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..
Is the PageHead thing really "likely"? We're usually dealing with
order>0 pages here?
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