[PATCH 00/32] tracing: Inter-event (e.g. latency) support

Tom Zanussi
2017-06-26T22:49:43Z

This patchset adds support for 'inter-event' quantities to the trace
event subsystem. The most important example of inter-event quantities
are latencies, or the time differences between two events.

This is a follow-up to the initial RFC patchset and subsequent
discussion at ELC. I think I've covered all the basic suggestions
from both those sources - thanks to everyone who reviewed and
contributed suggestions. Also, thanks to Vedang Patel for testing and
providing real-user input and scripts - a number of the new patches
such as extended error reporting, cpu fields, and aliases address his
input directly.

The current patchset has been fairly well tested and I'm not aware of
any major problems with it, but there is one problem/feature to be
aware of, which has to do with the fact that currently the tracing map
can naturally have duplicate entries. Normally this is expected and
isn't a problem for histograms without variables attached, as
duplicates are merged on output. If a histogram has variables though
and gets a duplicate entry, the variable associated with that record
can't be trusted, since it can't be retrieved again. Luckily
duplicates are normally very rare occurrences - I've never personally
seen one in practice - but it does mean that you always need to check
that the 'Duplicates' value on the histogram summary is 0 if that
histogram has variables attached. We're currently working on a 'fix'
for this that basically eliminates the possibility of duplicates, but
for now the 'Duplicates' total will alert users to that rare situation
should it occur.

One of the main motivations for adding this capability is to provide a
general-purpose base that existing existing tools such as the -RT
latency_hist patchset can be built upon, while at the same time
providing a simple way for users to track latencies (or any
inter-event quantity) generically between any two events.

Previous -RT latency_hist patchsets that take advantage of the trace
event subsystem have been submitted, but they essentially hard-code
special-case tracepoints and application logic in ways that can't be
reused. It seemed to me that rather than providing a one-off patchset
devoted specifically to generating the specific histograms in the
latency_hist patchset, it should be possible to build the same
functionality on top of a generic layer allowing users to do similar
things for other non-latency_hist applications.

In addition to preliminary patches that add some basic missing
functionality such as a common ringbuffer-derived timestamp and
dynamically-creatable tracepoints, the overall patchset is divided up
into a few different areas that combine to produce the overall goal
(The Documentation patch explains all the details):

- variables and simple expressions required to calculate a latency

In order to calculate a latency or any inter-event value,
something from one event needs to be saved and later retrieved,
and some operation such as subtraction or addition is performed on
it. This means some minimal form of variables and expressions,
which the first set of patches implements. Saving and retrieving
events to use in a latency calculation is normally done using a
hash table, and that's exactly what we have with trace event hist
triggers, so that's where variables are instantiated, set, and
retrieved. Basically, variables are set on one entry and
retrieved and used by a 'matching' event.

- 'synthetic' events, combining variables from other events

The trace event interface is based on pseudo-files associated with
individual events, so it wouldn't really make sense to have
quantities derived from multiple events attached to any one of
those events. For that reason, the patchset implements a means of
combining variables from other events into a separate 'synthetic'
event, which can be treated as if it were just like any other
trace event in the system.

- 'actions' generating synthetic events, among other things

Variables and synthetic events provide the data and data structure
for new events, but something still needs to actually generate an
event using that data. 'Actions' are expanded to provide that
capability. Though it hasn't been explicitly called as much
before, the default 'action' currently for a hist trigger is to
update the matching histogram entry's sum values. This patchset
essentially expands that to provide a new 'onmatch.event(params)'
action that can be used to have one event generate another. The
mechanism is extensible to other actions, and in fact the patchset
also includes another, 'onmax(var).save(field,...)' that can be
used to save context whenever a value exceeds the previous maximum
(something also needed by latency_hist).

Here are some examples that should make things less abstract.

====
Example - wakeup latency
====

This basically implements the -RT latency_hist 'wakeup_latency'
histogram using the synthetic events, variables, and actions
described. The output below is from a run of cyclictest using the
following command:

# rt-tests/cyclictest -p 80 -n -s -t 2

What we're measuring the latency of is the time between when a
thread (of cyclictest) is awakened and when it's scheduled in. To
do that we add triggers to sched_wakeup and sched_switch with the
appropriate variables, and on a matching sched_switch event,
generate a synthetic 'wakeup_latency' event. Since it's just
another trace event like any other, we can also define a histogram
on that event, the output of which is what we see displayed when
reading the wakeup_latency 'hist' file.

