Re: regression caused by bb6ab52f2bef ("intel_pstate: Do not set utilization update hook too early")

Jisheng Zhang
2016-06-27T06:13:07Z

Dear Rafael,

On Sat, 25 Jun 2016 02:14:28 +0200 "Rafael J. Wysocki" wrote:

> On Friday, June 17, 2016 04:09:33 PM Jisheng Zhang wrote:
> > Dear all,
> >
> > If using acpi-cpufreq instead, v4.6, v4.6-rc3, v4.7-rc3 can't reproduce the issue. It seems
> > only intel_pstate is impacted.
>
> Which is quite obvious, since the commit your bisection led to was
> intel_pstate-specific. :-)
>
> If the issue is what I'm thinking it is, the patch below should help, so
> can you please test it?
>
> Thanks,
> Rafael
>
> ---
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Subject: [PATCH] intel_pstate: Do not clear utilization update hooks on policy changes
>
> intel_pstate_set_policy() is invoked by the cpufreq core during
> driver initialization, on changes of policy attributes (minimim and
> maximum frequency, for example) via sysfs and via CPU notifications
> from the platform firmware. On some platforms the latter may occur
> relatively often.
>
> Commit bb6ab52f2bef (intel_pstate: Do not set utilization update hook
> too early) made intel_pstate_set_policy() clear the CPU's utilization
> update hook before updating the policy attributes for it (and set the
> hook again after doind that), but that involves invoking
> synchronize_sched() and adds overhead to the CPU notifications
> mentioned above and to the sched-RCU handling in general.
>
> That extra overhead is arguably not necessary, because updating
> policy attributes when the CPU's utilization update hook is active
> should not lead to any adverse effects, so drop the clearing of
> the hook from intel_pstate_set_policy() and make it check if
> the hook has been set already when attempting to set it.

This patch works!

on a 32 snb cores server:

80 wakeups/s w/o the patch

7 wakeups/s w/ the patch

on a 4 snb cores laptop:

22 wakeups/s w/o the patch

6 wakeups/s w/ the patch

Thank you so much for fixing it ;)

>
> Fixes: bb6ab52f2bef (intel_pstate: Do not set utilization update hook too early)
> Reported-by: Jisheng Zhang <jszhang@marvell.com>

So feel free to add

Tested-by: Jisheng Zhang <jszhang@marvell.com>

> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> drivers/cpufreq/intel_pstate.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -1440,6 +1440,9 @@ static void intel_pstate_set_update_util
> {
> struct cpudata *cpu = all_cpu_data[cpu_num];
>
> + if (cpu->update_util_set)
> + return;
> +
> /* Prevent intel_pstate_update_util() from using stale data. */
> cpu->sample.time = 0;
> cpufreq_add_update_util_hook(cpu_num, &cpu->update_util,
> @@ -1480,8 +1483,6 @@ static int intel_pstate_set_policy(struc
> if (!policy->cpuinfo.max_freq)
> return -ENODEV;
>
> - intel_pstate_clear_update_util_hook(policy->cpu);
> -
> pr_debug("set_policy cpuinfo.max %u policy->max %u\n",
> policy->cpuinfo.max_freq, policy->max);
>
>