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


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