tracing/filters: add filter Documentation
Documentation for event filters and formats. v2 changes: fix a few problems noticed by Randy Dunlap. Reviewed-by: Li Zefan <lizf@cn.fujitsu.com> Acked-by: Randy Dunlap <randy.dunlap@oracle.com> Signed-off-by: Tom Zanussi <tzanussi@gmail.com> LKML-Reference: <1252642431.8016.9.camel@tropicana> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Tom Zanussi
Steven Rostedt
parent
558e6547e4
commit
95b696088c
1 changed files with 183 additions and 1 deletions
|
|
@ -1,7 +1,7 @@
|
|||
Event Tracing
|
||||
|
||||
Documentation written by Theodore Ts'o
|
||||
Updated by Li Zefan
|
||||
Updated by Li Zefan and Tom Zanussi
|
||||
|
||||
1. Introduction
|
||||
===============
|
||||
|
|
@ -97,3 +97,185 @@ The format of this boot option is the same as described in section 2.1.
|
|||
|
||||
See The example provided in samples/trace_events
|
||||
|
||||
4. Event formats
|
||||
================
|
||||
|
||||
Each trace event has a 'format' file associated with it that contains
|
||||
a description of each field in a logged event. This information can
|
||||
be used to parse the binary trace stream, and is also the place to
|
||||
find the field names that can be used in event filters (see section 5).
|
||||
|
||||
It also displays the format string that will be used to print the
|
||||
event in text mode, along with the event name and ID used for
|
||||
profiling.
|
||||
|
||||
Every event has a set of 'common' fields associated with it; these are
|
||||
the fields prefixed with 'common_'. The other fields vary between
|
||||
events and correspond to the fields defined in the TRACE_EVENT
|
||||
definition for that event.
|
||||
|
||||
Each field in the format has the form:
|
||||
|
||||
field:field-type field-name; offset:N; size:N;
|
||||
|
||||
where offset is the offset of the field in the trace record and size
|
||||
is the size of the data item, in bytes.
|
||||
|
||||
For example, here's the information displayed for the 'sched_wakeup'
|
||||
event:
|
||||
|
||||
# cat /debug/tracing/events/sched/sched_wakeup/format
|
||||
|
||||
name: sched_wakeup
|
||||
ID: 60
|
||||
format:
|
||||
field:unsigned short common_type; offset:0; size:2;
|
||||
field:unsigned char common_flags; offset:2; size:1;
|
||||
field:unsigned char common_preempt_count; offset:3; size:1;
|
||||
field:int common_pid; offset:4; size:4;
|
||||
field:int common_tgid; offset:8; size:4;
|
||||
|
||||
field:char comm[TASK_COMM_LEN]; offset:12; size:16;
|
||||
field:pid_t pid; offset:28; size:4;
|
||||
field:int prio; offset:32; size:4;
|
||||
field:int success; offset:36; size:4;
|
||||
field:int cpu; offset:40; size:4;
|
||||
|
||||
print fmt: "task %s:%d [%d] success=%d [%03d]", REC->comm, REC->pid,
|
||||
REC->prio, REC->success, REC->cpu
|
||||
|
||||
This event contains 10 fields, the first 5 common and the remaining 5
|
||||
event-specific. All the fields for this event are numeric, except for
|
||||
'comm' which is a string, a distinction important for event filtering.
|
||||
|
||||
5. Event filtering
|
||||
==================
|
||||
|
||||
Trace events can be filtered in the kernel by associating boolean
|
||||
'filter expressions' with them. As soon as an event is logged into
|
||||
the trace buffer, its fields are checked against the filter expression
|
||||
associated with that event type. An event with field values that
|
||||
'match' the filter will appear in the trace output, and an event whose
|
||||
values don't match will be discarded. An event with no filter
|
||||
associated with it matches everything, and is the default when no
|
||||
filter has been set for an event.
|
||||
|
||||
5.1 Expression syntax
|
||||
---------------------
|
||||
|
||||
A filter expression consists of one or more 'predicates' that can be
|
||||
combined using the logical operators '&&' and '||'. A predicate is
|
||||
simply a clause that compares the value of a field contained within a
|
||||
logged event with a constant value and returns either 0 or 1 depending
|
||||
on whether the field value matched (1) or didn't match (0):
|
||||
|
||||
field-name relational-operator value
|
||||
|
||||
Parentheses can be used to provide arbitrary logical groupings and
|
||||
double-quotes can be used to prevent the shell from interpreting
|
||||
operators as shell metacharacters.
