Event Histograms
Documentation written by Tom Zanussi
1. Introduction
===============
Histogram triggers are special event triggers that can be used to
aggregate trace event data into histograms. For information on
trace events and event triggers, see Documentation/trace/events.txt.
2. Histogram Trigger Command
============================
A histogram trigger command is an event trigger command that
aggregates event hits into a hash table keyed on one or more trace
event format fields (or stacktrace) and a set of running totals
derived from one or more trace event format fields and/or event
counts (hitcount).
The format of a hist trigger is as follows:
hist:keys=<field1[,field2,...]>[:values=<field1[,field2,...]>]
[:sort=<field1[,field2,...]>][:size=#entries][:pause][:continue]
[:clear][:name=histname1] [if <filter>]
When a matching event is hit, an entry is added to a hash table
using the key(s) and value(s) named. Keys and values correspond to
fields in the event's format description. Values must correspond to
numeric fields - on an event hit, the value(s) will be added to a
sum kept for that field. The special string 'hitcount' can be used
in place of an explicit value field - this is simply a count of
event hits. If 'values' isn't specified, an implicit 'hitcount'
value will be automatically created and used as the only value.
Keys can be any field, or the special string 'stacktrace', which
will use the event's kernel stacktrace as the key. The keywords
'keys' or 'key' can be used to specify keys, and the keywords
'values', 'vals', or 'val' can be used to specify values. Compound
keys consisting of up to t