Re: [RFC PATCH v2 2/8] cpuidle: record the overhead of idle entry

Rafael J. Wysocki
2017-10-14T00:44:58Z

On Saturday, September 30, 2017 9:20:28 AM CEST Aubrey Li wrote:
> Record the overhead of idle entry in micro-second
>

What is this needed for?

> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
> ---
> drivers/cpuidle/cpuidle.c | 33 +++++++++++++++++++++++++++++++++
> include/linux/cpuidle.h | 14 ++++++++++++++
> kernel/sched/idle.c | 8 +++++++-
> 3 files changed, 54 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 60bb64f..4066308 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -302,6 +302,39 @@ void cpuidle_reflect(struct cpuidle_device *dev, int index)
> cpuidle_curr_governor->reflect(dev, index);
> }
>
> +/* cpuidle_entry_start - record idle entry start */

A proper kerneldoc, please.

> +void cpuidle_entry_start(void)
> +{
> + struct cpuidle_device *dev = cpuidle_get_device();
> +
> + if (dev)
> + dev->idle_stat.entry_start = local_clock();
> +}
> +
> +/*
> + * cpuidle_entry_end - record idle entry end, and maintain
> + * the entry overhead average in micro-second
> + */

A proper kerneldoc, please.

> +void cpuidle_entry_end(void)
> +{
> + struct cpuidle_device *dev = cpuidle_get_device();
> + u64 overhead;
> + s64 diff;
> +
> + if (dev) {
> + dev->idle_stat.entry_end = local_clock();
> + overhead = div_u64(dev->idle_stat.entry_end -
> + dev->idle_stat.entry_start, NSEC_PER_USEC);

Is the conversion really necessary?

If so, then why?

And if there is a good reason, what about using right shift to do
an approximate conversion to avoid the extra division here?

> + diff = overhead - dev->idle_stat.overhead;
> + dev->idle_stat.overhead += diff >> 3;

Can you please explain what happens in the two lines above?

> + /*
> + * limit overhead to 1us
> + */
> + if (dev->idle_stat.overhead == 0)
> + dev->idle_stat.overhead = 1;
> + }
> +}
> +
> /**
> * cpuidle_install_idle_handler - installs the cpuidle idle loop handler
> */
> diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> index fc1e5d7..cad9b71 100644
> --- a/include/linux/cpuidle.h
> +++ b/include/linux/cpuidle.h
> @@ -72,6 +72,15 @@ struct cpuidle_device_kobj;
> struct cpuidle_state_kobj;
> struct cpuidle_driver_kobj;
>
> +struct cpuidle_stat {
> + u64 entry_start; /* nanosecond */
> + u64 entry_end; /* nanosecond */
> + u64 overhead; /* nanosecond */
> + unsigned int predicted_us; /* microsecond */
> + bool predicted; /* ever predicted? */
> + bool fast_idle; /* fast idle? */

Some of the fields here are never used in the code after this patch.

Also it would be good to add a comment describing the meaning of the
fields.

> +};
> +
> struct cpuidle_device {
> unsigned int registered:1;
> unsigned int enabled:1;
> @@ -89,6 +98,7 @@ struct cpuidle_device {
> cpumask_t coupled_cpus;
> struct cpuidle_coupled *coupled;
> #endif
> + struct cpuidle_stat idle_stat;
> };
>
> DECLARE_PER_CPU(struct cpuidle_device *, cpuidle_devices);
> @@ -131,6 +141,8 @@ extern bool cpuidle_not_available(struct cpuidle_driver *drv,
>
> extern int cpuidle_select(struct cpuidle_driver *drv,
> struct cpuidle_device *dev);
> +extern void cpuidle_entry_start(void);
> +extern void cpuidle_entry_end(void);
> extern int cpuidle_enter(struct cpuidle_driver *drv,
> struct cpuidle_device *dev, int index);
> extern void cpuidle_reflect(struct cpuidle_device *dev, int index);
> @@ -164,6 +176,8 @@ static inline bool cpuidle_not_available(struct cpuidle_driver *drv,
> static inline int cpuidle_select(struct cpuidle_driver *drv,
> struct cpuidle_device *dev)
> {return -ENODEV; }
> +static inline void cpuidle_entry_start(void) { }
> +static inline void cpuidle_entry_end(void) { }
> static inline int cpuidle_enter(struct cpuidle_driver *drv,
> struct cpuidle_device *dev, int index)
> {return -ENODEV; }
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 6c23e30..0951dac 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -210,6 +210,12 @@ static void cpuidle_idle_call(void)
> static void do_idle(void)
> {
> /*
> + * we record idle entry overhead now, so any deferrable items
> + * in idle entry path need to be placed between cpuidle_entry_start()
> + * and cpuidle_entry_end()
> + */
> + cpuidle_entry_start();
> + /*
> * If the arch has a polling bit, we maintain an invariant:
> *
> * Our polling bit is clear if we're not scheduled (i.e. if rq->curr !=
> @@ -217,10 +223,10 @@ static void do_idle(void)
> * then setting need_resched is guaranteed to cause the CPU to
> * reschedule.
> */
> -
> __current_set_polling();
> quiet_vmstat();
> tick_nohz_idle_enter();
> + cpuidle_entry_end();
>
> while (!need_resched()) {
> check_pgt_cache();
>