Message-ID: <2511262.5Lce4mW4Bc@aspire.rjw.lan>
Date:   Mon, 10 Jul 2017 15:33:19 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Derek Basehore <dbasehore@...omium.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>,
        x86@...nel.org, platform-driver-x86@...r.kernel.org,
        Len Brown <len.brown@...el.com>, linux-pm@...r.kernel.org
Subject: Re: [PATCH v5 5/5] intel_idle: Add S0ix validation

On Friday, July 07, 2017 05:03:03 PM Derek Basehore wrote:
> This adds validation of S0ix entry and enables it on Skylake. Using
> the new tick_set_freeze_event function, we program the CPU to wake up
> X seconds after entering freeze. After X seconds, it will wake the CPU
> to check the S0ix residency counters and make sure we entered the
> lowest power state for suspend-to-idle.
>
> It exits freeze and reports an error to userspace when the SoC does
> not enter S0ix on suspend-to-idle.
>
 
Honestly, I'm totally unsure about this ATM, as it seems to assume that it
doesn't make senes to stay suspended if SLP_S0 residency is not there, but
that totally may not be the case.

First of all, there are systems in which SLP_S0 is related to about 10%-20% of
the total power draw reduction whereas the remaining 80%-90% comes from PC10
alone.  So, if you can get to PC10, being unable to get SLP_S0 residency on top
of that may not be a big deal after all.  Of course, you may argue that 10%-20%
of battery life while suspended is "a lot", but that really depends on the
possible alternatives.

Second, as far as the alternatives go, it may not be rosy, because there are
systems that don't support S3 (or any other ACPI sleep states at all for that
matter) and suspend-to-idle is the only suspend mechanism available there.
On those systems it still may make sense to use it even though it may not
reduce the power draw that much.  And from some experiments, suspend-to-idle
still extends battery life by 100% over runtime idle even if the system is not
able to get to PC10 most of the time.

While I understand the use case, I don't think it is a binary "yes"-"no" thing
and the focus on just SLP_S0 may be misguided.

> One example of a bug that can prevent a Skylake CPU from entering S0ix
> (suspend-to-idle) is a leaked reference count to one of the i915 power

Suspend-to-idle is not S0ix.  Suspend-to-idle is the state the kernel puts the
system into after echoing "freeze" to /sys/power/state and S0ix is a platform
power state that may or may not be entered as a result of that.

> wells. The CPU will not be able to enter Package C10 and will
> therefore use about 4x as much power for the entire system. The issue
> is not specific to the i915 power wells though.

Well, fair enough, but what if the SoC can enter PC10, but without SLP_S0
residency on top of it?

> Signed-off-by: Derek Basehore <dbasehore@...omium.org>
> ---
>  drivers/idle/intel_idle.c | 142 +++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 134 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index ebed3f804291..d38621da6e54 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -61,10 +61,12 @@
>  #include <linux/notifier.h>
>  #include <linux/cpu.h>
>  #include <linux/moduleparam.h>
> +#include <linux/suspend.h>
>  #include <asm/cpu_device_id.h>
>  #include <asm/intel-family.h>
>  #include <asm/mwait.h>
>  #include <asm/msr.h>
> +#include <asm/pmc_core.h>
>  
>  #define INTEL_IDLE_VERSION "0.4.1"
>  
> @@ -93,12 +95,29 @@ struct idle_cpu {
>  	bool disable_promotion_to_c1e;
>  };
>  
> +/*
> + * The limit for the exponential backoff for the freeze duration. At this point,
> + * power impact is is far from measurable. It's about 3uW based on scaling from
> + * waking up 10 times a second.
> + */
> +#define MAX_SLP_S0_SECONDS 1000
> +#define SLP_S0_EXP_BASE 10
> +
> +static bool slp_s0_check;
> +static unsigned int slp_s0_seconds;
> +
> +static DEFINE_SPINLOCK(slp_s0_check_lock);
> +static unsigned int slp_s0_num_cpus;
> +static bool slp_s0_check_inprogress;
> +
>  static const struct idle_cpu *icpu;
>  static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
>  static int intel_idle(struct cpuidle_device *dev,
>  			struct cpuidle_driver *drv, int index);
>  static int intel_idle_freeze(struct cpuidle_device *dev,
>  			     struct cpuidle_driver *drv, int index);
> +static int intel_idle_freeze_and_check(struct cpuidle_device *dev,
> +				       struct cpuidle_driver *drv, int index);
>  static struct cpuidle_state *cpuidle_state_table;
>  
>  /*
> @@ -597,7 +616,7 @@ static struct cpuidle_state skl_cstates[] = {
>  		.exit_latency = 2,
>  		.target_residency = 2,
>  		.enter = &intel_idle,
> -		.enter_freeze = intel_idle_freeze, },
> +		.enter_freeze = intel_idle_freeze_and_check, },

Why do you do this for anything lower than C6?