First, we create a synthetic event called wakeup_latency, that
creates 3 fields which will reference variables from other events:

# echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
/sys/kernel/debug/tracing/synthetic_events

Next we add a trigger to sched_wakeup, which saves the value of the
'$common_timestamp' when that event is hit in a variable, ts0. Note
that this happens only when 'comm==cyclictest'.

Also, '$common_timestamp' is a new field defined on every event (if
needed - if there are no users of timestamps in a trace, timestamps
won't be saved and there's no additional overhead from that).

# echo 'hist:keys=pid:ts0=$common_timestamp.usecs if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger

Next, we add a trigger to sched_switch. When the pid being switched
to matches the pid woken up by a previous sched_wakeup event, this
event grabs the ts0 variable saved on that event, takes the
difference between it and the current sched_switch's
$common_timestamp, and assigns it to a new 'wakeup_lat' variable.
It then generates the wakeup_latency synthetic event defined earlier
by 'invoking' it as a function using as parameters the wakeup_lat
variable and two sched_switch event fields directly:

# echo 'hist:keys=next_pid:wakeup_lat=$common_timestamp.usecs-$ts0: \
onmatch(sched.sched_wakeup).wakeup_latency($wakeup_lat,next_pid,next_prio) \
if next_comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger

Finally, all we have left to do is create a standard histogram
simply naming the fields of the wakeup_latency synthetic event:

# echo 'hist:keys=pid,prio,lat:sort=pid,lat' >> \
/sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger

At any time, we can see the histogram output by simply reading the
synthetic/wakeup_latency/hist file:

# cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist

# event histogram
#
# trigger info: hist:keys=pid,prio,lat:vals=hitcount:sort=pid,lat:size=2048 [active]
#

{ pid: 2519, prio: 120, lat: 1 } hitcount: 12
{ pid: 2519, prio: 120, lat: 2 } hitcount: 671
{ pid: 2519, prio: 120, lat: 3 } hitcount: 588
{ pid: 2519, prio: 120, lat: 4 } hitcount: 202
{ pid: 2519, prio: 120, lat: 5 } hitcount: 28
{ pid: 2519, prio: 120, lat: 6 } hitcount: 13
{ pid: 2519, prio: 120, lat: 7 } hitcount: 12
{ pid: 2519, prio: 120, lat: 8 } hitcount: 7
{ pid: 2519, prio: 120, lat: 9 } hitcount: 12
{ pid: 2519, prio: 120, lat: 10 } hitcount: 11
{ pid: 2519, prio: 120, lat: 11 } hitcount: 7
{ pid: 2519, prio: 120, lat: 12 } hitcount: 6
{ pid: 2519, prio: 120, lat: 13 } hitcount: 1
{ pid: 2519, prio: 120, lat: 17 } hitcount: 1
{ pid: 2519, prio: 120, lat: 18 } hitcount: 3
{ pid: 2519, prio: 120, lat: 19 } hitcount: 2
{ pid: 2519, prio: 120, lat: 22 } hitcount: 2
{ pid: 2519, prio: 120, lat: 23 } hitcount: 1
{ pid: 2519, prio: 120, lat: 24 } hitcount: 1
{ pid: 2519, prio: 120, lat: 27 } hitcount: 1
{ pid: 2519, prio: 120, lat: 34 } hitcount: 1
{ pid: 2519, prio: 120, lat: 53 } hitcount: 1
{ pid: 2519, prio: 120, lat: 67 } hitcount: 1
{ pid: 2519, prio: 120, lat: 69 } hitcount: 1
{ pid: 2521, prio: 19, lat: 1 } hitcount: 735
{ pid: 2521, prio: 19, lat: 2 } hitcount: 8978
{ pid: 2521, prio: 19, lat: 3 } hitcount: 4798
{ pid: 2521, prio: 19, lat: 4 } hitcount: 716
{ pid: 2521, prio: 19, lat: 5 } hitcount: 298
{ pid: 2521, prio: 19, lat: 6 } hitcount: 136
{ pid: 2521, prio: 19, lat: 7 } hitcount: 93
{ pid: 2521, prio: 19, lat: 8 } hitcount: 51
{ pid: 2521, prio: 19, lat: 9 } hitcount: 20
{ pid: 2521, prio: 19, lat: 10 } hitcount: 18
{ pid: 2521, prio: 19, lat: 11 } hitcount: 3
{ pid: 2521, prio: 19, lat: 12 } hitcount: 1
{ pid: 2521, prio: 19, lat: 13 } hitcount: 3
{ pid: 2521, prio: 19, lat: 15 } hitcount: 1
{ pid: 2521, prio: 19, lat: 17 } hitcount: 1
{ pid: 2521, prio: 19, lat: 18 } hitcount: 1
{ pid: 2521, prio: 19, lat: 20 } hitcount: 2
{ pid: 2521, prio: 19, lat: 22 } hitcount: 1
{ pid: 2521, prio: 19, lat: 25 } hitcount: 1
{ pid: 2521, prio: 19, lat: 26 } hitcount: 1
{ pid: 2522, prio: 19, lat: 1 } hitcount: 392
{ pid: 2522, prio: 19, lat: 2 } hitcount: 5376
{ pid: 2522, prio: 19, lat: 3 } hitcount: 3982
{ pid: 2522, prio: 19, lat: 4 } hitcount: 500
{ pid: 2522, prio: 19, lat: 5 } hitcount: 202
{ pid: 2522, prio: 19, lat: 6 } hitcount: 67
{ pid: 2522, prio: 19, lat: 7 } hitcount: 35
{ pid: 2522, prio: 19, lat: 8 } hitcount: 12
{ pid: 2522, prio: 19, lat: 9 } hitcount: 9
{ pid: 2522, prio: 19, lat: 10 } hitcount: 4
{ pid: 2522, prio: 19, lat: 11 } hitcount: 3
{ pid: 2522, prio: 19, lat: 12 } hitcount: 1
{ pid: 2522, prio: 19, lat: 13 } hitcount: 1
{ pid: 2522, prio: 19, lat: 16 } hitcount: 1
{ pid: 2522, prio: 19, lat: 18 } hitcount: 2
{ pid: 2522, prio: 19, lat: 19 } hitcount: 1
{ pid: 2522, prio: 19, lat: 21 } hitcount: 2
{ pid: 2522, prio: 19, lat: 22 } hitcount: 1
{ pid: 2522, prio: 19, lat: 23 } hitcount: 1
{ pid: 2522, prio: 19, lat: 45 } hitcount: 1
{ pid: 2522, prio: 19, lat: 82 } hitcount: 1

Totals:
Hits: 28037
Entries: 65
Dropped: 0
Duplicates: 0

The above output uses the .usecs modifier to common_timestamp, so
the latencies are reported in microseconds. The default, without
the modifier, is nanoseconds, but that's too fine-grained to put
directly into a histogram - for that however we can use the .log2
modifier on the 'lat' key. Otherwise the rest is the same:

# echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
/sys/kernel/debug/tracing/synthetic_events

# echo 'hist:keys=pid:ts0=$common_timestamp if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger

# echo 'hist:keys=next_pid:wakeup_lat=$common_timestamp-$ts0: \
onmatch(sched.sched_wakeup).wakeup_latency($wakeup_lat,next_pid,next_prio) \
if next_comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger

