Re: [PATCH v6 1/4] PM: Add sysfs files to represent time spent in hardware sleep state

["Box, David E" <david.e.box@intel.com>]

Hi Mario,

On Mon, 2023-04-03 at 16:18 -0500, Mario Limonciello wrote:
> Userspace can't easily discover how much of a sleep cycle was spent in a
> hardware sleep state without using kernel tracing and vendor specific sysfs
> or debugfs files.
> 
> To make this information more discoverable, introduce two new sysfs files:
> 1) The time spent in a hw sleep state for last cycle.
> 2) The time spent in a hw sleep state since the kernel booted
> Both of these files will be present only if the system supports s2idle.
> 
> Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> ---
> v5->v6:
>  * Add total attribute as well
>  * Change text for documentation
>  * Adjust flow of is_visible callback.
>  * If overflow was detected in total attribute return -EOVERFLOW
>  * Rename symbol
>  * Add stub for symbol for builds without CONFIG_PM_SLEEP
> v4->v5:
>  * Provide time in microseconds instead of percent. Userspace can convert
>    this if desirable.
> ---
>  Documentation/ABI/testing/sysfs-power | 27 ++++++++++++++++
>  include/linux/suspend.h               |  5 +++
>  kernel/power/main.c                   | 45 +++++++++++++++++++++++++++
>  3 files changed, 77 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-power
> b/Documentation/ABI/testing/sysfs-power
> index f99d433ff311..37240575d282 100644
> --- a/Documentation/ABI/testing/sysfs-power
> +++ b/Documentation/ABI/testing/sysfs-power
> @@ -413,6 +413,33 @@ Description:
>                 The /sys/power/suspend_stats/last_failed_step file contains
>                 the last failed step in the suspend/resume path.
>  
> +What:          /sys/power/suspend_stats/last_hw_sleep
> +Date:          June 2023
> +Contact:       Mario Limonciello <mario.limonciello@amd.com>
> +Description:
> +               The /sys/power/suspend_stats/last_hw_sleep file
> +               contains the duration of time spent in a hardware sleep
> +               state in the most recent system suspend-resume cycle.
> +               This number is measured in microseconds.
> +
> +               NOTE: Limitations in the size of the hardware counters may
> +               cause this value to be inaccurate in longer sleep cycles.
> +
> +What:          /sys/power/suspend_stats/total_hw_sleep
> +Date:          June 2023
> +Contact:       Mario Limonciello <mario.limonciello@amd.com>
> +Description:
> +               The /sys/power/suspend_stats/total_hw_sleep file
> +               contains the aggregate of time spent in a hardware sleep
> +               state since the kernel was booted. This number
> +               is measured in microseconds.
> +
> +               NOTE: Limitations in the size of the hardware counters may
> +               cause this value to be inaccurate in longer sleep cycles.
> +
> +               If an overflow has been detected this file will return
> +               -EOVERFLOW.
> +
>  What:          /sys/power/sync_on_suspend
>  Date:          October 2019
>  Contact:       Jonas Meurer <jonas@freesources.org>
> diff --git a/include/linux/suspend.h b/include/linux/suspend.h
> index cfe19a028918..069ef0c0ae57 100644
> --- a/include/linux/suspend.h
> +++ b/include/linux/suspend.h
> @@ -68,6 +68,8 @@ struct suspend_stats {
>         int     last_failed_errno;
>         int     errno[REC_FAILED_NUM];
>         int     last_failed_step;
> +       u64     last_hw_sleep;
> +       u64     total_hw_sleep;
>         enum suspend_stat_step  failed_steps[REC_FAILED_NUM];
>  };
>  
> @@ -489,6 +491,7 @@ void restore_processor_state(void);
>  extern int register_pm_notifier(struct notifier_block *nb);
>  extern int unregister_pm_notifier(struct notifier_block *nb);
>  extern void ksys_sync_helper(void);
> +extern void pm_report_hw_sleep_time(u64 t);
>  
>  #define pm_notifier(fn, pri) {                         \
>         static struct notifier_block fn##_nb =                  \
> @@ -526,6 +529,8 @@ static inline int unregister_pm_notifier(struct
> notifier_block *nb)
>         return 0;
>  }
>  
> +static inline void pm_report_hw_sleep_time(u64 t) {};
> +
>  static inline void ksys_sync_helper(void) {}
>  
>  #define pm_notifier(fn, pri)   do { (void)(fn); } while (0)
> diff --git a/kernel/power/main.c b/kernel/power/main.c
> index 31ec4a9b9d70..ffd4dd43cbdd 100644
> --- a/kernel/power/main.c
> +++ b/kernel/power/main.c
> @@ -6,6 +6,7 @@
>   * Copyright (c) 2003 Open Source Development Lab
>   */
>  
> +#include <linux/acpi.h>
>  #include <linux/export.h>
>  #include <linux/kobject.h>
>  #include <linux/string.h>
> @@ -83,6 +84,18 @@ int unregister_pm_notifier(struct notifier_block *nb)
>  }
>  EXPORT_SYMBOL_GPL(unregister_pm_notifier);
>  
> +void pm_report_hw_sleep_time(u64 t)
> +{
> +       suspend_stats.last_hw_sleep = t;
> +       if (suspend_stats.total_hw_sleep == -EOVERFLOW)
> +               return;
> +       if (suspend_stats.total_hw_sleep + t < suspend_stats.total_hw_sleep)
> +               suspend_stats.total_hw_sleep = -EOVERFLOW;
> +       else
> +               suspend_stats.total_hw_sleep += t;

