On 05/06/2015 01:41 PM, Andrew Morton wrote:
On Wed, 06 May 2015 13:27:43 -0700 Alexander Duyck <alexander.h.duyck@xxxxxxxxxx> wrote:
+void skb_free_frag(void *head)
+{
+ struct page *page = virt_to_head_page(head);
+
+ if (unlikely(put_page_testzero(page))) {
+ if (likely(PageHead(page)))
+ __free_pages_ok(page, compound_order(page));
+ else
+ free_hot_cold_page(page, false);
+ }
+}
Why are we testing for PageHead in here? If the code were to simply do
if (unlikely(put_page_testzero(page)))
__free_pages_ok(page, compound_order(page));
that would still work?
My assumption was that there was a performance difference between
__free_pages_ok and free_hot_cold_page for order 0 pages. From what I
can tell free_hot_cold_page will do bulk cleanup via free_pcppages_bulk
while __free_pages_ok just calls free_one_page.
Could be. Plus there's hopefully some performance advantage if the
page is genuinely cache-hot. I don't think that anyone has verified
the benefits of the hot/cold optimisation in the last decade or two,
and it was always pretty marginal..
Either way it doesn't make much difference. If you would prefer I can
probably just call __free_pages_ok for all cases.
Is the PageHead thing really "likely"? We're usually dealing with
order>0 pages here?
On any system that only supports 4K pages the default is to allocate an
order 3 page (32K) and then pull the fragments out of that. So if
__free_pages_ok works for an order 0 page I'll just call it since it
shouldn't be a very common occurrence anyway unless we are under memory
pressure.
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