From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755557Ab1DOKSC (ORCPT ); Fri, 15 Apr 2011 06:18:02 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:47431 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752096Ab1DOKSA (ORCPT ); Fri, 15 Apr 2011 06:18:00 -0400 Date: Fri, 15 Apr 2011 12:17:12 +0200 From: Ingo Molnar To: Len Brown Cc: x86@kernel.org, linux-pm@lists.linux-foundation.org, linux-kernel@vger.kernel.org, "H. Peter Anvin" , Thomas Gleixner , Alan Cox , Linus Torvalds , Andrew Morton , Arjan van de Ven Subject: Re: [PATCH] x86 intel power: Initialize MSR_IA32_ENERGY_PERF_BIAS Message-ID: <20110415101712.GB28007@elte.hu> References: <20110414090854.GB9248@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.3.1 -2.0 BAYES_00 BODY: Bayes spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Len Brown wrote: > > > > From: Len Brown > > > > > > Since 2.6.35 (23016bf0d25), Linux prints the existence of "epb" in /proc/cpuinfo, > > > Since 2.6.38 (d5532ee7b40), the x86_energy_perf_policy(8) utility is available > > > in-tree to update MSR_IA32_ENERGY_PERF_BIAS. > > > > > > However, the typical BIOS fails to initialize the MSR, > > > and the typical Linux distro neglects to invoke x86_energy_perf_policy(8). > > > > > > The result is that some modern hardware is running in hardware default, > > > which is "performance" mode, rather than the intended design default > > > of "normal" mode. > > > > > > Initialize the MSR to the "normal" setting during kernel boot. > > > > > > Of course, x86_energy_perf_policy(8) is available to change > > > the default after boot, should the user have a policy preference. > > > > > > cc: stable@kernel.org > > > Signed-off-by: Len Brown > > > --- > > > arch/x86/include/asm/msr-index.h | 3 +++ > > > arch/x86/kernel/cpu/intel.c | 14 ++++++++++++++ > > > 2 files changed, 17 insertions(+), 0 deletions(-) > > > > ... > > > > Dunno, this patch appears to silently modify the system to be slower than it > > was before under Linux. > > > > Won't people report this as a regression if this change reduces performance for > > them? > > > > They wont be able to see your comments in the code and in the changelog either, > > when this happens to them. They might look into /proc/cpuinfo and see 'epb' > > there but it wont tell them anything. They wont know about a utility available > > in tools/power/x86/ either. > > This patch makes no change to the epb feature indicator > /proc/cpuinfo. I know. I reacted to this bit in the changelog: > > > Since 2.6.35 (23016bf0d25), Linux prints the existence of "epb" in /proc/cpuinfo, Printing the existence of a CPU feature does nothing to inform users. > > So this patch has 'future trouble' written all over it i'm afraid. > > EPB is limited to SNB and later. > So the installed base as yet is small. > (it also exists on WSM-EP, but doesn't do so much there) > EPB will have a more significant effect on future hardware. > > Linux currently trails competing operating systems in energy > efficiency on SNB due to this setting, and Linux will trail > competing operating systems even more on future hardware > if this default is not fixed. > > Will it be possible to measure a performance difference between > "performance" and "normal"? Yes, it will be possible. > Will 99.9% of users notice? Nope. More likely they'll notice > the the power savings that are disabled in "performance" mode. > > I should have called it "benchmark" mode instead of "performance" mode... That's all fair but does not address the concerns i raised. A silent change during bootup is asking for trouble. So how about informing users, how about making it non-silent? An informative printk that also mentions the power configuration tool, etc. This solves the concerns i mentioned. Thanks, Ingo