Re: [PATCH -tip v6 00/22] kprobes: introduce NOKPROBE_SYMBOL(), cleanupand fixes crash bugs

[Masami Hiramatsu]

(2014/02/09 23:37), Ingo Molnar wrote:
> 
> * Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
> 
>> I guess the second reason is why the stap takes so long time to set 
>> probes. stap tries to register kprobes without disabled flag, that 
>> means we enables thousands of probes (overheads).
>>
>> So the similar thing happens when we enables events as below;
>>
>>   # for i in /sys/kernel/debug/tracing/events/kprobes/* ; do date; echo 1 > $i; done
>>   Wed Jan 29 10:44:50 UTC 2014
>>   ...
>>
>> I tried it and canceled after 4 min passed. It enabled about 17k 
>> events and slowed down my system very much(I almost got hang check 
>> timer).
> 
> Ok, I guess that's the slowdown bug that Frank reported.
> 
>> I think we should have some performance statistics (hit count?) and 
>> if it goes over a threshold, we should stop enabling other events.
> 
> That really feels like a hack. How about fixing the root cause? Does 
> the enabling of all probes have to be so slow?

When I tried to use perf top, most of the time was consumed in
kprobe_ftrace_handler and optimized_callback, both of them
are the handler of kprobes. Since I just tried on a VM guest and it
didn't support NMI nor PMU, thus I have to use a bare metal machine
for deeper investigation (and I'll do).
And yes, it seems that the performance problem comes from probing
and tracing itself.

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx


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