linux-kernel - Re: [PATCH] tracing: Have hist_debug show what function a field uses

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Message-ID: <b6239d2550f028216b7cfe5e0ced0f0fc324c3a1.camel@kernel.org>
Date: Sun, 25 Jan 2026 13:21:19 -0600
From: Tom Zanussi <zanussi@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>, LKML
 <linux-kernel@...r.kernel.org>,  Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
	 <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] tracing: Have hist_debug show what function a field uses

Hi Steve,

On Thu, 2026-01-22 at 20:38 -0500, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@...dmis.org>
> 
> When CONFIG_HIST_TRIGGERS_DEBUG is enabled, each trace event has a
> "hist_debug" file that explains the histogram internal data. This is very
> useful for debugging histograms.
> 
> One bit of data that was missing from this file was what function a
> histogram field uses to process its data. The hist_field structure now has
> a fn_num that is used by a switch statement in hist_fn_call() to call a
> function directly (to avoid spectre mitigations).
> 
> Instead of displaying that number, create a string array that maps to the
> histogram function enums so that the function for a field may be
> displayed:
> 
>  ~# cat /sys/kernel/tracing/events/sched/sched_switch/hist_debug
> [..]
> hist_data: 0000000043d62762
> 
>   n_vals: 2
>   n_keys: 1
>   n_fields: 3
> 
>   val fields:
> 
>     hist_data->fields[0]:
>       flags:
>         VAL: HIST_FIELD_FL_HITCOUNT
>       type: u64
>       size: 8
>       is_signed: 0
>       function: hist_field_counter()
> 
>     hist_data->fields[1]:
>       flags:
>         HIST_FIELD_FL_VAR
>       var.name: __arg_3921_2
>       var.idx (into tracing_map_elt.vars[]): 0
>       type: unsigned long[]
>       size: 128
>       is_signed: 0
>       function: hist_field_nop()
> 
>   key fields:
> 
>     hist_data->fields[2]:
>       flags:
>         HIST_FIELD_FL_KEY
>       ftrace_event_field name: prev_pid
>       type: pid_t
>       size: 8
>       is_signed: 1
>       function: hist_field_s32()
> 
> The "function:" field above is added.
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

Nice and useful addition, thanks!

Reviewed-by: Tom Zanussi <zanussi@...nel.org>
Tested-by: Tom Zanussi <zanussi@...nel.org>

Tom

> ---
>  kernel/trace/trace_events_hist.c | 75 +++++++++++++++++++-------------
>  1 file changed, 44 insertions(+), 31 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index 4a7f945e368d..24441b30a1a4 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -105,38 +105,44 @@ enum field_op_id {
>  	FIELD_OP_MULT,
>  };
>  
> +#define FIELD_FUNCS					\
> +	C(NOP,			"nop"),			\
> +	C(VAR_REF,		"var_ref"),		\
> +	C(COUNTER,		"counter"),		\
> +	C(CONST,		"const"),		\
> +	C(LOG2,			"log2"),		\
> +	C(BUCKET,		"bucket"),		\
> +	C(TIMESTAMP,		"timestamp"),		\
> +	C(CPU,			"cpu"),			\
> +	C(COMM,			"comm"),		\
> +	C(STRING,		"string"),		\
> +	C(DYNSTRING,		"dynstring"),		\
> +	C(RELDYNSTRING,		"reldynstring"),	\
> +	C(PSTRING,		"pstring"),		\
> +	C(S64,			"s64"),			\
> +	C(U64,			"u64"),			\
> +	C(S32,			"s32"),			\
> +	C(U32,			"u32"),			\
> +	C(S16,			"s16"),			\
> +	C(U16,			"u16"),			\
> +	C(S8,			"s8"),			\
> +	C(U8,			"u8"),			\
> +	C(UMINUS,		"uminus"),		\
> +	C(MINUS,		"minus"),		\
> +	C(PLUS,			"plus"),		\
> +	C(DIV,			"div"),			\
> +	C(MULT,			"mult"),		\
> +	C(DIV_POWER2,		"div_power2"),		\
> +	C(DIV_NOT_POWER2,	"div_not_power2"),	\
> +	C(DIV_MULT_SHIFT,	"div_mult_shift"),	\
> +	C(EXECNAME,		"execname"),		\
> +	C(STACK,		"stack"),
> +
> +#undef C
> +#define C(a, b)		HIST_FIELD_FN_##a
> +
>  enum hist_field_fn {
> -	HIST_FIELD_FN_NOP,
> -	HIST_FIELD_FN_VAR_REF,
> -	HIST_FIELD_FN_COUNTER,
> -	HIST_FIELD_FN_CONST,
> -	HIST_FIELD_FN_LOG2,
> -	HIST_FIELD_FN_BUCKET,
> -	HIST_FIELD_FN_TIMESTAMP,
> -	HIST_FIELD_FN_CPU,
> -	HIST_FIELD_FN_COMM,
> -	HIST_FIELD_FN_STRING,
> -	HIST_FIELD_FN_DYNSTRING,
> -	HIST_FIELD_FN_RELDYNSTRING,
> -	HIST_FIELD_FN_PSTRING,
> -	HIST_FIELD_FN_S64,
> -	HIST_FIELD_FN_U64,
> -	HIST_FIELD_FN_S32,
> -	HIST_FIELD_FN_U32,
> -	HIST_FIELD_FN_S16,
> -	HIST_FIELD_FN_U16,
> -	HIST_FIELD_FN_S8,
> -	HIST_FIELD_FN_U8,
> -	HIST_FIELD_FN_UMINUS,
> -	HIST_FIELD_FN_MINUS,
> -	HIST_FIELD_FN_PLUS,
> -	HIST_FIELD_FN_DIV,
> -	HIST_FIELD_FN_MULT,
> -	HIST_FIELD_FN_DIV_POWER2,
> -	HIST_FIELD_FN_DIV_NOT_POWER2,
> -	HIST_FIELD_FN_DIV_MULT_SHIFT,
> -	HIST_FIELD_FN_EXECNAME,
> -	HIST_FIELD_FN_STACK,
> +	FIELD_FUNCS
>  };
>  
>  /*
> @@ -5845,6 +5851,12 @@ const struct file_operations event_hist_fops = {
>  };
>  
>  #ifdef CONFIG_HIST_TRIGGERS_DEBUG
> +
> +#undef C
> +#define C(a, b)		b
> +
> +static const char * const field_funcs[] = { FIELD_FUNCS };
> +
>  static void hist_field_debug_show_flags(struct seq_file *m,
>  					unsigned long flags)
>  {
> @@ -5909,6 +5921,7 @@ static int hist_field_debug_show(struct seq_file *m,
>  	seq_printf(m, "      type: %s\n", field->type);
>  	seq_printf(m, "      size: %u\n", field->size);
>  	seq_printf(m, "      is_signed: %u\n", field->is_signed);
> +	seq_printf(m, "      function: hist_field_%s()\n", field_funcs[field->fn_num]);
>  
>  	return 0;
>  }