Re: [PATCH -tip tracing/kprobes 0/9] tracing/kprobes, perf: perf probe and kprobe-tracer bugfixes

[Masami Hiramatsu <mhiramat at redhat dot com>]

Ingo Molnar wrote:
> The 'perf probe --list schedule' sub-tool i outlined would display 
> relative line numbers for the function - starting at 0:
> 
> 000	/*
> 001	 * schedule() is the main scheduler function.
> 002	 */
> 003	asmlinkage void __sched schedule(void)
> 004	{
> 005		struct task_struct *prev, *next;
> 006		unsigned long *switch_count;
> 007		struct rq *rq;
> 008		int cpu;
> 009
> 010	need_resched:
> 011		preempt_disable();
> 012		cpu = smp_processor_id();
> 013		rq = cpu_rq(cpu);
> 014		rcu_sched_qs(cpu);
> 015		prev = rq->curr;
> 016		switch_count = &prev->nivcsw;
> 017
> 018		release_kernel_lock(prev);

Ah, I've gotten what you said, and many lines will be optimized out
it would be better to show line numbers which can be probed, as below.

000	asmlinkage void __sched schedule(void)
	{
		struct task_struct *prev, *next;
		unsigned long *switch_count;
		struct rq *rq;
		int cpu;

007	need_resched:
008		preempt_disable();
009		cpu = smp_processor_id();
010		rq = cpu_rq(cpu);
011		rcu_sched_qs(cpu);
012		prev = rq->curr;
013		switch_count = &prev->nivcsw;

015		release_kernel_lock(prev);


> That way the following two are equivalent:
> 
>   perf probe schedule
>   perf probe schedule+0
> 
> The advantage of relative line numbers is that they are much more 
> version invariant than absolute line numbers. Relative line numbers into 
> schedule() will only change if the function itself changes.
> 
> This means that expressions like 'schedule+16' will have a lot longer 
> life-time than absolute line number driven probes. You can quote it in 
> an email and chances are that it will still be valid even a few kernel 
> releases down the road.

Hmm, I imagines 'schedule() + 16 byte offset' from 'schedule+16', because
many in-kernel(and oops) messages means that. :-)
So I'd like to use 'schedule:16'.

>>> We also want to have functionality that helps people find probe 
>>> spots within a function:
>>>
>>>   perf probe --list-lines schedule
>>>
>>> Would list the line numbers and source code of the schedule() 
>>> function. (similar to how GDB 'list' works) That way someone can 
>>> have an ad-hoc session of deciding what place to probe, and the line 
>>> numbers make for an easy ID of the statement to probe.
>>
>> Agreed!
> 
> Furthermore - to answer another question you raised above - the 
> following syntax:
> 
>    perf probe schedule:'switch_count = &prev->nivcsw'
> 
> is basically a 'fuzzy string match' based probe to within a function.
> 
> For example you might want to probe the point within schedule that calls 
> switch_mm() - this could be done via:
> 
>    perf probe schedule@switch_mm
> 
> Or the point where 'next' gets assigned? Sure, you dont need to even 
> open the editor, if you know the rough outline of the function you can 
> probe it via:
> 
>    perf probe schedule@'next ='
> 
> Note that i was able to specify both probes without having opened an 
> editor - just based on the general knowledge of the scheduler.

It may be useful for return probe too :-)

 perf probe schedule@return

> The point is to prefer intuitive, non-mechanic, fundamentally human 
> expressions of information above mechanic ones (absolute line numbers, 
> addresses, ways of probing, etc.) - and to have a rich variety of them.
> 
> String based pattern matching and intuitive syntax that reuses existing 
> paradigms of arithmetics and pattern-matching is good - limited syntax 
> and extra, arbitrary syntactic hoops to jump through is bad.
> 
> If we provide all that, people will start using this stuff - and i'd 
> only like to merge this upstream once it's clear that people like me 
> will (be able to) use this facility for ad-hoc probe insertion.
> 
> In other words: this facility has to 'live within' our source code and 
> has to be able to interact with it on a very broad basis - for it to be 
> maximally useful for everyday development.

Hmm, so you mean perf-probe should work with source-code?
Without source code (but with debuginfo), maybe we can't use
string matching, is that OK?

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com