# echo 'hist:keys=pid,prio,lat.log2:sort=pid,lat' >> \
/sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger

# cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist

# event histogram
#
# trigger info: hist:keys=pid,prio,lat.log2:vals=hitcount:sort=pid,lat.log2:size=2048 [active]
#

{ pid: 2585, prio: 120, lat: ~ 2^10 } hitcount: 1
{ pid: 2585, prio: 120, lat: ~ 2^11 } hitcount: 379
{ pid: 2585, prio: 120, lat: ~ 2^12 } hitcount: 1008
{ pid: 2585, prio: 120, lat: ~ 2^13 } hitcount: 42
{ pid: 2585, prio: 120, lat: ~ 2^14 } hitcount: 18
{ pid: 2585, prio: 120, lat: ~ 2^15 } hitcount: 3
{ pid: 2585, prio: 120, lat: ~ 2^16 } hitcount: 1
{ pid: 2586, prio: 19, lat: ~ 2^11 } hitcount: 4715
{ pid: 2586, prio: 19, lat: ~ 2^12 } hitcount: 9161
{ pid: 2586, prio: 19, lat: ~ 2^13 } hitcount: 632
{ pid: 2586, prio: 19, lat: ~ 2^14 } hitcount: 47
{ pid: 2586, prio: 19, lat: ~ 2^15 } hitcount: 3
{ pid: 2586, prio: 19, lat: ~ 2^17 } hitcount: 1
{ pid: 2587, prio: 19, lat: ~ 2^11 } hitcount: 3398
{ pid: 2587, prio: 19, lat: ~ 2^12 } hitcount: 5762
{ pid: 2587, prio: 19, lat: ~ 2^13 } hitcount: 505
{ pid: 2587, prio: 19, lat: ~ 2^14 } hitcount: 58
{ pid: 2587, prio: 19, lat: ~ 2^15 } hitcount: 3
{ pid: 2587, prio: 19, lat: ~ 2^17 } hitcount: 1

Totals:
Hits: 25738
Entries: 19
Dropped: 0
Duplicates: 0

====
Example - wakeup latency with onmax()
====

This example is the same as the previous ones, but here we're
additionally using the onmax() action to save some context (several
fields of the sched_switch event) whenever the latency (wakeup_lat)
exceeds the previous maximum.

As with the similar functionality of the -RT latency_hist
histograms, it's useful to be able to capture information about the
previous process, which potentially could have contributed to the
maximum latency that was saved.

# echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
/sys/kernel/debug/tracing/synthetic_events

# echo 'hist:keys=pid:ts0=$common_timestamp.usecs if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger

Here we add an onmax() action that saves some important fields of
the sched_switch event along with the maximum, in addition to
sending some of the same data to the synthetic event:

# echo 'hist:keys=next_pid:wakeup_lat=$common_timestamp.usecs-$ts0: \
onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm): \
onmatch(sched.sched_wakeup).wakeup_latency($wakeup_lat,next_pid,next_prio) \
if next_comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger

# echo 'hist:keys=pid,prio,lat:sort=pid,lat' >> \
/sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger

To see the maximums and associated data for each pid, cat the
sched_switch event, as that's the event the onmax() action is
associated with:

# cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist

# event histogram
#

{ next_pid: 6173 } hitcount: 199
max: 14 next_comm: cyclictest prev_pid: 0 \
prev_prio: 120 prev_comm: swapper/2
{ next_pid: 6175 } hitcount: 1327
max: 15 next_comm: cyclictest prev_pid: 0 \
prev_prio: 120 prev_comm: swapper/2
{ next_pid: 6174 } hitcount: 1985
max: 30 next_comm: cyclictest prev_pid: 0 \
prev_prio: 120 prev_comm: swapper/3

Totals:
Hits: 17555
Entries: 3
Dropped: 0
Duplicates: 0

And, verifying, we can see that the max latencies captured above
match the highest latencies for each thread in the wakeup_latency
histogram:

# cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist

# event histogram
#
# trigger info: hist:keys=pid,prio,lat:vals=hitcount:sort=pid,lat:size=2048 [active]
#

{ pid: 6173, prio: 120, lat: 1 } hitcount: 2
{ pid: 6173, prio: 120, lat: 2 } hitcount: 82
{ pid: 6173, prio: 120, lat: 3 } hitcount: 84
...
{ pid: 6173, prio: 120, lat: 14 } hitcount: 1
{ pid: 6174, prio: 19, lat: 1 } hitcount: 23
{ pid: 6174, prio: 19, lat: 2 } hitcount: 749
...
{ pid: 6174, prio: 19, lat: 30 } hitcount: 1
{ pid: 6175, prio: 19, lat: 1 } hitcount: 20
{ pid: 6175, prio: 19, lat: 2 } hitcount: 633
...
{ pid: 6175, prio: 19, lat: 15 } hitcount: 1

Totals:
Hits: 3511
Entries: 38
Dropped: 0
Duplicates: 0

====
Example - combined wakeup and switchtime (wakeupswitch) latency
====

Finally, this example is quite a bit more involved, but that's
because it implements 3 latencies, one which is a combination of the
other two. This also, is something that the -RT latency_hist
patchset does and which this patchset adds generic support for.

