Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752403AbdF3Riz (ORCPT ); Fri, 30 Jun 2017 13:38:55 -0400 Received: from canardo.mork.no ([148.122.252.1]:55145 "EHLO canardo.mork.no" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752302AbdF3Riy (ORCPT ); Fri, 30 Jun 2017 13:38:54 -0400 From: =?utf-8?Q?Bj=C3=B8rn_Mork?= To: Jim Baxter Cc: , , , Oliver Neukum , David Laight Subject: Re: [PATCH V2 1/1] net: cdc_ncm: Reduce memory use when kernel memory low Organization: m References: <1498682129-9129-1-git-send-email-jim_baxter@mentor.com> <1498682129-9129-2-git-send-email-jim_baxter@mentor.com> Date: Fri, 30 Jun 2017 19:38:34 +0200 In-Reply-To: <1498682129-9129-2-git-send-email-jim_baxter@mentor.com> (Jim Baxter's message of "Wed, 28 Jun 2017 21:35:29 +0100") Message-ID: <87bmp5s7dx.fsf@miraculix.mork.no> User-Agent: Gnus/5.130015 (Ma Gnus v0.15) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id v5UHd0td024846 Content-Length: 2263 Lines: 50 Jim Baxter writes: > The CDC-NCM driver can require large amounts of memory to create > skb's and this can be a problem when the memory becomes fragmented. > > This especially affects embedded systems that have constrained > resources but wish to maximise the throughput of CDC-NCM with 16KiB > NTB's. > > The issue is after running for a while the kernel memory can become > fragmented and it needs compacting. > If the NTB allocation is needed before the memory has been compacted > the atomic allocation can fail which can cause increased latency, > large re-transmissions or disconnections depending upon the data > being transmitted at the time. > This situation occurs for less than a second until the kernel has > compacted the memory but the failed devices can take a lot longer to > recover from the failed TX packets. > > To ease this temporary situation I modified the CDC-NCM TX path to > temporarily switch into a reduced memory mode which allocates an NTB > that will fit into a USB_CDC_NCM_NTB_MIN_OUT_SIZE (default 2048 Bytes) > sized memory block and only transmit NTB's with a single network frame > until the memory situation is resolved. > Each time this issue occurs we wait for an increasing number of > reduced size allocations before requesting a full size one to not > put additional pressure on a low memory system. > > Once the memory is compacted the CDC-NCM data can resume transmitting > at the normal tx_max rate once again. > > Signed-off-by: Jim Baxter This looks good to me. I would still be happier if we didn't need something like this, but I understand that we do. And this patch looks as clean as it can get. I haven't tested the patch under any sort of memory pressure, but I did a basic runtime test on a single MBIM device. As expected, I did not notice any changes with this patch applied. But regarding noticable effects: The patch adds no printks, counters or sysfs attributes which could tell the user that the initial buffer allocation has failed. Maybe add some sort of debug helper(s) in a followup patch? How did you verify the patch operation while testing it? Anyway, that's no show stopper of course. So FWIW: Reviewed-by: Bjørn Mork