From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-lj1-f198.google.com (mail-lj1-f198.google.com [209.85.208.198]) by kanga.kvack.org (Postfix) with ESMTP id 10A196B0289 for ; Sat, 10 Nov 2018 18:54:30 -0500 (EST) Received: by mail-lj1-f198.google.com with SMTP id q185-v6so1588630ljb.14 for ; Sat, 10 Nov 2018 15:54:30 -0800 (PST) Received: from smtp7.iq.pl (smtp7.iq.pl. [86.111.240.244]) by mx.google.com with ESMTPS id d2-v6si10653604lfh.31.2018.11.10.15.54.27 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sat, 10 Nov 2018 15:54:28 -0800 (PST) Subject: Re: [PATCH 1/2] mm/page_alloc: free order-0 pages through PCP in page_frag_free() References: <20181105085820.6341-1-aaron.lu@intel.com> From: =?UTF-8?Q?Pawe=c5=82_Staszewski?= Message-ID: Date: Sun, 11 Nov 2018 00:54:27 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: pl Sender: owner-linux-mm@kvack.org List-ID: To: Alexander Duyck , aaron.lu@intel.com Cc: linux-mm , LKML , Netdev , Andrew Morton , Jesper Dangaard Brouer , Eric Dumazet , Tariq Toukan , ilias.apalodimas@linaro.org, yoel@kviknet.dk, Mel Gorman , Saeed Mahameed , Michal Hocko , Vlastimil Babka , dave.hansen@linux.intel.com W dniu 05.11.2018 oA 16:44, Alexander Duyck pisze: > On Mon, Nov 5, 2018 at 12:58 AM Aaron Lu wrote: >> 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); >> > One thing I would suggest for Pawel to try would be to reduce the Tx > qdisc size on his transmitting interfaces, Reduce the Tx ring size, > and possibly increase the Tx interrupt rate. Ideally we shouldn't have > too many packets in-flight and I suspect that is the issue that Pawel > is seeing that is leading to the page pool allocator freeing up the > memory. I know we like to try to batch things but the issue is > processing too many Tx buffers in one batch leads to us eating up too > much memory and causing evictions from the cache. Ideally the Rx and > Tx rings and queues should be sized as small as possible while still > allowing us to process up to our NAPI budget. Usually I run things > with a 128 Rx / 128 Tx setup and then reduce the Tx queue length so we > don't have more buffers stored there than we can place in the Tx ring. > Then we can avoid the extra thrash of having to pull/push memory into > and out of the freelists. Essentially the issue here ends up being > another form of buffer bloat. Thanks Aleksandar - yes it can be - but in my scenario setting RX buffer <4096 producing more interface rx drops - and no_rx_buffer on network controller that is receiving more packets So i need to stick with 3000-4000 on RX - and yes i was trying to lower the TX buff on connectx4 - but that changed nothing before Aaron patch After Aaron patch - decreasing TX buffer influencing total bandwidth that can be handled by the router/server Dono why before this patch there was no difference there no matter what i set there there was always page_alloc/slowpath on top in perf Currently testing RX4096/TX256 - this helps with bandwidth like +10% more bandwidth with less interrupts... > > With that said this change should be mostly harmless and does address > the fact that we can have both regular order 0 pages and page frags > used for skb->head. > > Acked-by: Alexander Duyck >