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

[Jisheng Zhang]

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@xxxxxxxxx>
> 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@xxxxxxxxxxx>

So feel free to add

Tested-by: Jisheng Zhang <jszhang@xxxxxxxxxxx>

> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
>  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);
>  
>