From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Pallipadi, Venkatesh" Subject: [PATCH] ACPI: cap off P-state transition latency from buggy BIOSes Date: Thu, 19 Mar 2009 14:41:40 -0700 Message-ID: <20090319214140.GA28868@linux-os.sc.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mga11.intel.com ([192.55.52.93]:19570 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753274AbZCSVls (ORCPT ); Thu, 19 Mar 2009 17:41:48 -0400 Content-Disposition: inline Sender: linux-acpi-owner@vger.kernel.org List-Id: linux-acpi@vger.kernel.org To: Len Brown Cc: linux-acpi , mjg59@srcf.ucam.org Some BIOSes report very high frequency transition latency which are plainly wrong on CPus that can change frequency using native MSR interface. One such system is IBM T42 (2327-8ZU) as reported by Owen Taylor and Rik van Riel. cpufreq_ondemand driver uses this transition latency to come up with a reasonable sampling interval to sample CPU usage and with such high latency value, ondemand sampling interval ends up being very high (0.5 sec, in this particular case), resulting in performance impact due to slow response to increasing frequency. Fix it by capping-off the transition latency to 20uS for native MSR based frequency transitions. Signed-off-by: Venkatesh Pallipadi --- arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) Index: linux-2.6/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c =================================================================== --- linux-2.6.orig/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c 2008-05-02 09:45:23.000000000 -0700 +++ linux-2.6/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c 2008-06-30 12:08:32.000000000 -0700 @@ -659,6 +659,18 @@ static int acpi_cpufreq_cpu_init(struct perf->states[i].transition_latency * 1000; } + /* Check for high latency (>20uS) from buggy BIOSes, like on T42 */ + if (perf->control_register.space_id == ACPI_ADR_SPACE_FIXED_HARDWARE && + policy->cpuinfo.transition_latency > 20 * 1000) { + static int print_once; + policy->cpuinfo.transition_latency = 20 * 1000; + if (!print_once) { + print_once = 1; + printk(KERN_INFO "Capping off P-state tranision latency" + " at 20 uS\n"); + } + } + data->max_freq = perf->states[0].core_frequency * 1000; /* table init */ for (i=0; istate_count; i++) {