linux-kernel - Re: [PATCH 0/1] uprobes/tracing: Don't pass addr=ip to perf_trace_buf

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Date:	Fri, 12 Apr 2013 14:01:46 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	"yrl.pp-manager.tt@...achi.com" <yrl.pp-manager.tt@...achi.com>
Subject: Re: [PATCH 0/1] uprobes/tracing: Don't pass addr=ip to
 perf_trace_buf_submit()

On Thu, 2013-04-11 at 13:59 +0200, Oleg Nesterov wrote:
> On 04/11, Masami Hiramatsu wrote:
> >
> > (2013/04/10 23:58), Oleg Nesterov wrote:
> >
> > > And... Cough, another question ;) To simplify, lets discuss kprobe_perf_func()
> > > only. Suppose that a task hits the kprobe but this task/cpu doesn't have
> > > a counter. Can't we avoid perf_trace_buf_prepare/submit in this case?
> > > IOW, what do you think about the change below?
> >
> > Hmm, I'm not so sure how frequently this happens.
> 
> Suppose that you do, say, "perf record -e probe:some_func workload". Only
> "workload" will have the active counter, any other task which hits the
> probed some_func() will do perf_trace_buf_prepare/perf_trace_buf_submit
> just to realize that nobody wants perf_swevent_event().

Wow, you're right. Seems that perf goes through a lot of work for every
time a tracepoint is hit for *all tasks*.

> 
> Simple test-case:
> 
> 	#include <unistd.h>
> 
> 	int main(void)
> 	{
> 		int n;
> 
> 		for (n = 0; n < 1000 * 1000; ++n)
> 			getppid();
> 
> 		return 0;
> 	}
> 
> Without kprobe:
> 
> 	# time ./ppid
> 
> 	real    0m0.663s
> 	user    0m0.163s
> 	sys     0m0.500s
> 
> Activate the probe:
> 
> 	# perf probe sys_getppid
> 
> 	# perf record -e probe:sys_getppid sleep 1000 &
> 	[1] 546
> 
> Test it again 3 times:
> 
> 	# time ./ppid
> 
> Before the patch:
> 
> 	real    0m9.727s
> 	user    0m0.177s
> 	sys     0m9.547s
> 
> 	real    0m9.752s
> 	user    0m0.180s
> 	sys     0m9.573s
> 
> 	real    0m9.761s
> 	user    0m0.187s
> 	sys     0m9.573s
> 
> After the patch:
> 
> 	real    0m9.605s
> 	user	0m0.163s
> 	sys	0m9.437s
> 
> 	real	0m9.592s
> 	user	0m0.167s
> 	sys	0m9.423s
> 
> 	real	0m9.613s
> 	user	0m0.183s
> 	sys	0m9.427s
> 
> So the difference looks measurable but small, and I did the testing
> under qemu so I do not really know if we can trust the numbers.
> 
> > And, is this right way to
> > handle that case?
> 
> If only I was sure ;) I am asking.
> 
> And, to clarify, it is not that I think this change can really
> improve the perfomance. Just I am trying to understand what I have
> missed.
> 
> > If so, we can do same thing also on trace_events.
> > (perf_trace_##call in include/trace/ftrace.h)
> 
> Yes, yes, this is not kprobe-specific. It seems that more users of
> perf_trace_buf_submit() could be changed the same way.

Yeah, looks like include/trace/ftrace.h needs an update.

Frederic?

-- Steve


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