linux-kernel - Re: [PATCH 13/15] perf_counter: provide generic callchain bits

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Date:	Wed, 01 Apr 2009 16:25:24 -0700
From:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/15] perf_counter: provide generic callchain bits

Peter Zijlstra wrote:
> On Wed, 2009-04-01 at 14:48 +1100, Paul Mackerras wrote:
>> Peter Zijlstra writes:
>>
>>> That still has the record and read things separate, but as one unified
>>> overflow output.
>> I take it PERF_EVENT_OVERFLOW refers to counter overflow, not ring
>> buffer overflow?  That had me confused for a bit, so more explicit
>> naming, or at least some comments, would be good.
> 
> Ah, yes, I see how that can confuse. PERF_EVENT_COUNTER_OVERFLOW then?
> 
> I was thinking about doing splice() support and that could also generate
> actual event overflow events ;-)
> 
>>>  /*
>>> + * Bits that can be set in hw_event.record_type to request information
>>> + * in the overflow packets.
>>> + */
>>> +enum perf_counter_record_format {
>>> +	PERF_RECORD_IP		= 1U << 0,
>>> +	PERF_RECORD_TID		= 1U << 1,
>>> +	PERF_RECORD_GROUP	= 1U << 2,
>>> +	PERF_RECORD_CALLCHAIN	= 1U << 3,
>>> +};
>> [snip]
>>>  enum perf_event_type {
>>>  
>>> -	PERF_EVENT_GROUP	= 1,
>>> -
>>> -	PERF_EVENT_MMAP		= 2,
>>> -	PERF_EVENT_MUNMAP	= 3,
>>> +	PERF_EVENT_MMAP		= 1,
>>> +	PERF_EVENT_MUNMAP	= 2,
>>>  
>>>  	PERF_EVENT_OVERFLOW	= 1UL << 31,
>>>  	__PERF_EVENT_IP		= 1UL << 30,
>>>  	__PERF_EVENT_TID	= 1UL << 29,
>>> -	__PERF_EVENT_CALLCHAIN  = 1UL << 28,
>>> +	__PERF_EVENT_GROUP	= 1UL << 28,
>>> +	__PERF_EVENT_CALLCHAIN  = 1UL << 27,
>> Could we use the same value (and even the same name) for
>> PERF_RECORD_IP/__PERF_EVENT_IP, PERF_RECORD_TID/__PERF_EVENT_TID,
>> etc.?
> 
> I suppose we could.
> 
>> Also, I haven't looked at the callchain stuff much, but does the
>> callchain info contain a recognizable end delimiter?  At present the
>> callchain comes last, but if we add more output elements they'll
>> presumably go after it, so working out where the callchain ends may
>> become tricky if we're not careful.
> 
> It writes:
> 
> struct callchain_event {
> 	u64 nr
> 	u64 ips[nr]
> };

Looking at the current version of perf_counter.h in the -tip tree, this 
struct definition is not visible to userspace (it's in the #ifdef KERNEL 
section).

- Corey

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