On Thu, 19 Mar 2009, Pallipadi, Venkatesh wrote:
>
> 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 <venkatesh.pallipadi@xxxxxxxxx>
>
> ---
> 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; i<perf->state_count; i++) {
> --
Sort of makes you wonder if Windows is using this bogus info, or
ignoring it...
applied.
thanks,
Len Brown, Intel Open Source Technology Center
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