Re: [PATCH] tracing: Add common_comm to histograms

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 7 Apr 2025 15:49:12 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> If one wants to trace the name of the task that wakes up a process and
> pass that to the synthetic events, there's nothing currently that lets the
> synthetic events do that. Add a "common_comm" to the histogram logic that
> allows histograms save the current->comm as a variable that can be passed
> through and added to a synthetic event:
> 
>  # cd /sys/kernel/tracing
>  # echo 's:wake_lat char[] waker; char[] wakee; u64 delta;' >> dynamic_events
>  # echo 'hist:keys=pid:comm=common_comm:ts=common_timestamp.usecs if !(common_flags & 0x18)' > events/sched/sched_waking/trigger
>  # echo 'hist:keys=next_pid:wake_comm=$comm:delta=common_timestamp.usecs-$ts:onmatch(sched.sched_waking).trace(wake_lat,$wake_comm,next_comm,$delta)' > events/sched/sched_switch/trigger
> 
> The above will create a synthetic trace event that will save both the name
> of the waker and the wakee but only if the wakeup did not happen in a hard
> or soft interrupt context.
> 
> The "common_comm" is used to save the task->comm at the time of the
> initial event and is passed via the "comm" variable to the second event,
> and that is saved as the "waker" field in the "wake_lat" synthetic event.
> 

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

BTW, the string field size must always be MAX_FILTER_STR_VAL (for comparing
with other string filters, which checks the size at first.)
Somewhere we should comment it.

