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    SubjectRe: [PATCH 19/32] tracing: Add variable reference handling to hist triggers
    On Mon, Jun 26, 2017 at 05:49:20PM -0500, Tom Zanussi wrote:
    > Add the necessary infrastructure to allow the variables defined on one
    > event to be referenced in another. This allows variables set by a
    > previous event to be referenced and used in expressions combining the
    > variable values saved by that previous event and the event fields of
    > the current event. For example, here's how a latency can be
    > calculated and saved into yet another variable named 'wakeup_lat':
    >
    > # echo 'hist:keys=pid,prio:ts0=common_timestamp ...
    > # echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...
    >
    > In the first event, the event's timetamp is saved into the variable
    > ts0. In the next line, ts0 is subtracted from the second event's
    > timestamp to produce the latency.
    >
    > Further users of variable references will be described in subsequent
    > patches, such as for instance how the 'wakeup_lat' variable above can
    > be displayed in a latency histogram.
    >
    > Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
    > ---

    I think it'd be better spliting this into 3 parts: add hist_elt_data,
    pass tracing_map_elt to hist field func and add var ref logic.


    [SNIP]
    > @@ -241,6 +270,324 @@ static u64 hist_field_timestamp(struct hist_field *hist_field, void *event,
    > return ts;
    > }
    >
    > +static LIST_HEAD(hist_var_list);
    > +
    > +struct hist_var_data {
    > + struct list_head list;
    > + struct hist_trigger_data *hist_data;
    > +};

    Hmm.. I'm confused whether this list maintains reference of variable
    or defintion (or both?). It seems an entry is added only by
    save_hist_vars() which is for definition. But find_any_var_ref()
    looks it up for references.. What am I missing?

    Also I think it'd be better keeping this list for each instance rather
    than globally.


    [SNIP]
    > @@ -2186,10 +2613,32 @@ static void hist_unregister_trigger(char *glob, struct event_trigger_ops *ops,
    > tracing_set_time_stamp_abs(file->tr, false);
    > }
    >
    > +static bool hist_file_check_refs(struct trace_event_file *file)
    > +{
    > + struct hist_trigger_data *hist_data;
    > + struct event_trigger_data *test;
    > +
    > + printk("func: %s\n", __func__);

    Please remove this.

    Thanks,
    Namhyung


    > +
    > + list_for_each_entry_rcu(test, &file->triggers, list) {
    > + if (test->cmd_ops->trigger_type == ETT_EVENT_HIST) {
    > + hist_data = test->private_data;
    > + if (check_var_refs(hist_data))
    > + return true;
    > + break;
    > + }
    > + }
    > +
    > + return false;
    > +}

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