The latency_hist patchset creates a few individual latency
histograms but also combines them into larger overall combined
histograms. For example, the time between when a thread is awakened
and when it actually continues executing in userspace is something
covered by a histogram, but it's also broken down into two
sub-histograms, one covering the time between sched_wakeup and the
time the thread is scheduled in (wakeup_latency as above), and the
time between when the thread is scheduled in and the time it
actually begins executing again (return from sys_nanosleep), covered
by a separate switchtime_latency histogram.

The below combines the wakeup_latency histogram from before, adds a
new switchtime_latency histogram, and another, wakeupswitch_latency,
that's a combination of the other two.

There isn't anything really new here, other than that the use of the
addition operator to add two latencies to produce the
wakeupswitch_latency.

First, we create the familiar wakeup_latency histogram:

# echo 'wakeup_latency u64 lat; pid_t pid' >> \
/sys/kernel/debug/tracing/synthetic_events

# echo 'hist:keys=pid:ts0=$common_timestamp.usecs \
if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger

# echo 'hist:keys=next_pid:wakeup_lat=$common_timestamp.usecs-$ts0:\
onmatch(sched.sched_wakeup).wakeup_latency($wakeup_lat,next_pid) \
if next_comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger

Here we save the wakeup_latency lat value as wakeup_lat for use
later in the combined latency:

# echo 'hist:keys=pid,lat:wakeup_lat=lat:sort=pid,lat' \
>> /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger

Next, we create the switchtime_latency histogram:

# echo 'switchtime_latency u64 lat; pid_t pid' >> \
/sys/kernel/debug/tracing/synthetic_events

Here we save the sched_switch next_pid field as 'pid'. This is so
we can access the next_pid in the matching sys_exit_nanosleep event.

# echo 'hist:keys=next_pid:pid=next_pid:ts0=$common_timestamp.usecs \
if next_comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger

# echo 'hist:keys=common_pid:switchtime_lat=$common_timestamp.usecs-$ts0: \
onmatch(sched.sched_switch).switchtime_latency($switchtime_lat,$pid)' \
>> /sys/kernel/debug/tracing/events/syscalls/sys_exit_nanosleep/trigger

# echo 'hist:keys=pid,lat:sort=pid,lat' \
>> /sys/kernel/debug/tracing/events/synthetic/switchtime_latency/trigger

Finally, we create the combined wakeupswitch_latency:

# echo 'wakeupswitch_latency u64 lat; pid_t pid' >> \
/sys/kernel/debug/tracing/synthetic_events

Here we calculate the combined latency using the saved $wakeup_lat
variable from the wakeup_latency histogram and the lat value of the
switchtime_latency, save it as ws_lat and then use it to generate
the combined wakeupswitch latency:

# echo 'hist:keys=pid,lat:sort=pid,lat:ws_lat=$wakeup_lat+lat: \
onmatch(synthetic.wakeup_latency).wakeupswitch_latency($ws_lat,pid)' \
>> /sys/kernel/debug/tracing/events/synthetic/switchtime_latency/trigger

# echo 'hist:keys=pid,lat:sort=pid,lat' >> \
/sys/kernel/debug/tracing/events/synthetic/wakeupswitch_latency/trigger

After running our cyclictest workload, we can now look at each
histogram, starting with wakeup_latency:

# cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist

# event histogram
#
# trigger info: hist:keys=pid,lat:vals=hitcount:wakeup_lat=lat:sort=pid,lat:size=2048 [active]
#

{ pid: 6852, lat: 1 } hitcount: 2
{ pid: 6852, lat: 2 } hitcount: 88
{ pid: 6852, lat: 3 } hitcount: 72
{ pid: 6852, lat: 4 } hitcount: 19
{ pid: 6852, lat: 5 } hitcount: 10
{ pid: 6852, lat: 6 } hitcount: 1
{ pid: 6852, lat: 7 } hitcount: 4
{ pid: 6852, lat: 9 } hitcount: 2
{ pid: 6852, lat: 10 } hitcount: 1
{ pid: 6853, lat: 1 } hitcount: 1
{ pid: 6853, lat: 2 } hitcount: 535
{ pid: 6853, lat: 3 } hitcount: 1007
...
{ pid: 6853, lat: 19 } hitcount: 1
{ pid: 6853, lat: 20 } hitcount: 1
{ pid: 6853, lat: 29 } hitcount: 1
{ pid: 6854, lat: 1 } hitcount: 5
{ pid: 6854, lat: 2 } hitcount: 552
{ pid: 6854, lat: 3 } hitcount: 532
{ pid: 6854, lat: 4 } hitcount: 135
{ pid: 6854, lat: 5 } hitcount: 48
{ pid: 6854, lat: 6 } hitcount: 18
{ pid: 6854, lat: 7 } hitcount: 12
{ pid: 6854, lat: 8 } hitcount: 5
...
{ pid: 6854, lat: 29 } hitcount: 2
{ pid: 6854, lat: 48 } hitcount: 1

Totals:
Hits: 3501
Entries: 46
Dropped: 0
Duplicates: 0

Here's the switchtime histogram:

# cat /sys/kernel/debug/tracing/events/synthetic/switchtime_latency/hist

# event histogram
#
# trigger info: hist:keys=pid,lat:vals=hitcount:sort=pid,lat:size=2048 [active]
#

{ pid: 6852, lat: 1 } hitcount: 1
{ pid: 6852, lat: 2 } hitcount: 33
{ pid: 6852, lat: 3 } hitcount: 73
{ pid: 6852, lat: 4 } hitcount: 49
{ pid: 6852, lat: 5 } hitcount: 28
{ pid: 6852, lat: 6 } hitcount: 4
...
{ pid: 6852, lat: 16 } hitcount: 1
{ pid: 6853, lat: 1 } hitcount: 76
{ pid: 6853, lat: 2 } hitcount: 1190
{ pid: 6853, lat: 3 } hitcount: 475
{ pid: 6853, lat: 4 } hitcount: 125
{ pid: 6853, lat: 5 } hitcount: 40
{ pid: 6853, lat: 6 } hitcount: 21
{ pid: 6853, lat: 7 } hitcount: 13
{ pid: 6853, lat: 8 } hitcount: 5
{ pid: 6853, lat: 9 } hitcount: 11
...
{ pid: 6853, lat: 20 } hitcount: 2
{ pid: 6854, lat: 1 } hitcount: 13
{ pid: 6854, lat: 2 } hitcount: 799
{ pid: 6854, lat: 3 } hitcount: 332
{ pid: 6854, lat: 4 } hitcount: 97
{ pid: 6854, lat: 5 } hitcount: 29
{ pid: 6854, lat: 6 } hitcount: 16
...
{ pid: 6854, lat: 22 } hitcount: 1
{ pid: 6854, lat: 26 } hitcount: 1

Totals:
Hits: 3501
Entries: 48
Dropped: 0
Duplicates: 0

And here's the combined wakeupswitch latency histogram:

