Return-path: Received: from nbd.name ([46.4.11.11]:42686 "EHLO nbd.name" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753832Ab2CSKgL (ORCPT ); Mon, 19 Mar 2012 06:36:11 -0400 Message-ID: <4F670C16.9030009@openwrt.org> (sfid-20120319_113614_058353_53EA295C) Date: Mon, 19 Mar 2012 11:36:06 +0100 From: Felix Fietkau MIME-Version: 1.0 To: Helmut Schaa CC: Johannes Berg , linux-wireless@vger.kernel.org, linville@tuxdriver.com Subject: Re: [PATCH 3/3] mac80211: optimize aggregation session timeout handling References: <1332025254-5048-1-git-send-email-nbd@openwrt.org> <1332025254-5048-2-git-send-email-nbd@openwrt.org> <1332025254-5048-3-git-send-email-nbd@openwrt.org> <1332065875.3609.3.camel@jlt3.sipsolutions.net> <4F65C374.2060505@openwrt.org> <1332146368.3359.12.camel@jlt3.sipsolutions.net> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-wireless-owner@vger.kernel.org List-ID: On 2012-03-19 10:29 AM, Helmut Schaa wrote: > Hi, > > On Mon, Mar 19, 2012 at 9:39 AM, Johannes Berg > wrote: >> On Sun, 2012-03-18 at 12:13 +0100, Felix Fietkau wrote: >>> On 2012-03-18 11:17 AM, Johannes Berg wrote: >>> > On Sun, 2012-03-18 at 00:00 +0100, Felix Fietkau wrote: >>> >> Calling mod_timer from the rx/tx hotpath is somewhat expensive, and the >>> >> timeout doesn't need to be so precise. >>> >> >>> >> Switch to a different strategy: Schedule the timer initially, store jiffies >>> >> of all last rx/tx activity which would previously modify the timer, and >>> >> let the timer re-arm itself after checking the last rx/tx timestamp. >>> > >>> > I don't like this. It's not the optimisation you think it is on other >>> > ("embedded") systems where firing a timer is more expensive. >>> > >>> > You're trading power consumption against CPU utilisation by causing the >>> > timer to wake up. >>> I considered that was well, but didn't think one wakeup every 5 seconds >>> or so would be significant. Would you take the patch if I change the >>> timer to be deferrable, so that it doesn't cause wakeups by itself? >> >> I'm not really convinced, for making them deferrable we should analyse >> the consequences of that more carefully, for example it seems possible >> that the system wakes up to send a packet, and then the first thing that >> happens is a few aggregation handshakes ... that wastes a lot of time >> and power. > > I like the idea of getting rid of the mod_timer overhead. Looking at the timer > code, if the timer value is unchanged mod_timer is not that expensive. > > So, instead of calling mod_timer for every successive frame with a slightly > different timeout we could just use round_jiffies to round the timeout to the > next full second. This would in most cases take the quick path through > mod_timer and only update the timer once every second. > > See code (untested, not even compile tested) below. I would still like to avoid the overhead of apply_slack(), which is called early by mod_timer(). It was visible in both CPU cycles and icache misses when I did some profiling under high tx load. - Felix