From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755187AbdERKDa (ORCPT ); Thu, 18 May 2017 06:03:30 -0400 Received: from mx2.suse.de ([195.135.220.15]:42612 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1754288AbdERKD2 (ORCPT ); Thu, 18 May 2017 06:03:28 -0400 Message-ID: <1495101712.6672.4.camel@suse.com> Subject: Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory low From: Oliver Neukum To: David Miller , bjorn@mork.no Cc: jim_baxter@mentor.com, linux-kernel@vger.kernel.org, linux-usb@vger.kernel.org, netdev@vger.kernel.org Date: Thu, 18 May 2017 12:01:52 +0200 In-Reply-To: <20170517.141819.1307166900606639947.davem@davemloft.net> References: <1494956480-6127-1-git-send-email-jim_baxter@mentor.com> <1494956480-6127-2-git-send-email-jim_baxter@mentor.com> <87shk4fynp.fsf@miraculix.mork.no> <20170517.141819.1307166900606639947.davem@davemloft.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.20.5 Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Am Mittwoch, den 17.05.2017, 14:18 -0400 schrieb David Miller: > From: Bjørn Mork > Date: Tue, 16 May 2017 20:24:10 +0200 > > > 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. > >> Once the memory is compacted the CDC-NCM data can resume transmitting > >> at the normal tx_max rate once again. > > > > I must say that I don't like the additional complexity added here.  If > > there are memory issues and you can reduce the buffer size to > > USB_CDC_NCM_NTB_MIN_OUT_SIZE, then why don't you just set a lower tx_max > > buffer size in the first place? > > > >   echo 2048 > /sys/class/net/wwan0/cdc_ncm/tx_max > > When there isn't memory pressure this will hurt performance of > course. > > It is a quite common paradigm to back down to 0 order memory requests > when higher order ones fail, so this isn't such a bad change from the > perspective. > > However, one negative about it is that when the system is under memory > stress it doesn't help at all to keep attemping high order allocations > when the system hasn't recovered yet.  In fact, this can make it > worse. This makes me wonder why there is no notifier chain for this. Or am I just too stupid to find it? Regards Oliver