From: Tom Zanussi zanussi@kernel.org

Original changelog from Steve Rostedt (except last sentence which explains the problem, and the Fixes: tag):

I performed a three way histogram with the following commands:

echo 'irq_lat u64 lat pid_t pid' > synthetic_events echo 'wake_lat u64 lat u64 irqlat pid_t pid' >> synthetic_events echo 'hist:keys=common_pid:irqts=common_timestamp.usecs if function == 0xffffffff81200580' > events/timer/hrtimer_start/trigger echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$irqts:onmatch(timer.hrtimer_start).irq_lat($lat,pid) if common_flags & 1' > events/sched/sched_waking/trigger echo 'hist:keys=pid:wakets=common_timestamp.usecs,irqlat=lat' > events/synthetic/irq_lat/trigger echo 'hist:keys=next_pid:lat=common_timestamp.usecs-$wakets,irqlat=$irqlat:onmatch(synthetic.irq_lat).wake_lat($lat,$irqlat,next_pid)' > events/sched/sched_switch/trigger echo 1 > events/synthetic/wake_lat/enable

Basically I wanted to see:

hrtimer_start (calling function tick_sched_timer)

Note:

# grep tick_sched_timer /proc/kallsyms ffffffff81200580 t tick_sched_timer

And save the time of that, and then record sched_waking if it is called in interrupt context and with the same pid as the hrtimer_start, it will record the latency between that and the waking event.

I then look at when the task that is woken is scheduled in, and record the latency between the wakeup and the task running.

At the end, the wake_lat synthetic event will show the wakeup to scheduled latency, as well as the irq latency in from hritmer_start to the wakeup. The problem is that I found this:

<idle>-0 [007] d... 190.485261: wake_lat: lat=27 irqlat=190485230 pid=698 <idle>-0 [005] d... 190.485283: wake_lat: lat=40 irqlat=190485239 pid=10 <idle>-0 [002] d... 190.488327: wake_lat: lat=56 irqlat=190488266 pid=335 <idle>-0 [005] d... 190.489330: wake_lat: lat=64 irqlat=190489262 pid=10 <idle>-0 [003] d... 190.490312: wake_lat: lat=43 irqlat=190490265 pid=77 <idle>-0 [005] d... 190.493322: wake_lat: lat=54 irqlat=190493262 pid=10 <idle>-0 [005] d... 190.497305: wake_lat: lat=35 irqlat=190497267 pid=10 <idle>-0 [005] d... 190.501319: wake_lat: lat=50 irqlat=190501264 pid=10

The irqlat seemed quite large! Investigating this further, if I had enabled the irq_lat synthetic event, I noticed this:

<idle>-0 [002] d.s. 249.429308: irq_lat: lat=164968 pid=335 <idle>-0 [002] d... 249.429369: wake_lat: lat=55 irqlat=249429308 pid=335

Notice that the timestamp of the irq_lat "249.429308" is awfully similar to the reported irqlat variable. In fact, all instances were like this. It appeared that:

irqlat=$irqlat

Wasn't assigning the old $irqlat to the new irqlat variable, but instead was assigning the $irqts to it.

The issue is that assigning the old $irqlat to the new irqlat variable creates a variable reference alias, but the alias creation code forgets to make sure the alias uses the same var_ref_idx to access the reference.

Link: http://lkml.kernel.org/r/1567375321.5282.12.camel@kernel.org

Cc: Linux Trace Devel linux-trace-devel@vger.kernel.org Cc: linux-rt-users linux-rt-users@vger.kernel.org Cc: stable@vger.kernel.org Fixes: 7e8b88a30b085 ("tracing: Add hist trigger support for variable reference aliases") Reported-by: Steven Rostedt (VMware) rostedt@goodmis.org Signed-off-by: Tom Zanussi zanussi@kernel.org Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- kernel/trace/trace_events_hist.c | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c index 3a6e42aa08e6..9468bd8d44a2 100644 --- a/kernel/trace/trace_events_hist.c +++ b/kernel/trace/trace_events_hist.c @@ -2804,6 +2804,8 @@ static struct hist_field *create_alias(struct hist_trigger_data *hist_data, return NULL; }

+ alias->var_ref_idx = var_ref->var_ref_idx; + return alias; }