From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758987AbYDSHOz (ORCPT ); Sat, 19 Apr 2008 03:14:55 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752306AbYDSHOq (ORCPT ); Sat, 19 Apr 2008 03:14:46 -0400 Received: from www.tglx.de ([62.245.132.106]:59839 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751964AbYDSHOq (ORCPT ); Sat, 19 Apr 2008 03:14:46 -0400 Date: Sat, 19 Apr 2008 09:13:40 +0200 (CEST) From: Thomas Gleixner To: David Brownell cc: linux-pm@lists.linux-foundation.org, "Woodruff, Richard" , Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [linux-pm] Higer latency with dynamic tick (need for an io-ondemand govenor?) In-Reply-To: <200804182045.56893.david-b@pacbell.net> Message-ID: References: <3B6D69C3A9EBCA4BA5DA60D913027429010253CD@dlee13.ent.ti.com> <1179474567.12981.53.camel@chaos> <3B6D69C3A9EBCA4BA5DA60D91302742904131BED@dlee13.ent.ti.com> <200804182045.56893.david-b@pacbell.net> User-Agent: Alpine 1.10 (LFD 962 2008-03-14) MIME-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY="8323328-307532848-1208589223=:3261" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --8323328-307532848-1208589223=:3261 Content-Type: TEXT/PLAIN; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT On Fri, 18 Apr 2008, David Brownell wrote: > On Friday 18 April 2008, Woodruff, Richard wrote: > > When capturing some traces with dynamic tick we were noticing the > > interrupt latency seems to go up a good amount. If you look at the trace > > the gpio IRQ is now offset a good amount.  Good news I guess is its > > pretty predictable. > > That is, about 24 usec on this CPU ... an ARM v7, which I'm guessing > is an OMAP34xx running fairly fast (order of 4x faster than most ARMs). > > Similar issues were noted, also using ETM trace, on an ARM920 core [1] > from Atmel. There, the overhead of NO_HZ was observed to be more like > 150 usec of per-IRQ overhead, which is enough to make NO_HZ non-viable > in some configurations. > > > > I was wondering what thoughts of optimizing this might be. > > Cutting down the math implied by jiffies updates might help. > The 64 bit math for ktime structs isn't cheap; purely by eyeball, > that was almost 1/3 the cost of that 24 usec (mostly __do_div64). Hmm, I have no real good idea to avoid the div64 in the case of a long idle sleep. Any brilliant patches are welcome :) Thanks, tglx --8323328-307532848-1208589223=:3261--