From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f198.google.com (mail-pf1-f198.google.com [209.85.210.198]) by kanga.kvack.org (Postfix) with ESMTP id 018626B0007 for ; Mon, 5 Nov 2018 03:58:26 -0500 (EST) Received: by mail-pf1-f198.google.com with SMTP id 129-v6so8825700pfx.11 for ; Mon, 05 Nov 2018 00:58:25 -0800 (PST) Received: from mga06.intel.com (mga06.intel.com. [134.134.136.31]) by mx.google.com with ESMTPS id 16si17376912pgh.58.2018.11.05.00.58.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 05 Nov 2018 00:58:24 -0800 (PST) From: Aaron Lu Subject: [PATCH 1/2] mm/page_alloc: free order-0 pages through PCP in page_frag_free() Date: Mon, 5 Nov 2018 16:58:19 +0800 Message-Id: <20181105085820.6341-1-aaron.lu@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org Cc: Andrew Morton , =?UTF-8?q?Pawe=C5=82=20Staszewski?= , Jesper Dangaard Brouer , Eric Dumazet , Tariq Toukan , Ilias Apalodimas , Yoel Caspersen , Mel Gorman , Saeed Mahameed , Michal Hocko , Vlastimil Babka , Dave Hansen page_frag_free() calls __free_pages_ok() to free the page back to Buddy. This is OK for high order page, but for order-0 pages, it misses the optimization opportunity of using Per-Cpu-Pages and can cause zone lock contention when called frequently. PaweA? Staszewski recently shared his result of 'how Linux kernel handles normal traffic'[1] and from perf data, Jesper Dangaard Brouer found the lock contention comes from page allocator: mlx5e_poll_tx_cq | --16.34%--napi_consume_skb | |--12.65%--__free_pages_ok | | | --11.86%--free_one_page | | | |--10.10%--queued_spin_lock_slowpath | | | --0.65%--_raw_spin_lock | |--1.55%--page_frag_free | --1.44%--skb_release_data Jesper explained how it happened: mlx5 driver RX-page recycle mechanism is not effective in this workload and pages have to go through the page allocator. The lock contention happens during mlx5 DMA TX completion cycle. And the page allocator cannot keep up at these speeds.[2] I thought that __free_pages_ok() are mostly freeing high order pages and thought this is an lock contention for high order pages but Jesper explained in detail that __free_pages_ok() here are actually freeing order-0 pages because mlx5 is using order-0 pages to satisfy its page pool allocation request.[3] The free path as pointed out by Jesper is: skb_free_head() -> skb_free_frag() -> skb_free_frag() -> page_frag_free() And the pages being freed on this path are order-0 pages. Fix this by doing similar things as in __page_frag_cache_drain() - send the being freed page to PCP if it's an order-0 page, or directly to Buddy if it is a high order page. With this change, PaweA? hasn't noticed lock contention yet in his workload and Jesper has noticed a 7% performance improvement using a micro benchmark and lock contention is gone. [1]: https://www.spinics.net/lists/netdev/msg531362.html [2]: https://www.spinics.net/lists/netdev/msg531421.html [3]: https://www.spinics.net/lists/netdev/msg531556.html Reported-by: PaweA? Staszewski Analysed-by: Jesper Dangaard Brouer Signed-off-by: Aaron Lu --- mm/page_alloc.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index ae31839874b8..91a9a6af41a2 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4555,8 +4555,14 @@ void page_frag_free(void *addr) { struct page *page = virt_to_head_page(addr); - if (unlikely(put_page_testzero(page))) - __free_pages_ok(page, compound_order(page)); + if (unlikely(put_page_testzero(page))) { + unsigned int order = compound_order(page); + + if (order == 0) + free_unref_page(page); + else + __free_pages_ok(page, order); + } } EXPORT_SYMBOL(page_frag_free); -- 2.17.2