Re: [PATCH v9 3.2 7/9] tracing: uprobes trace_event interface

[Srikar Dronamraju]

> 
> Have you tried to use 'perf probe' to achieve any useful 
> instrumentation on a real app?
> 
> I just tried out the 'glibc:free' usecase and it's barely 
> usable.
> 

Thanks for trying.

Yes, I have actually place probes on free and malloc while building a
kernel and it has worked for me.

> Firstly, the recording very frequently produces overruns:
> 
>  $ perf record -e probe_libc:free -aR sleep 1
>  [ perf record: Woken up 169 times to write data ]
>  [ perf record: Captured and wrote 89.674 MB perf.data (~3917919 samples) ]
>  Warning:Processed 1349133 events and lost 1 chunks!
> 

I have seen the "lost chunks" but thats not always and mostly when I
have run perf record for a very long time. But I have seen this even
with just kernel probes too. So I didnt debug that.

> Using -m 4096 made it work better.
> 
> Adding -g for call-graph profiling caused 'perf report' to lock 
> up:
> 
>   perf record -m 4096 -e probe_libc:free -agR sleep 1
>   perf report
>   [ loops forever ]
> 

This actually works for me.
I had CONFIG_PREEMPT not set. So I set it and tried with and without
Jiri's fix. It still worked for me. Can you pass me your config that
shows this hang so that I can try and fix it.

> I've sent a testcase to Arnaldo separately. Note that perf 
> report --stdio appears to work.

There are no changes in perf report because of uprobes. So the data
collected from uprobes could trigger the hang. I was speculating that
not disabling preemption that Jiri pointed out could be causing this. So
I tried with and without but couldnt reproduce. Can I request you to see
if Jiri's patch helps?

Today with -g option, on one machine I do see a 
$ sudo perf report
perf: Floating point exception

I will take a look at this too. Other machines dont seem to show this
error.

> Regular '-e cycles -g' works fine, so this is a uprobes specific 
> bug.
> 

-- 
Thanks and Regards
Srikar

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