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authorPallipadi, Venkatesh <venkatesh.pallipadi@intel.com>2009-03-19 14:41:40 -0700
committerLen Brown <len.brown@intel.com>2009-03-27 21:21:09 -0400
commita59d1637eb0e0a37ee0e5c92800c60abe3624e24 (patch)
tree8cc6a7235fc746bd77da4e756f784d80aff316b0 /arch/x86/kernel/cpu
parent8e0ee43bc2c3e19db56a4adaa9a9b04ce885cd84 (diff)
ACPI: cap off P-state transition latency from buggy BIOSes
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. mjg: We've confirmed that this also helps on the X31 Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com> Acked-by: Matthew Garrett <mjg@redhat.com> Signed-off-by: Len Brown <len.brown@intel.com>
Diffstat (limited to 'arch/x86/kernel/cpu')
-rw-r--r--arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c12
1 files changed, 12 insertions, 0 deletions
diff --git a/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c b/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c
index 4b1c319d30c..89c676d6caf 100644
--- a/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c
+++ b/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.c
@@ -680,6 +680,18 @@ static int acpi_cpufreq_cpu_init(struct cpufreq_policy *policy)
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; i<perf->state_count; i++) {