>  	{
>  		.name = "C1E",
>  		.desc = "MWAIT 0x01",
> @@ -605,7 +624,7 @@ static struct cpuidle_state skl_cstates[] = {
>  		.exit_latency = 10,
>  		.target_residency = 20,
>  		.enter = &intel_idle,
> -		.enter_freeze = intel_idle_freeze, },
> +		.enter_freeze = intel_idle_freeze_and_check, },
>  	{
>  		.name = "C3",
>  		.desc = "MWAIT 0x10",
> @@ -613,7 +632,7 @@ static struct cpuidle_state skl_cstates[] = {
>  		.exit_latency = 70,
>  		.target_residency = 100,
>  		.enter = &intel_idle,
> -		.enter_freeze = intel_idle_freeze, },
> +		.enter_freeze = intel_idle_freeze_and_check, },
>  	{
>  		.name = "C6",
>  		.desc = "MWAIT 0x20",
> @@ -621,7 +640,7 @@ static struct cpuidle_state skl_cstates[] = {
>  		.exit_latency = 85,
>  		.target_residency = 200,
>  		.enter = &intel_idle,
> -		.enter_freeze = intel_idle_freeze, },
> +		.enter_freeze = intel_idle_freeze_and_check, },
>  	{
>  		.name = "C7s",
>  		.desc = "MWAIT 0x33",
> @@ -629,7 +648,7 @@ static struct cpuidle_state skl_cstates[] = {
>  		.exit_latency = 124,
>  		.target_residency = 800,
>  		.enter = &intel_idle,
> -		.enter_freeze = intel_idle_freeze, },
> +		.enter_freeze = intel_idle_freeze_and_check, },
>  	{
>  		.name = "C8",
>  		.desc = "MWAIT 0x40",
> @@ -637,7 +656,7 @@ static struct cpuidle_state skl_cstates[] = {
>  		.exit_latency = 200,
>  		.target_residency = 800,
>  		.enter = &intel_idle,
> -		.enter_freeze = intel_idle_freeze, },
> +		.enter_freeze = intel_idle_freeze_and_check, },
>  	{
>  		.name = "C9",
>  		.desc = "MWAIT 0x50",
> @@ -645,7 +664,7 @@ static struct cpuidle_state skl_cstates[] = {
>  		.exit_latency = 480,
>  		.target_residency = 5000,
>  		.enter = &intel_idle,
> -		.enter_freeze = intel_idle_freeze, },
> +		.enter_freeze = intel_idle_freeze_and_check, },
>  	{
>  		.name = "C10",
>  		.desc = "MWAIT 0x60",
> @@ -653,7 +672,7 @@ static struct cpuidle_state skl_cstates[] = {
>  		.exit_latency = 890,
>  		.target_residency = 5000,
>  		.enter = &intel_idle,
> -		.enter_freeze = intel_idle_freeze, },
> +		.enter_freeze = intel_idle_freeze_and_check, },
>  	{
>  		.enter = NULL }
>  };
> @@ -940,6 +959,8 @@ static __cpuidle int intel_idle(struct cpuidle_device *dev,
>   * @dev: cpuidle_device
>   * @drv: cpuidle driver
>   * @index: state index
> + *
> + * @return 0 for success, no failure state
>   */
>  static int intel_idle_freeze(struct cpuidle_device *dev,
>  			     struct cpuidle_driver *drv, int index)
> @@ -952,6 +973,101 @@ static int intel_idle_freeze(struct cpuidle_device *dev,
>  	return 0;
>  }
>  
> +static int check_slp_s0(u32 slp_s0_saved_count)
> +{
> +	u32 slp_s0_new_count;
> +
> +	if (intel_pmc_slp_s0_counter_read(&slp_s0_new_count)) {
> +		pr_warn("Unable to read SLP S0 residency counter\n");
> +		return -EIO;
> +	}
> +
> +	if (slp_s0_saved_count == slp_s0_new_count) {
> +		pr_warn("CPU did not enter SLP S0 for suspend-to-idle.\n");
> +		return -EIO;
> +	}
> +
> +	return 0;
> +}
> +
> +/**
> + * intel_idle_freeze_and_check - enters suspend-to-idle and validates the power
> + * state
> + *
> + * This function enters suspend-to-idle with intel_idle_freeze, but also sets up
> + * a timer to check that S0ix (low power state for suspend-to-idle on Intel
> + * CPUs) is properly entered.
> + *
> + * @dev: cpuidle_device
> + * @drv: cpuidle_driver
> + * @index: state index
