linux-kernel - Re: [PATCH 12/32] tracing: Add hist trigger timestamp support

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Date:   Mon, 17 Jul 2017 11:57:26 -0500
From:   Tom Zanussi <tom.zanussi@...ux.intel.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     rostedt@...dmis.org, tglx@...utronix.de, mhiramat@...nel.org,
        vedang.patel@...el.com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH 12/32] tracing: Add hist trigger timestamp support

Hi Namhyung,

On Mon, 2017-07-17 at 15:00 +0900, Namhyung Kim wrote:
> On Mon, Jun 26, 2017 at 05:49:13PM -0500, Tom Zanussi wrote:
> > Add support for a timestamp event field.  This is actually a 'pseudo-'
> > event field in that it behaves like it's part of the event record, but
> > is really part of the corresponding ring buffer event.
> > 
> > To make use of the timestamp field, users can specify
> > "$common_timestamp" as a field name for any histogram.  Note that this
> > doesn't make much sense on its own either as either a key or value,
> > but needs to be supported even so, since follow-on patches will add
> > support for making use of this field in time deltas.  The '$' is used
> > as a prefix on the variable name to indicate that it's not an bonafide
> > event field - so you won't find it in the event description - but
> > rather it's a synthetic field that can be used like a real field).
> > 
> > Note that the use of this field requires the ring buffer be put into
> > TIME_EXTEND_ABS mode, which saves the complete timestamp for each
> > event rather than an offset.  This mode will be enabled if and only if
> > a histogram makes use of the "$common_timestamp" field.
> > 
> > Signed-off-by: Tom Zanussi <tom.zanussi@...ux.intel.com>
> > ---
> >  kernel/trace/trace_events_hist.c | 90 +++++++++++++++++++++++++++++-----------
> >  1 file changed, 66 insertions(+), 24 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> > index dab6ff6..aeae3b4 100644
> > --- a/kernel/trace/trace_events_hist.c
> > +++ b/kernel/trace/trace_events_hist.c
> 
> [SNIP]
> > @@ -756,7 +783,7 @@ static int create_sort_keys(struct hist_trigger_data *hist_data)
> >  			break;
> >  		}
> >  
> > -		if (strcmp(field_name, "hitcount") == 0) {
> > +		if ((strcmp(field_name, "hitcount") == 0)) {
> 
> Seems like an unnecessary change.
> 
> 
> >  			descending = is_descending(field_str);
> >  			if (descending < 0) {
> >  				ret = descending;
> > @@ -816,6 +843,9 @@ static int create_tracing_map_fields(struct hist_trigger_data *hist_data)
> >  
> >  			if (hist_field->flags & HIST_FIELD_FL_STACKTRACE)
> >  				cmp_fn = tracing_map_cmp_none;
> > +			else if (!field)
> > +				cmp_fn = tracing_map_cmp_num(hist_field->size,
> > +							     hist_field->is_signed);
> 
> I couldn't find where the hist_field->is_signed is set.

It's set in a later patch (the one that adds onmatch).  I'll move the
code that defines and sets the field to the same patch.

> 
> 
> >  			else if (is_string_field(field))
> >  				cmp_fn = tracing_map_cmp_string;
> >  			else
> 
> [SNIP]
> > @@ -1534,6 +1570,9 @@ static int hist_register_trigger(char *glob, struct event_trigger_ops *ops,
> >  
> >  	update_cond_flag(file);
> >  
> > +	if (hist_data->enable_timestamps)
> > +		tracing_set_time_stamp_abs(file->tr, true);
> > +
> >  	if (trace_event_trigger_enable_disable(file, 1) < 0) {
> >  		list_del_rcu(&data->list);
> >  		update_cond_flag(file);
> > @@ -1568,6 +1607,9 @@ static void hist_unregister_trigger(char *glob, struct event_trigger_ops *ops,
> >  
> >  	if (unregistered && test->ops->free)
> >  		test->ops->free(test->ops, test);
> > +
> > +	if (hist_data->enable_timestamps)
> > +		tracing_set_time_stamp_abs(file->tr, false);
> 
> I think it needs some kind of refcount to disable the absolute
> timestamp.  Otherwise unregistering a timestamp-enabled hist might
> corrupt others IMHO.
> 

Good point, I'll add that.  Thanks,

Tom

> Thanks,
> Namhyung
> 
> 
> >  }
> >  
> >  static void hist_unreg_all(struct trace_event_file *file)
> > -- 
> > 1.9.3
> >