Re: [PATCH v2] cpufreq: pcc-cpufreq: Disable dynamic scaling on many-CPU systems

Andreas Herrmann
2018-07-17T20:13:23Z

On Tue, Jul 17, 2018 at 06:14:58PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> The firmware interface used by the pcc-cpufreq driver is
> fundamentally not scalable and using it for dynamic CPU performance
> scaling on systems with many CPUs leads to degraded performance.
>
> For this reason, disable dynamic CPU performance scaling on systems
> with pcc-cpufreq where the number of CPUs present at the driver init
> time is greater than 4. Also make the driver print corresponding
> complaints to the kernel log.
>
> Reported-by: Andreas Herrmann <aherrmann@suse.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>
> -> v2: Rework the messages printed in the problematic case.

I've tested this patch. Effect is as expected: driver loads but use of
ondemand governor is not allowed. Sample output:

[ 40.757519] pcc-cpufreq: (v1.10.00) driver loaded with frequency limits: 1200 MHz, 2800 MHz
[ 40.831705] pcc_cpufreq_init: Too many CPUs, dynamic performance scaling disabled
[ 40.898353] pcc_cpufreq_init: Try to enable a different scaling driver through BIOS settings
[ 40.972327] pcc_cpufreq_init: and complain to the system vendor
[ 41.025620] cpufreq: Can't use ondemand governor as dynamic switching is disallowed. Fallback to performance governor
...
[ 41.187928] cpufreq: Can't use ondemand governor as dynamic switching is disallowed. Fallback to performance governor

Last message is shown for each online CPU in the system (ie. 120x).

Looks good to me.

Andreas

> ---
> drivers/cpufreq/pcc-cpufreq.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> Index: linux-pm/drivers/cpufreq/pcc-cpufreq.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/pcc-cpufreq.c
> +++ linux-pm/drivers/cpufreq/pcc-cpufreq.c
> @@ -589,6 +589,15 @@ static int __init pcc_cpufreq_init(void)
> return ret;
> }
>
> + if (num_present_cpus() > 4) {
> + pcc_cpufreq_driver.flags |= CPUFREQ_NO_AUTO_DYNAMIC_SWITCHING;
> + pr_err("%s: Too many CPUs, dynamic performance scaling disabled\n",
> + __func__);
> + pr_err("%s: Try to enable a different scaling driver through BIOS settings\n",
> + __func__);
> + pr_err("%s: and complain to the system vendor\n", __func__);
> + }
> +
> ret = cpufreq_register_driver(&pcc_cpufreq_driver);
>
> return ret;
>
>