> + * @return 0 for success, -EERROR if S0ix was not entered.
> + */
> +static int intel_idle_freeze_and_check(struct cpuidle_device *dev,
> +				       struct cpuidle_driver *drv, int index)
> +{
> +	bool check_on_this_cpu = false;
> +	u32 slp_s0_saved_count;
> +	unsigned long flags;
> +	int cpu = smp_processor_id();
> +	int ret;
> +
> +	/* The last CPU to freeze sets up checking SLP S0 assertion. */
> +	spin_lock_irqsave(&slp_s0_check_lock, flags);
> +	slp_s0_num_cpus++;
> +	if (slp_s0_seconds &&
> +	    slp_s0_num_cpus == num_online_cpus() &&
> +	    !slp_s0_check_inprogress &&
> +	    !intel_pmc_slp_s0_counter_read(&slp_s0_saved_count)) {
> +		ret = tick_set_freeze_event(cpu, ktime_set(slp_s0_seconds, 0));
> +		if (ret < 0) {
> +			spin_unlock_irqrestore(&slp_s0_check_lock, flags);
> +			goto out;
> +		}
> +
> +		/*
> +		 * Make sure check_slp_s0 isn't scheduled on another CPU if it
> +		 * were to leave freeze and enter it again before this CPU
> +		 * leaves freeze.
> +		 */
> +		slp_s0_check_inprogress = true;
> +		check_on_this_cpu = true;
> +	}
> +	spin_unlock_irqrestore(&slp_s0_check_lock, flags);
> +
> +	ret = intel_idle_freeze(dev, drv, index);
> +	if (ret < 0)
> +		goto out;
> +
> +	if (check_on_this_cpu && tick_clear_freeze_event(cpu))
> +		ret = check_slp_s0(slp_s0_saved_count);
> +
> +out:
> +	spin_lock_irqsave(&slp_s0_check_lock, flags);
> +	if (check_on_this_cpu) {
> +		slp_s0_check_inprogress = false;
> +		slp_s0_seconds = min_t(unsigned int,
> +				       SLP_S0_EXP_BASE * slp_s0_seconds,
> +				       MAX_SLP_S0_SECONDS);
> +	}
> +	slp_s0_num_cpus--;
> +	spin_unlock_irqrestore(&slp_s0_check_lock, flags);
> +	return ret;
> +}
> +
> +static int slp_s0_check_prepare(struct notifier_block *nb, unsigned long action,
> +				void *data)
> +{
> +	if (action == PM_SUSPEND_PREPARE)
> +		slp_s0_seconds = slp_s0_check ? 1 : 0;
> +
> +	return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block intel_slp_s0_check_nb = {
> +	.notifier_call = slp_s0_check_prepare,
> +};
> +
>  static void __setup_broadcast_timer(bool on)
>  {
>  	if (on)
> @@ -1454,6 +1570,13 @@ static int __init intel_idle_init(void)
>  		goto init_driver_fail;
>  	}
>  
> +	retval = register_pm_notifier(&intel_slp_s0_check_nb);
> +	if (retval) {
> +		free_percpu(intel_idle_cpuidle_devices);
> +		cpuidle_unregister_driver(&intel_idle_driver);
> +		goto pm_nb_fail;
> +	}
> +
>  	if (boot_cpu_has(X86_FEATURE_ARAT))	/* Always Reliable APIC Timer */
>  		lapic_timer_reliable_states = LAPIC_TIMER_ALWAYS_RELIABLE;
>  
> @@ -1469,6 +1592,8 @@ static int __init intel_idle_init(void)
>  
>  hp_setup_fail:
>  	intel_idle_cpuidle_devices_uninit();
> +	unregister_pm_notifier(&intel_slp_s0_check_nb);
> +pm_nb_fail:
>  	cpuidle_unregister_driver(&intel_idle_driver);
>  init_driver_fail:
>  	free_percpu(intel_idle_cpuidle_devices);
> @@ -1484,3 +1609,4 @@ device_initcall(intel_idle_init);
>   * is the easiest way (currently) to continue doing that.
>   */
>  module_param(max_cstate, int, 0444);
> +module_param(slp_s0_check, bool, 0644);

This has to be documented somehow.

Also, if it is not set, there is a useless overhead every time
intel_idle_freeze_and_check() is called.  It looks like you could use
a static key or similar to avoid that.

Moreover, the notifier is not necessary then as well.

Thanks,
Rafael

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