# cat /sys/kernel/debug/tracing/events/synthetic/wakeupswitch_latency/hist

# event histogram
#
# trigger info: hist:keys=pid,lat:vals=hitcount:sort=pid,lat:size=2048 [active]
#

{ pid: 6852, lat: 4 } hitcount: 24
{ pid: 6852, lat: 5 } hitcount: 47
{ pid: 6852, lat: 6 } hitcount: 53
{ pid: 6852, lat: 7 } hitcount: 29
...
{ pid: 6852, lat: 25 } hitcount: 1
{ pid: 6853, lat: 3 } hitcount: 5
{ pid: 6853, lat: 4 } hitcount: 504
{ pid: 6853, lat: 5 } hitcount: 753
{ pid: 6853, lat: 6 } hitcount: 337
{ pid: 6853, lat: 7 } hitcount: 149
{ pid: 6853, lat: 8 } hitcount: 66
{ pid: 6853, lat: 9 } hitcount: 45
{ pid: 6853, lat: 10 } hitcount: 24
{ pid: 6853, lat: 11 } hitcount: 26
{ pid: 6853, lat: 12 } hitcount: 9
{ pid: 6853, lat: 13 } hitcount: 13
{ pid: 6853, lat: 14 } hitcount: 9
{ pid: 6853, lat: 15 } hitcount: 3
{ pid: 6853, lat: 17 } hitcount: 3
{ pid: 6853, lat: 18 } hitcount: 5
{ pid: 6853, lat: 19 } hitcount: 2
...
{ pid: 6853, lat: 37 } hitcount: 1
{ pid: 6853, lat: 38 } hitcount: 1
{ pid: 6854, lat: 3 } hitcount: 7
{ pid: 6854, lat: 4 } hitcount: 418
{ pid: 6854, lat: 5 } hitcount: 493
{ pid: 6854, lat: 6 } hitcount: 165
{ pid: 6854, lat: 7 } hitcount: 81
{ pid: 6854, lat: 8 } hitcount: 57
{ pid: 6854, lat: 9 } hitcount: 27
{ pid: 6854, lat: 10 } hitcount: 15
{ pid: 6854, lat: 11 } hitcount: 13
{ pid: 6854, lat: 12 } hitcount: 7
{ pid: 6854, lat: 13 } hitcount: 10
{ pid: 6854, lat: 14 } hitcount: 6
{ pid: 6854, lat: 15 } hitcount: 5
{ pid: 6854, lat: 16 } hitcount: 5
{ pid: 6854, lat: 17 } hitcount: 2
{ pid: 6854, lat: 18 } hitcount: 1
...
{ pid: 6854, lat: 55 } hitcount: 1

Totals:
Hits: 3501
Entries: 70
Dropped: 0
Duplicates: 0

Finally, just to show that synthetic events are indeed just like any
other event as far as the event subsystem is concerned, we can
enable the synthetic events and see the events appear in the trace
buffer:

# echo 1 > /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/enable
# echo 1 > /sys/kernel/debug/tracing/events/synthetic/switchtime_latency/enable
# echo 1 > /sys/kernel/debug/tracing/events/synthetic/wakeupswitch_latency/enable

Below is a snippet of the contents of the trace file produced when
the above histograms were generated:

# cat /sys/kernel/debug/tracing/trace

# tracer: nop
#
# entries-in-buffer/entries-written: 10503/10503 #P:4
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
<idle>-0 [001] d..2 23532.240146: wakeup_latency: lat=4, pid=6853
cyclictest-6853 [001] .... 23532.240153: switchtime_latency: lat=7, pid=6853
cyclictest-6853 [001] .... 23532.240157: wakeupswitch_latency: lat=11, pid=6853
gnome-terminal--2500 [001] d..2 23532.240672: wakeup_latency: lat=5, pid=6854
cyclictest-6854 [001] .... 23532.240676: switchtime_latency: lat=4, pid=6854
cyclictest-6854 [001] .... 23532.240677: wakeupswitch_latency: lat=9, pid=6854
gnome-terminal--2500 [001] d..2 23532.241169: wakeup_latency: lat=4, pid=6853
cyclictest-6853 [001] .... 23532.241172: switchtime_latency: lat=3, pid=6853
cyclictest-6853 [001] .... 23532.241174: wakeupswitch_latency: lat=7, pid=6853
<idle>-0 [001] d..2 23532.242189: wakeup_latency: lat=6, pid=6853
cyclictest-6853 [001] .... 23532.242195: switchtime_latency: lat=8, pid=6853
<idle>-0 [000] d..2 23532.242196: wakeup_latency: lat=12, pid=6854
cyclictest-6853 [001] .... 23532.240146: wakeupswitch_latency: lat=14, pid=6853
cyclictest-6854 [000] .... 23532.242196: switchtime_latency: lat=4, pid=6854
<idle>-0 [001] d..2 23532.240146: wakeup_latency: lat=2, pid=6853
cyclictest-6854 [000] .... 23532.242196: wakeupswitch_latency: lat=16, pid=6854
cyclictest-6853 [001] .... 23532.240146: switchtime_latency: lat=3, pid=6853
...

