linux-kernel - Re: [PATCH 20/20] function_graph: Use bitmask to loop on fgraph entry

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Date: Mon, 27 May 2024 09:09:49 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Alexei Starovoitov
 <alexei.starovoitov@...il.com>, Florent Revest <revest@...omium.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, bpf <bpf@...r.kernel.org>, Sven
 Schnelle <svens@...ux.ibm.com>, Alexei Starovoitov <ast@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Arnaldo Carvalho de Melo <acme@...nel.org>, Daniel
 Borkmann <daniel@...earbox.net>, Alan Maguire <alan.maguire@...cle.com>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner
 <tglx@...utronix.de>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH 20/20] function_graph: Use bitmask to loop on fgraph
 entry

On Fri, 24 May 2024 22:37:12 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> Instead of looping through all the elements of fgraph_array[] to see if
> there's an gops attached to one and then calling its gops->func(). Create
> a fgraph_array_bitmask that sets bits when an index in the array is
> reserved (via the simple lru algorithm). Then only the bits set in this
> bitmask needs to be looked at where only elements in the array that have
> ops registered need to be looked at.
> 
> Note, we do not care about races. If a bit is set before the gops is
> assigned, it only wastes time looking at the element and ignoring it (as
> it did before this bitmask is added).

This is OK because anyway we check gops == &fgraph_stub.
By the way, shouldn't we also make "if (gops == &fgraph_stub)"
check unlikely()?

This change looks good to me.

Reviewed-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thank you,

> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/fgraph.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 5e8e13ffcfb6..1aae521e5997 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -173,6 +173,7 @@ DEFINE_STATIC_KEY_FALSE(kill_ftrace_graph);
>  int ftrace_graph_active;
>  
>  static struct fgraph_ops *fgraph_array[FGRAPH_ARRAY_SIZE];
> +static unsigned long fgraph_array_bitmask;
>  
>  /* LRU index table for fgraph_array */
>  static int fgraph_lru_table[FGRAPH_ARRAY_SIZE];
> @@ -197,6 +198,8 @@ static int fgraph_lru_release_index(int idx)
>  
>  	fgraph_lru_table[fgraph_lru_last] = idx;
>  	fgraph_lru_last = (fgraph_lru_last + 1) % FGRAPH_ARRAY_SIZE;
> +
> +	clear_bit(idx, &fgraph_array_bitmask);
>  	return 0;
>  }
>  
> @@ -211,6 +214,8 @@ static int fgraph_lru_alloc_index(void)
>  
>  	fgraph_lru_table[fgraph_lru_next] = -1;
>  	fgraph_lru_next = (fgraph_lru_next + 1) % FGRAPH_ARRAY_SIZE;
> +
> +	set_bit(idx, &fgraph_array_bitmask);
>  	return idx;
>  }
>  
> @@ -632,7 +637,8 @@ int function_graph_enter(unsigned long ret, unsigned long func,
>  	if (offset < 0)
>  		goto out;
>  
> -	for (i = 0; i < FGRAPH_ARRAY_SIZE; i++) {
> +	for_each_set_bit(i, &fgraph_array_bitmask,
> +			 sizeof(fgraph_array_bitmask) * BITS_PER_BYTE) {
>  		struct fgraph_ops *gops = fgraph_array[i];
>  		int save_curr_ret_stack;
>  
> -- 
> 2.43.0
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>