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

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Date:	Sat, 25 Jun 2016 15:09:46 +0000
From:	"Pandruvada, Srinivas" <srinivas.pandruvada@...el.com>
To:	"jszhang@...vell.com" <jszhang@...vell.com>,
	"viresh.kumar@...aro.org" <viresh.kumar@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"rafael@...nel.org" <rafael@...nel.org>
Subject: Re: regression caused by bb6ab52f2bef ("intel_pstate: Do not set
 utilization update hook too early")

On Sat, 2016-06-25 at 02:14 +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. :-)
> 
We should also check why the set_policy callback is getting called
quite often. May be some thermal zone is tripping quite often.

echo 'file thermal_core.c +p' > /sys/kernel/debug/dynamic_debug/control

may give us some clue.

Thanks,
Srinivas

> 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@...el.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.
> 
> Fixes: bb6ab52f2bef (intel_pstate: Do not set utilization update hook
> too early)
> Reported-by: Jisheng Zhang <jszhang@...vell.com>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.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);
>  
>