Re: [RFC v2 2/7] perf: add AUX area to ring buffer for raw data streams

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

Peter Zijlstra <peterz@infradead.org> writes:

> On Wed, Jun 11, 2014 at 06:41:45PM +0300, Alexander Shishkin wrote:
>> +	/*
>> +	 * Set up pmu-private data structures for an AUX area
>> +	 */
>> +	void *(*setup_aux)		(int cpu, void **pages,
>> +					 int nr_pages, bool overwrite);
>> +					/* optional */
>> +
>> +	/*
>> +	 * Free pmu-private AUX data structures
>> +	 */
>> +	void (*free_aux)		(void *aux); /* optional */
>
> I was hoping you could replace those with a PERF_CAP_AUX or something
> and then have one generic allocation routine like you provide in the
> subsequent patches.

I need these to allocate pmu-specific SG tables now, which I don't see
how to generalize nicely. User-visible aux_pages are allocated in the
generic rb_alloc_aux().

As for sg tables, the alternative would be to allocate them in the
event::pmu::add() path, which is probably not very good for
performance, unless I'm missing something? Like, I can allocate stuff in
the first add() and then free it in event::destroy().

Or, we can assume a generic sg table format and allocate them in the
generic code, but then we'll need more capabilities to indicate at least
the size of a table entry. PT needs 64 bits per entry, ARM's TMC needs
32 and the non-SG PT will still need some tables. But I'd rather leave
this part to the pmu drivers. What do you think?

Regards,
--
Alex