Re: [PATCH v6 29/37] tracing: Add cpu field for hist triggers

[Namhyung Kim]

On Fri, Nov 17, 2017 at 02:33:08PM -0600, Tom Zanussi wrote:
> A common key to use in a histogram is the cpuid - add a new cpu
> 'synthetic' field for that purpose.  This field is named cpu rather
> than $cpu or $common_cpu because 'cpu' already exists as a special
> filter field and it makes more sense to match that rather than add
> another name for the same thing.
> 
> Signed-off-by: Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx>
> ---
>  Documentation/trace/histogram.txt | 17 +++++++++++++++++
>  kernel/trace/trace_events_hist.c  | 28 +++++++++++++++++++++++++++-
>  2 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/trace/histogram.txt b/Documentation/trace/histogram.txt
> index d1d92ed..cd3ec00 100644
> --- a/Documentation/trace/histogram.txt
> +++ b/Documentation/trace/histogram.txt
> @@ -172,6 +172,23 @@
>    The examples below provide a more concrete illustration of the
>    concepts and typical usage patterns discussed above.
>  
> +  'special' event fields
> +  ------------------------
> +
> +  There are a number of 'special event fields' available for use as
> +  keys or values in a hist trigger.  These look like and behave as if
> +  they were actual event fields, but aren't really part of the event's
> +  field definition or format file.  They are however available for any
> +  event, and can be used anywhere an actual event field could be.
> +  'Special' field names are always prefixed with a '$' character to
> +  indicate that they're not normal fields (with the exception of
> +  'cpu', for compatibility with existing filter usage):

But it also could make a confusion to variables.  How about removing
'$' character at all?


> +
> +    $common_timestamp      u64 - timestamp (from ring buffer) associated
> +                                 with the event, in nanoseconds.  May be
> +				 modified by .usecs to have timestamps
> +				 interpreted as microseconds.
> +    cpu                    int - the cpu on which the event occurred.
>  
>  6.2 'hist' trigger examples
>  ---------------------------
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index 121f7ef..afbfa9c 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -227,6 +227,7 @@ enum hist_field_flags {
>  	HIST_FIELD_FL_VAR		= 1 << 12,
>  	HIST_FIELD_FL_EXPR		= 1 << 13,
>  	HIST_FIELD_FL_VAR_REF		= 1 << 14,
> +	HIST_FIELD_FL_CPU		= 1 << 15,
>  };
>  
>  struct var_defs {
> @@ -1177,6 +1178,16 @@ static u64 hist_field_timestamp(struct hist_field *hist_field,
>  	return ts;
>  }
>  
> +static u64 hist_field_cpu(struct hist_field *hist_field,
> +			  struct tracing_map_elt *elt,
> +			  struct ring_buffer_event *rbe,
> +			  void *event)
> +{
> +	int cpu = smp_processor_id();
> +
> +	return cpu;
> +}
> +
>  static struct hist_field *
>  check_field_for_var_ref(struct hist_field *hist_field,
>  			struct hist_trigger_data *var_data,
> @@ -1622,6 +1633,8 @@ static const char *hist_field_name(struct hist_field *field,
>  		field_name = hist_field_name(field->operands[0], ++level);
>  	else if (field->flags & HIST_FIELD_FL_TIMESTAMP)
>  		field_name = "$common_timestamp";
> +	else if (field->flags & HIST_FIELD_FL_CPU)
> +		field_name = "cpu";
>  	else if (field->flags & HIST_FIELD_FL_EXPR ||
>  		 field->flags & HIST_FIELD_FL_VAR_REF) {
>  		if (field->system) {
> @@ -2125,6 +2138,15 @@ static struct hist_field *create_hist_field(struct hist_trigger_data *hist_data,
>  		goto out;
>  	}
>  
> +	if (flags & HIST_FIELD_FL_CPU) {
> +		hist_field->fn = hist_field_cpu;
> +		hist_field->size = sizeof(int);
> +		hist_field->type = kstrdup("int", GFP_KERNEL);

Is it unsigned?

Thanks,
Namhyung


> +		if (!hist_field->type)
> +			goto free;
> +		goto out;
> +	}
> +
>  	if (WARN_ON_ONCE(!field))
>  		goto out;
>  
> @@ -2343,7 +2365,9 @@ static struct hist_field *parse_var_ref(struct hist_trigger_data *hist_data,
>  		hist_data->enable_timestamps = true;
>  		if (*flags & HIST_FIELD_FL_TIMESTAMP_USECS)
>  			hist_data->attrs->ts_in_usecs = true;
> -	} else {
> +	} else if (strcmp(field_name, "cpu") == 0)
> +		*flags |= HIST_FIELD_FL_CPU;
> +	else {
>  		field = trace_find_event_field(file->event_call, field_name);
>  		if (!field || !field->size) {
>  			field = ERR_PTR(-EINVAL);
> @@ -4612,6 +4636,8 @@ static void hist_field_print(struct seq_file *m, struct hist_field *hist_field)
>  
>  	if (hist_field->flags & HIST_FIELD_FL_TIMESTAMP)
>  		seq_puts(m, "$common_timestamp");
> +	else if (hist_field->flags & HIST_FIELD_FL_CPU)
> +		seq_puts(m, "cpu");
>  	else if (field_name) {
>  		if (hist_field->flags & HIST_FIELD_FL_VAR_REF)
>  			seq_putc(m, '$');
> -- 
> 1.9.3
>