From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qk0-f182.google.com (mail-qk0-f182.google.com [209.85.220.182]) by kanga.kvack.org (Postfix) with ESMTP id 9D9DF6B006E for ; Wed, 6 May 2015 16:55:47 -0400 (EDT) Received: by qkgx75 with SMTP id x75so15009602qkg.1 for ; Wed, 06 May 2015 13:55:47 -0700 (PDT) Received: from mx1.redhat.com (mx1.redhat.com. [209.132.183.28]) by mx.google.com with ESMTPS id d63si1302564qgd.60.2015.05.06.13.55.45 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 06 May 2015 13:55:46 -0700 (PDT) Message-ID: <554A7FC9.5010506@redhat.com> Date: Wed, 06 May 2015 13:55:37 -0700 From: Alexander Duyck MIME-Version: 1.0 Subject: Re: [net-next PATCH 1/6] net: Add skb_free_frag to replace use of put_page in freeing skb->head 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> In-Reply-To: <20150506134102.b01faad32e07ff3d308e1a09@linux-foundation.org> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: linux-mm@kvack.org, netdev@vger.kernel.org, davem@davemloft.net, Eric Dumazet 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org