total_hw_sleep is u64. At microsecond granularity it will never realistically
overflow and isn't worth the check IMO. The overflow concern comes from the u32
hardware counter, but I don't think there's a good way to detect it.

You could just report the maximum hardware counter time as max_hw_sleep so users
know the value isn't reliable if suspended for longer than that.

David

> +}
> +EXPORT_SYMBOL_GPL(pm_report_hw_sleep_time);
> +
>  int pm_notifier_call_chain_robust(unsigned long val_up, unsigned long
> val_down)
>  {
>         int ret;
> @@ -377,6 +390,22 @@ static ssize_t last_failed_step_show(struct kobject
> *kobj,
>  }
>  static struct kobj_attribute last_failed_step = __ATTR_RO(last_failed_step);
>  
> +static ssize_t last_hw_sleep_show(struct kobject *kobj,
> +               struct kobj_attribute *attr, char *buf)
> +{
> +       return sysfs_emit(buf, "%llu\n", suspend_stats.last_hw_sleep);
> +}
> +static struct kobj_attribute last_hw_sleep = __ATTR_RO(last_hw_sleep);
> +
> +static ssize_t total_hw_sleep_show(struct kobject *kobj,
> +               struct kobj_attribute *attr, char *buf)
> +{
> +       if (suspend_stats.total_hw_sleep == -EOVERFLOW)
> +               return suspend_stats.total_hw_sleep;
> +       return sysfs_emit(buf, "%llu\n", suspend_stats.total_hw_sleep);
> +}
> +static struct kobj_attribute total_hw_sleep = __ATTR_RO(total_hw_sleep);
> +
>  static struct attribute *suspend_attrs[] = {
>         &success.attr,
>         &fail.attr,
> @@ -391,12 +420,28 @@ static struct attribute *suspend_attrs[] = {
>         &last_failed_dev.attr,
>         &last_failed_errno.attr,
>         &last_failed_step.attr,
> +       &last_hw_sleep.attr,
> +       &total_hw_sleep.attr,
>         NULL,
>  };
>  
> +static umode_t suspend_attr_is_visible(struct kobject *kobj, struct attribute
> *attr, int idx)
> +{
> +       if (attr != &last_hw_sleep.attr &&
> +           attr != &total_hw_sleep.attr)
> +               return 0444;
> +
> +#ifdef CONFIG_ACPI
> +       if (acpi_gbl_FADT.flags & ACPI_FADT_LOW_POWER_S0)
> +               return 0444;
> +#endif
> +       return 0;
> +}
> +
>  static const struct attribute_group suspend_attr_group = {
>         .name = "suspend_stats",
>         .attrs = suspend_attrs,
> +       .is_visible = suspend_attr_is_visible,
>  };
>  
>  #ifdef CONFIG_DEBUG_FS