Messages in this thread | | | From | Alexander Shishkin <> | Subject | Re: [PATCH v4 02/22] perf: Add AUX area to ring buffer for raw data streams | Date | Mon, 06 Oct 2014 12:08:19 +0300 |
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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?
Regards, -- Alex
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