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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 06, 2014 at 12:08:19PM +0300, Alexander Shishkin wrote:
> Peter Zijlstra <peterz@infradead.org> 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?

No, and I think I realized this after sending that email, but at the
time I figured there was another way to do it. Of course, now that
several weeks have passed I cannot for the life of me remember what it
was.

Lemme go over these patches again to refresh my mind and maybe I'll
remember.