One quick note about usage - the introduction of variables and
actions obviously makes it harder to determine the cause of a hist
trigger command failure - 'Invalid argument' is no longer sufficient
in many cases.

For that reason, a new 'extended error' mechanism has been added to
hist triggers, initially focused on variable and action-related
errors, but something that could possibly expanded to other error
conditions later.

To make use of it, simply read the 'hist' file of the event that was
the target of the command.

In this example, we've entered the same command twice, resulting in
an attempt to define the same variable (ts0) twice. After seeing
the 'Invalid argument' error for the command, we read the same
event's hist file and see a message to that effect at the bottom of
the file:

# echo 'hist:keys=pid:ts0=$common_timestamp.usecs if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger

# echo 'hist:keys=pid:ts0=$common_timestamp.usecs if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_wakeup/trigger

-su: echo: write error: Invalid argument

# cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/hist
# event histogram
#
#

Totals:
Hits: 0
Entries: 0
Dropped: 0
Duplicates: 0

ERROR: Variable already defined: ts0
Last command: keys=pid:ts0=$common_timestamp.usecs if comm=="cyclictest"

The following changes since commit 681bec0367c2606b6154060310a2ffa543175980:

tracing: Rename update the enum_map file (2017-06-13 17:13:06 -0400)

are available in the git repository at:

https://github.com/tzanussi/linux-trace-inter-event.git tzanussi/inter-event-v01
https://github.com/tzanussi/linux-trace-inter-event/tree/tzanussi/inter-event-v01

Tom Zanussi (32):
tracing: Add hist_field_name() accessor
tracing: Reimplement log2
ring-buffer: Add interface for setting absolute time stamps
ring-buffer: Redefine the unimplemented RINGBUF_TIME_TIME_STAMP
tracing: Give event triggers access to ring_buffer_event
tracing: Add ring buffer event param to hist field functions
tracing: Increase tracing map KEYS_MAX size
tracing: Break out hist trigger assignment parsing
tracing: Make traceprobe parsing code reusable
tracing: Add NO_DISCARD event file flag
tracing: Add post-trigger flag to hist trigger command
tracing: Add hist trigger timestamp support
tracing: Add per-element variable support to tracing_map
tracing: Add hist_data member to hist_field
tracing: Add usecs modifier for hist trigger timestamps
tracing: Add variable support to hist triggers
tracing: Account for variables in named trigger compatibility
tracing: Add simple expression support to hist triggers
tracing: Add variable reference handling to hist triggers
tracing: Add support for dynamic tracepoints
tracing: Add hist trigger action hook
tracing: Add support for 'synthetic' events
tracing: Add 'onmatch' hist trigger action support
tracing: Add 'onmax' hist trigger action support
tracing: Allow whitespace to surround hist trigger filter
tracing: Make duplicate count from tracing_map available
tracing: Add cpu field for hist triggers
tracing: Add hist trigger support for variable reference aliases
tracing: Add 'last error' error facility for hist triggers
tracing: Add inter-event hist trigger Documentation
tracing: Make tracing_set_clock() non-static
tracing: Add a clock attribute for hist triggers

Documentation/trace/events.txt | 422 ++++
include/linux/ring_buffer.h | 14 +-
include/linux/trace_events.h | 17 +-
include/linux/tracepoint.h | 11 +-
kernel/trace/ring_buffer.c | 118 +-
kernel/trace/trace.c | 121 +-
kernel/trace/trace.h | 36 +-
kernel/trace/trace_events.c | 4 +-
kernel/trace/trace_events_hist.c | 4107 ++++++++++++++++++++++++++++++++---
kernel/trace/trace_events_trigger.c | 69 +-
kernel/trace/trace_kprobe.c | 18 +-
kernel/trace/trace_probe.c | 86 -
kernel/trace/trace_probe.h | 7 -
kernel/trace/trace_uprobe.c | 2 +-
kernel/trace/tracing_map.c | 125 +-
kernel/trace/tracing_map.h | 16 +-
kernel/tracepoint.c | 42 +-
17 files changed, 4684 insertions(+), 531 deletions(-)

--
1.9.3