From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932949Ab1KCOtu (ORCPT ); Thu, 3 Nov 2011 10:49:50 -0400 Received: from www.linutronix.de ([62.245.132.108]:58805 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932108Ab1KCOtt (ORCPT ); Thu, 3 Nov 2011 10:49:49 -0400 Date: Thu, 3 Nov 2011 15:49:45 +0100 (CET) From: Thomas Gleixner To: John Stultz cc: LKML , Yong Zhang , David Daney Subject: Re: [PATCH] clocksource: Avoid selecting mult values that might overflow when adjusted In-Reply-To: <1320329371.3681.20.camel@js-netbook> Message-ID: References: <1320264087-3413-1-git-send-email-john.stultz@linaro.org> <1320325819.2892.1.camel@js-netbook> <1320328869.3681.17.camel@js-netbook> <1320329371.3681.20.camel@js-netbook> User-Agent: Alpine 2.02 (LFD 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 3 Nov 2011, John Stultz wrote: > On Thu, 2011-11-03 at 10:01 -0400, John Stultz wrote: > > On Thu, 2011-11-03 at 14:26 +0100, Thomas Gleixner wrote: > > > On Thu, 3 Nov 2011, John Stultz wrote: > > > > On Thu, 2011-11-03 at 13:05 +0100, Thomas Gleixner wrote: > > > > > On Wed, 2 Nov 2011, John Stultz wrote: > > > > > > > > > > > > + WARN_ONCE(timekeeper.mult+adj > > > > > > > + timekeeper.clock->mult + timekeeper.clock->maxadj, > > > > > > + "Adjusting more then 11%%"); > > > > > > > > > > Shouldn't we rather limit the update instead of just warn and overflow ? > > > > > > > > Well, I'm hesitant to commit to that, just yet. So I figured I'd start > > > > with the warning. > > > > > > OTOH, we know right there that we might warp 32bit and confuse the > > > hell out of timekeeping, which is not a real good thing either. > > > > Oh certainly, but two things: > > 1) The 11% max is not the actual overflow edge. Its just calculated as > > safe. The overflow could as far out as ~22%. > > > > 2) This is the first case in however many years I've heard of of mult > > overflowing. So before we go changing the NTP code (which is really > > terribly complex, but has been working fairly well for awhile) I want to > > have some sense that the 11% max adjustment assumption is really > > correct. > > > > But maybe I'm being too conservative? If we do limit the adjustment > > keeping the warning, I guess we'd know why things blew up on previously > > working machines. > > Oh, and the other bit is that not all clocksources have been converted > over to using clocksource_register_hz/khz, so some may be using very > small shift values, which could more easily hit large % mult adjustment > (due to the resulting coarseness of each integer change) that wouldn't > cause overflows. Fair enough. I'm queuing it.