netdev - Re: order-0 vs order-N driver allocation. Was: [PATCH v10 07/12] net/mlx4_en: add page recycle to prepare rx ring for tx support

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Date:	Fri, 05 Aug 2016 09:15:33 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Brenden Blanco <bblanco@...mgrid.com>, davem@...emloft.net,
	netdev@...r.kernel.org, Jamal Hadi Salim <jhs@...atatu.com>,
	Saeed Mahameed <saeedm@....mellanox.co.il>,
	Martin KaFai Lau <kafai@...com>, Ari Saha <as754m@....com>,
	Or Gerlitz <gerlitz.or@...il.com>, john.fastabend@...il.com,
	hannes@...essinduktion.org, Thomas Graf <tgraf@...g.ch>,
	Tom Herbert <tom@...bertland.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Tariq Toukan <ttoukan.linux@...il.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: order-0 vs order-N driver allocation. Was: [PATCH v10 07/12]
 net/mlx4_en: add page recycle to prepare rx ring for tx support

On Thu, 2016-08-04 at 18:19 +0200, Jesper Dangaard Brouer wrote:

> I actually agree, that we should switch to order-0 allocations.
> 
> *BUT* this will cause performance regressions on platforms with
> expensive DMA operations (as they no longer amortize the cost of
> mapping a larger page).

We much prefer reliable behavior, even it it is ~1 % slower than the
super-optimized thing that opens highways for attackers.

Anyway, in most cases pages are re-used, so we only call
dma_sync_single_range_for_cpu(), and there is no way to avoid this.

Using order-0 pages [1] is actually faster, since when we use high-order
pages (multiple frames per 'page') we can not reuse the pages.

[1] I had a local patch to allocate these pages using a very simple
allocator allocating max order (order-10) pages and splitting them into
order-0 ages, in order to lower TLB footprint. But I could not measure a
gain doing so on x86, at least on my lab machines.