On Thu, Sep 5, 2024 at 11:27 AM Christian Loehle
<christian.loehle@xxxxxxx> wrote:
>
> Remove CPU iowaiters influence on idle state selection.
> Remove the menu notion of performance multiplier which increased with
> the number of tasks that went to iowait sleep on this CPU and haven't
> woken up yet.
>
> Relying on iowait for cpuidle is problematic for a few reasons:
> 1. There is no guarantee that an iowaiting task will wake up on the
> same CPU.
> 2. The task being in iowait says nothing about the idle duration, we
> could be selecting shallower states for a long time.
> 3. The task being in iowait doesn't always imply a performance hit
> with increased latency.
> 4. If there is such a performance hit, the number of iowaiting tasks
> doesn't directly correlate.
> 5. The definition of iowait altogether is vague at best, it is
> sprinkled across kernel code.
>
> Signed-off-by: Christian Loehle <christian.loehle@xxxxxxx>
I promised feedback on this series.
As far as this particular patch is concerned, I generally agree with
all of the above, so I'm going to put it into linux-next right away
and see if anyone reports a problem with it.
> ---
> drivers/cpuidle/governors/menu.c | 76 ++++----------------------------
> 1 file changed, 9 insertions(+), 67 deletions(-)
>
> diff --git a/drivers/cpuidle/governors/menu.c b/drivers/cpuidle/governors/menu.c
> index f3c9d49f0f2a..28363bfa3e4c 100644
> --- a/drivers/cpuidle/governors/menu.c
> +++ b/drivers/cpuidle/governors/menu.c
> @@ -19,7 +19,7 @@
>
> #include "gov.h"
>
> -#define BUCKETS 12
> +#define BUCKETS 6
> #define INTERVAL_SHIFT 3
> #define INTERVALS (1UL << INTERVAL_SHIFT)
> #define RESOLUTION 1024
> @@ -29,12 +29,11 @@
> /*
> * Concepts and ideas behind the menu governor
> *
> - * For the menu governor, there are 3 decision factors for picking a C
> + * For the menu governor, there are 2 decision factors for picking a C
> * state:
> * 1) Energy break even point
> - * 2) Performance impact
> - * 3) Latency tolerance (from pmqos infrastructure)
> - * These three factors are treated independently.
> + * 2) Latency tolerance (from pmqos infrastructure)
> + * These two factors are treated independently.
> *
> * Energy break even point
> * -----------------------
> @@ -75,30 +74,6 @@
> * intervals and if the stand deviation of these 8 intervals is below a
> * threshold value, we use the average of these intervals as prediction.
> *
> - * Limiting Performance Impact
> - * ---------------------------
> - * C states, especially those with large exit latencies, can have a real
> - * noticeable impact on workloads, which is not acceptable for most sysadmins,
> - * and in addition, less performance has a power price of its own.
> - *
> - * As a general rule of thumb, menu assumes that the following heuristic
> - * holds:
> - * The busier the system, the less impact of C states is acceptable
> - *
> - * This rule-of-thumb is implemented using a performance-multiplier:
> - * If the exit latency times the performance multiplier is longer than
> - * the predicted duration, the C state is not considered a candidate
> - * for selection due to a too high performance impact. So the higher
> - * this multiplier is, the longer we need to be idle to pick a deep C
> - * state, and thus the less likely a busy CPU will hit such a deep
> - * C state.
> - *
> - * Currently there is only one value determining the factor:
> - * 10 points are added for each process that is waiting for IO on this CPU.
> - * (This value was experimentally determined.)
> - * Utilization is no longer a factor as it was shown that it never contributed
> - * significantly to the performance multiplier in the first place.
> - *
> */
>
> struct menu_device {
> @@ -112,19 +87,10 @@ struct menu_device {
> int interval_ptr;
> };
>
> -static inline int which_bucket(u64 duration_ns, unsigned int nr_iowaiters)
> +static inline int which_bucket(u64 duration_ns)
> {
> int bucket = 0;
>
> - /*
> - * We keep two groups of stats; one with no
> - * IO pending, one without.
> - * This allows us to calculate
> - * E(duration)|iowait
> - */
> - if (nr_iowaiters)
> - bucket = BUCKETS/2;
> -
> if (duration_ns < 10ULL * NSEC_PER_USEC)
> return bucket;
> if (duration_ns < 100ULL * NSEC_PER_USEC)
> @@ -138,19 +104,6 @@ static inline int which_bucket(u64 duration_ns, unsigned int nr_iowaiters)
> return bucket + 5;
> }
>
> -/*
> - * Return a multiplier for the exit latency that is intended
> - * to take performance requirements into account.
> - * The more performance critical we estimate the system
> - * to be, the higher this multiplier, and thus the higher
> - * the barrier to go to an expensive C state.
> - */
> -static inline int performance_multiplier(unsigned int nr_iowaiters)
> -{
> - /* for IO wait tasks (per cpu!) we add 10x each */
> - return 1 + 10 * nr_iowaiters;
> -}
> -
> static DEFINE_PER_CPU(struct menu_device, menu_devices);
>
> static void menu_update(struct cpuidle_driver *drv, struct cpuidle_device *dev);
> @@ -258,8 +211,6 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> struct menu_device *data = this_cpu_ptr(&menu_devices);
> s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
> u64 predicted_ns;
> - u64 interactivity_req;
> - unsigned int nr_iowaiters;
> ktime_t delta, delta_tick;
> int i, idx;
>
> @@ -268,8 +219,6 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> data->needs_update = 0;
> }
>
> - nr_iowaiters = nr_iowait_cpu(dev->cpu);
> -
> /* Find the shortest expected idle interval. */
> predicted_ns = get_typical_interval(data) * NSEC_PER_USEC;
> if (predicted_ns > RESIDENCY_THRESHOLD_NS) {
> @@ -283,7 +232,7 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> }
>
> data->next_timer_ns = delta;
> - data->bucket = which_bucket(data->next_timer_ns, nr_iowaiters);
> + data->bucket = which_bucket(data->next_timer_ns);
>
> /* Round up the result for half microseconds. */
> timer_us = div_u64((RESOLUTION * DECAY * NSEC_PER_USEC) / 2 +
> @@ -301,7 +250,7 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> */
> data->next_timer_ns = KTIME_MAX;
> delta_tick = TICK_NSEC / 2;
> - data->bucket = which_bucket(KTIME_MAX, nr_iowaiters);
> + data->bucket = which_bucket(KTIME_MAX);
> }
>
> if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
> @@ -328,15 +277,8 @@ static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
> */
> if (predicted_ns < TICK_NSEC)
> predicted_ns = data->next_timer_ns;
> - } else {
> - /*
> - * Use the performance multiplier and the user-configurable
> - * latency_req to determine the maximum exit latency.
> - */
> - interactivity_req = div64_u64(predicted_ns,
> - performance_multiplier(nr_iowaiters));
> - if (latency_req > interactivity_req)
> - latency_req = interactivity_req;
> + } else if (latency_req > predicted_ns) {
> + latency_req = predicted_ns;
> }
>
> /*
> --
> 2.34.1
>
