fgraph_stub;
> -		fgraph_lru_init();
> -	}
> +	fgraph_lru_init();
>  
>  	i = fgraph_lru_alloc_index();
>  	if (i < 0 || WARN_ON_ONCE(fgraph_array[i] != &fgraph_stub)) {