Thank you,

> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  kernel/trace/trace_events_hist.c | 51 ++++++++++++++++++++++++++------
>  1 file changed, 42 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index e85bc59c0421..58c9535f61df 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -114,6 +114,7 @@ enum hist_field_fn {
>  	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,
> @@ -506,6 +507,7 @@ enum hist_field_flags {
>  	HIST_FIELD_FL_CONST		= 1 << 18,
>  	HIST_FIELD_FL_PERCENT		= 1 << 19,
>  	HIST_FIELD_FL_GRAPH		= 1 << 20,
> +	HIST_FIELD_FL_COMM		= 1 << 21,
>  };
>  
>  struct var_defs {
> @@ -885,6 +887,15 @@ static u64 hist_field_cpu(struct hist_field *hist_field,
>  	return cpu;
>  }
>  
> +static u64 hist_field_comm(struct hist_field *hist_field,
> +			  struct tracing_map_elt *elt,
> +			  struct trace_buffer *buffer,
> +			  struct ring_buffer_event *rbe,
> +			  void *event)
> +{
> +	return (u64)(unsigned long)current->comm;
> +}
> +
>  /**
>   * check_field_for_var_ref - Check if a VAR_REF field references a variable
>   * @hist_field: The VAR_REF field to check
> @@ -1338,6 +1349,8 @@ static const char *hist_field_name(struct hist_field *field,
>  		field_name = hist_field_name(field->operands[0], ++level);
>  	else if (field->flags & HIST_FIELD_FL_CPU)
>  		field_name = "common_cpu";
> +	else if (field->flags & HIST_FIELD_FL_COMM)
> +		field_name = "common_comm";
>  	else if (field->flags & HIST_FIELD_FL_EXPR ||
>  		 field->flags & HIST_FIELD_FL_VAR_REF) {
>  		if (field->system) {
> @@ -2015,6 +2028,13 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
>  		goto out;
>  	}
>  
> +	if (flags & HIST_FIELD_FL_COMM) {
> +		hist_field->fn_num = HIST_FIELD_FN_COMM;
> +		hist_field->size = MAX_FILTER_STR_VAL;
> +		hist_field->type = "char[]";
> +		goto out;
> +	}
> +
>  	if (WARN_ON_ONCE(!field))
>  		goto out;
>  
> @@ -2359,9 +2379,11 @@ parse_field(struct hist_trigger_data *hist_data, struct trace_event_file *file,
>  			hist_data->attrs->ts_in_usecs = true;
>  	} else if (strcmp(field_name, "common_stacktrace") == 0) {
>  		*flags |= HIST_FIELD_FL_STACKTRACE;
> -	} else if (strcmp(field_name, "common_cpu") == 0)
> +	} else if (strcmp(field_name, "common_cpu") == 0) {
>  		*flags |= HIST_FIELD_FL_CPU;
> -	else if (strcmp(field_name, "hitcount") == 0)
> +	} else if (strcmp(field_name, "common_comm") == 0) {
> +		*flags |= HIST_FIELD_FL_COMM | HIST_FIELD_FL_STRING;
> +	} else if (strcmp(field_name, "hitcount") == 0)
>  		*flags |= HIST_FIELD_FL_HITCOUNT;
>  	else {
>  		field = trace_find_event_field(file->event_call, field_name);
> @@ -2377,6 +2399,8 @@ parse_field(struct hist_trigger_data *hist_data, struct trace_event_file *file,
>  				*flags |= HIST_FIELD_FL_CPU;
>  			} else if (field && field->filter_type == FILTER_STACKTRACE) {
>  				*flags |= HIST_FIELD_FL_STACKTRACE;
> +			} else if (field && field->filter_type == FILTER_COMM) {
> +				*flags |= HIST_FIELD_FL_COMM | HIST_FIELD_FL_STRING;
>  			} else {
>  				hist_err(tr, HIST_ERR_FIELD_NOT_FOUND,
>  					 errpos(field_name));
> @@ -4327,6 +4351,8 @@ static u64 hist_fn_call(struct hist_field *hist_field,
>  		return hist_field_timestamp(hist_field, elt, buffer, rbe, event);
>  	case HIST_FIELD_FN_CPU:
>  		return hist_field_cpu(hist_field, elt, buffer, rbe, event);
> +	case HIST_FIELD_FN_COMM:
> +		return hist_field_comm(hist_field, elt, buffer, rbe, event);
>  	case HIST_FIELD_FN_STRING:
>  		return hist_field_string(hist_field, elt, buffer, rbe, event);
>  	case HIST_FIELD_FN_DYNSTRING:
> @@ -5212,14 +5238,19 @@ static inline void add_to_key(char *compound_key, void *key,
>  	size_t size = key_field->size;
>  
>  	if (key_field->flags & HIST_FIELD_FL_STRING) {
> -		struct ftrace_event_field *field;
>  
> -		field = key_field->field;
> -		if (field->filter_type == FILTER_DYN_STRING ||
> -		    field->filter_type == FILTER_RDYN_STRING)
> -			size = *(u32 *)(rec + field->offset) >> 16;
> -		else if (field->filter_type == FILTER_STATIC_STRING)
> -			size = field->size;
> +		if (key_field->flags & HIST_FIELD_FL_COMM) {
> +			size = strlen((char *)key);
> +		} else {
> +			struct ftrace_event_field *field;
> +
> +			field = key_field->field;
> +			if (field->filter_type == FILTER_DYN_STRING ||
> +			    field->filter_type == FILTER_RDYN_STRING)
> +				size = *(u32 *)(rec + field->offset) >> 16;
> +			else if (field->filter_type == FILTER_STATIC_STRING)
> +				size = field->size;
> +		}
>  
>  		/* ensure NULL-termination */
>  		if (size > key_field->size - 1)
> @@ -6097,6 +6128,8 @@ static void hist_field_print(struct seq_file *m, struct hist_field *hist_field)
>  
>  	if (hist_field->flags & HIST_FIELD_FL_CPU)
>  		seq_puts(m, "common_cpu");
> +	if (hist_field->flags & HIST_FIELD_FL_COMM)
> +		seq_puts(m, "common_comm");
>  	else if (hist_field->flags & HIST_FIELD_FL_CONST)
>  		seq_printf(m, "%llu", hist_field->constant);
>  	else if (field_name) {
> -- 
> 2.47.2
> 


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