* Pallipadi, Venkatesh <venkatesh.pallipadi@xxxxxxxxx> 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");
> + }
btw.., in the next merge window we'll have printk_once():
f036be9: printk: introduce printk_once()
so then the above can be cleaned up to:
> + /* 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) {
> +
> + policy->cpuinfo.transition_latency = 20 * 1000;
> + printk_once(KERN_INFO
> + "Capping off P-state tranision latency at 20 uS\n");
> + }
Ingo
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