|
||||
|
||||
The field-names available for use in filters can be found in the
|
||||
'format' files for trace events (see section 4).
|
||||
|
||||
The relational-operators depend on the type of the field being tested:
|
||||
|
||||
The operators available for numeric fields are:
|
||||
|
||||
==, !=, <, <=, >, >=
|
||||
|
||||
And for string fields they are:
|
||||
|
||||
==, !=
|
||||
|
||||
Currently, only exact string matches are supported.
|
||||
|
||||
Currently, the maximum number of predicates in a filter is 16.
|
||||
|
||||
5.2 Setting filters
|
||||
-------------------
|
||||
|
||||
A filter for an individual event is set by writing a filter expression
|
||||
to the 'filter' file for the given event.
|
||||
|
||||
For example:
|
||||
|
||||
# cd /debug/tracing/events/sched/sched_wakeup
|
||||
# echo "common_preempt_count > 4" > filter
|
||||
|
||||
A slightly more involved example:
|
||||
|
||||
# cd /debug/tracing/events/sched/sched_signal_send
|
||||
# echo "((sig >= 10 && sig < 15) || sig == 17) && comm != bash" > filter
|
||||
|
||||
If there is an error in the expression, you'll get an 'Invalid
|
||||
argument' error when setting it, and the erroneous string along with
|
||||
an error message can be seen by looking at the filter e.g.:
|
||||
|
||||
# cd /debug/tracing/events/sched/sched_signal_send
|
||||
# echo "((sig >= 10 && sig < 15) || dsig == 17) && comm != bash" > filter
|
||||
-bash: echo: write error: Invalid argument
|
||||
# cat filter
|
||||
((sig >= 10 && sig < 15) || dsig == 17) && comm != bash
|
||||
^
|
||||
parse_error: Field not found
|
||||
|
||||
Currently the caret ('^') for an error always appears at the beginning of
|
||||
the filter string; the error message should still be useful though
|
||||
even without more accurate position info.
|
||||
|
||||
5.3 Clearing filters
|
||||
--------------------
|
||||
|
||||
To clear the filter for an event, write a '0' to the event's filter
|
||||
file.
|
||||
|
||||
To clear the filters for all events in a subsystem, write a '0' to the
|
||||
subsystem's filter file.
|
||||
|
||||
5.3 Subsystem filters
|
||||
---------------------
|
||||
|
||||
For convenience, filters for every event in a subsystem can be set or
|
||||
cleared as a group by writing a filter expression into the filter file
|
||||
at the root of the subsytem. Note however, that if a filter for any
|
||||
event within the subsystem lacks a field specified in the subsystem
|
||||
filter, or if the filter can't be applied for any other reason, the
|
||||
filter for that event will retain its previous setting. This can
|
||||
result in an unintended mixture of filters which could lead to
|
||||
confusing (to the user who might think different filters are in
|
||||
effect) trace output. Only filters that reference just the common
|
||||
fields can be guaranteed to propagate successfully to all events.
|
||||
|
||||
Here are a few subsystem filter examples that also illustrate the
|
||||
above points:
|
||||
|
||||
Clear the filters on all events in the sched subsytem:
|
||||
|
||||
# cd /sys/kernel/debug/tracing/events/sched
|
||||
# echo 0 > filter
|
||||
# cat sched_switch/filter
|
||||
none
|
||||
# cat sched_wakeup/filter
|
||||
none
|
||||
|
||||
Set a filter using only common fields for all events in the sched
|
||||
subsytem (all events end up with the same filter):
|
||||
|
||||
# cd /sys/kernel/debug/tracing/events/sched
|
||||
# echo common_pid == 0 > filter
|
||||
# cat sched_switch/filter
|
||||
common_pid == 0
|
||||
# cat sched_wakeup/filter
|
||||
common_pid == 0
|
||||
|
||||
Attempt to set a filter using a non-common field for all events in the
|
||||
sched subsytem (all events but those that have a prev_pid field retain
|
||||
their old filters):
|
||||
|
||||
# cd /sys/kernel/debug/tracing/events/sched
|
||||
# echo prev_pid == 0 > filter
|
||||
# cat sched_switch/filter
|
||||
prev_pid == 0
|
||||
# cat sched_wakeup/filter
|
||||
common_pid == 0
|
||||
|
|
|
|||
Loading…
Reference in a new issue