Documentation for event filters and formats.
Signed-off-by: Tom Zanussi <[email protected]>
Reviewed-by: Li Zefan <[email protected]>
---
Documentation/trace/events.txt | 184 +++++++++++++++++++++++++++++++++++++++-
1 files changed, 183 insertions(+), 1 deletions(-)
diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt
index 2bcc8d4..57e0a08 100644
--- a/Documentation/trace/events.txt
+++ b/Documentation/trace/events.txt
@@ -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 set at 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 was 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_switch/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
--
1.5.6.3
* Tom Zanussi <[email protected]> wrote:
> +# 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
Btw., are we willing to pin down this format in some sort of ABI?
If yes then this enumeration should probably also be represented in
/proc/events/ or /sys/kernel/events/.
Then we also need a TRACE_EVENT_ABI():
#define TRACE_EVENT_ABI TRACE_EVENT
To document and declare tracepoints that maintainers are willing to
maintain as an ABI component.
The scheduler tracepoints could certainly be propagated to that
status. (For that they need to be made 32-bit/64-bit safe first by
using u32/u64, etc. - but that's doable.)
Thoughts?
Ingo