Re: [PATCH v4 02/22] perf: Add AUX area to ring buffer for raw data streams

[Alexander Shishkin]

Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:

> On Mon, Sep 08, 2014 at 03:55:11PM +0300, Alexander Shishkin wrote:
>
>> Fair enough. Then I'd like to disable the ACTIVE ones before freeing AUX
>> stuff and then re-enabling them since perf_event_{en,dis}able() already
>> provide the convenient cross-cpu calls, which would also avoid
>> concurrency between pmu::{add,del} callbacks and this unmap path. Makes
>> sense?
>
> But why? The buffer stuff is RCU freed, so if the hardware observes
> pages and does get_page_unless_zero() on them its good. The memory will
> not be freed from underneath the hardware writer because of the
> get_page().
>
> Then when the buffer is full and we 'swap', we'll find there is no next
> buffer. At that point we can not provide a new buffer, effectively
> stopping the hardware writes and release the old buffer, freeing the
> memory.

There are several problems with this. Firstly, aux buffers can be quite
large, which means that we have to do get_page() on thousands of pages
on every pmu::add, which is a hot path and free_page() again in
pmu::del.

Secondly, all the sg bookkeeping that the driver keeps in aux_priv needs
to be refcounted. Right now, in the mmap_close path we just free
everything. But if we want to free the aux_pages in pmu::del, we need to
keep a list of these pages still around after mmap_close() and same goes
for the actual sg tables. I can see a way of doing that on the ring
buffer side (as opposed to the driver side), but are you quite sure we
should go down this road?

Regards,
--
Alex
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