From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Duyck Subject: Re: [net-next PATCH 1/6] net: Add skb_free_frag to replace use of put_page in freeing skb->head Date: Wed, 06 May 2015 13:55:37 -0700 Message-ID: <554A7FC9.5010506@redhat.com> References: <20150504231000.1538.70520.stgit@ahduyck-vm-fedora22> <20150504231448.1538.84164.stgit@ahduyck-vm-fedora22> <20150506123840.312f41000e8d46f1ef9ce046@linux-foundation.org> <554A793F.3070001@redhat.com> <20150506134102.b01faad32e07ff3d308e1a09@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Cc: linux-mm@kvack.org, netdev@vger.kernel.org, davem@davemloft.net, Eric Dumazet To: Andrew Morton Return-path: Received: from mx1.redhat.com ([209.132.183.28]:51278 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750845AbbEFUzr (ORCPT ); Wed, 6 May 2015 16:55:47 -0400 In-Reply-To: <20150506134102.b01faad32e07ff3d308e1a09@linux-foundation.org> Sender: netdev-owner@vger.kernel.org List-ID: On 05/06/2015 01:41 PM, Andrew Morton wrote: > On Wed, 06 May 2015 13:27:43 -0700 Alexander Duyck 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.