Re: [PATCH 4/6] perf_counter: Add PERF_COUNTER_IOC_SET_FILTER ioctl

Randy Dunlap
2009-09-10T23:01:57Z

On Tue, 08 Sep 2009 02:01:55 -0500 Tom Zanussi wrote:

> I've been meaning to write up something about the event filters - here's
> a first stab that hopefully helps explain them...
>
> Tom

Hi Tom,

Pretty good info. Thanks.
Just a few small comments & question(s):

> diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt
> index 2bcc8d4..50fe510 100644
> --- a/Documentation/trace/events.txt
> +++ b/Documentation/trace/events.txt
> @@ -97,3 +97,160 @@ 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
> +below).
> +
> +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.
> +
> +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
> +
> +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, which is the default when no
> +filter has been set for an event.
> +
> +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 above).
> +
> +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.

in a filter is 16.

> +
> +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

If there is an error

> +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

caret ('^')

> +the filter string; the error message should be still be useful though
> +even without more accurate position info.
> +
> +To clear a filter, write a '0' to the filter file.
> +
> +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.
> +
> +To clear the filters for all events in a subsystem, write a '0' to the
> +subsystem's filter file.
> +
> +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

One of the above should be "sched_wakeup", like the examples below?

> +
> +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

---
~Randy
LPC 2009, Sept. 23-25, Portland, Oregon