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Subject[PATCH 3/3] tracing: Add a check_val() check before updating cond_snapshot() track_val
Date
From: Tom Zanussi <tom.zanussi@linux.intel.com>

Without this check a snapshot is taken whenever a bucket's max is hit,
rather than only when the global max is hit, as it should be.

Before:

In this example, we do a first run of the workload (cyclictest),
examine the output, note the max ('triggering value') (347), then do
a second run and note the max again.

In this case, the max in the second run (39) is below the max in the
first run, but since we haven't cleared the histogram, the first max
is still in the histogram and is higher than any other max, so it
should still be the max for the snapshot. It isn't however - the
value should still be 347 after the second run.

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

# cyclictest -p 80 -n -s -t 2 -D 2

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

{ next_pid: 2143 } hitcount: 199
max: 44 next_prio: 120 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/4

{ next_pid: 2145 } hitcount: 1325
max: 38 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/2

{ next_pid: 2144 } hitcount: 1982
max: 347 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/6

Snapshot taken (see tracing/snapshot). Details:
triggering value { onmax($wakeup_lat) }: 347
triggered by event with key: { next_pid: 2144 }

# cyclictest -p 80 -n -s -t 2 -D 2

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

{ next_pid: 2143 } hitcount: 199
max: 44 next_prio: 120 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/4

{ next_pid: 2148 } hitcount: 199
max: 16 next_prio: 120 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/1

{ next_pid: 2145 } hitcount: 1325
max: 38 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/2

{ next_pid: 2150 } hitcount: 1326
max: 39 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/4

{ next_pid: 2144 } hitcount: 1982
max: 347 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/6

{ next_pid: 2149 } hitcount: 1983
max: 130 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/0

Snapshot taken (see tracing/snapshot). Details:
triggering value { onmax($wakeup_lat) }: 39
triggered by event with key: { next_pid: 2150 }

After:

In this example, we do a first run of the workload (cyclictest),
examine the output, note the max ('triggering value') (375), then do
a second run and note the max again.

In this case, the max in the second run is still 375, the highest in
any bucket, as it should be.

# cyclictest -p 80 -n -s -t 2 -D 2

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

{ next_pid: 2072 } hitcount: 200
max: 28 next_prio: 120 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/5

{ next_pid: 2074 } hitcount: 1323
max: 375 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/2

{ next_pid: 2073 } hitcount: 1980
max: 153 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/6

Snapshot taken (see tracing/snapshot). Details:
triggering value { onmax($wakeup_lat) }: 375
triggered by event with key: { next_pid: 2074 }

# cyclictest -p 80 -n -s -t 2 -D 2

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

{ next_pid: 2101 } hitcount: 199
max: 49 next_prio: 120 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/6

{ next_pid: 2072 } hitcount: 200
max: 28 next_prio: 120 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/5

{ next_pid: 2074 } hitcount: 1323
max: 375 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/2

{ next_pid: 2103 } hitcount: 1325
max: 74 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/4

{ next_pid: 2073 } hitcount: 1980
max: 153 next_prio: 19 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/6

{ next_pid: 2102 } hitcount: 1981
max: 84 next_prio: 19 next_comm: cyclictest
prev_pid: 12 prev_prio: 120 prev_comm: kworker/0:1

Snapshot taken (see tracing/snapshot). Details:
triggering value { onmax($wakeup_lat) }: 375
triggered by event with key: { next_pid: 2074 }

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
---
kernel/trace/trace_events_hist.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
index cb56c7c456fa..61b58c906489 100644
--- a/kernel/trace/trace_events_hist.c
+++ b/kernel/trace/trace_events_hist.c
@@ -3585,14 +3585,20 @@ static bool cond_snapshot_update(struct trace_array *tr, void *cond_data)
struct track_data *track_data = tr->cond_snapshot->cond_data;
struct hist_elt_data *elt_data, *track_elt_data;
struct snapshot_context *context = cond_data;
+ struct action_data *action;
u64 track_val;

if (!track_data)
return false;

+ action = track_data->action_data;
+
track_val = get_track_val(track_data->hist_data, context->elt,
track_data->action_data);

+ if (!action->track_data.check_val(track_data->track_val, track_val))
+ return false;
+
track_data->track_val = track_val;
memcpy(track_data->key, context->key, track_data->key_len);

--
2.14.1
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