Re: [net-next PATCH 1/6] net: Add skb_free_frag to replace use of put_page in freeing skb->head

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 06 May 2015 13:27:43 -0700 Alexander Duyck <alexander.h.duyck@redhat.com> 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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