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SubjectRe: [PATCH 5/5] cpuidle-pseries: Block Extended CEDE(1) which adds no additional value.
* Gautham R Shenoy <ego@linux.vnet.ibm.com> [2020-07-07 16:41:39]:

> From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
>
> The Extended CEDE state with latency-hint = 1 is only different from
> normal CEDE (with latency-hint = 0) in that a CPU in Extended CEDE(1)
> does not wakeup on timer events. Both CEDE and Extended CEDE(1) map to
> the same hardware idle state. Since we already get SMT folding from
> the normal CEDE, the Extended CEDE(1) doesn't provide any additional
> value. This patch blocks Extended CEDE(1).
>
> Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>

Reviewed-by: Vaidyanathan Srinivasan <svaidy@linux.ibm.com>

> ---
> drivers/cpuidle/cpuidle-pseries.c | 57 ++++++++++++++++++++++++++++++++++++---
> 1 file changed, 54 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle-pseries.c b/drivers/cpuidle/cpuidle-pseries.c
> index 6f893cd..be0b8b2 100644
> --- a/drivers/cpuidle/cpuidle-pseries.c
> +++ b/drivers/cpuidle/cpuidle-pseries.c
> @@ -350,6 +350,43 @@ static int pseries_cpuidle_driver_init(void)
> return 0;
> }
>
> +#define XCEDE1_HINT 1
> +#define ERR_NO_VALUE_ADD (-1)
> +#define ERR_NO_EE_WAKEUP (-2)
> +
> +/*
> + * Returns 0 if the Extende CEDE state with @hint is not blocked in
> + * cpuidle framework.
> + *
> + * Returns ERR_NO_EE_WAKEUP if the Extended CEDE state is blocked due
> + * to not being responsive to external interrupts.
> + *
> + * Returns ERR_NO_VALUE_ADD if the Extended CEDE state does not provide
> + * added value addition over the normal CEDE.
> + */
> +static int cpuidle_xcede_blocked(u8 hint, u64 latency_us, u8 responsive_to_irqs)
> +{
> +
> + /*
> + * We will only allow extended CEDE states that are responsive
> + * to irqs do not require an H_PROD to be woken up.
> + */
> + if (!responsive_to_irqs)
> + return ERR_NO_EE_WAKEUP;
> +
> + /*
> + * We already obtain SMT folding benefits from CEDE (which is
> + * CEDE with hint 0). Furthermore, CEDE is also responsive to
> + * timer-events, while XCEDE1 requires an external
> + * interrupt/H_PROD to be woken up. Hence, block XCEDE1 since
> + * it adds no further value.
> + */
> + if (hint == XCEDE1_HINT)
> + return ERR_NO_VALUE_ADD;
> +
> + return 0;
> +}
> +
> static int add_pseries_idle_states(void)
> {
> int nr_states = 2; /* By default we have snooze, CEDE */
> @@ -365,15 +402,29 @@ static int add_pseries_idle_states(void)
> char name[CPUIDLE_NAME_LEN];
> unsigned int latency_hint = xcede_records[i].latency_hint;
> u64 residency_us;
> + int rc;
> +
> + if (latency_us < min_latency_us)
> + min_latency_us = latency_us;
> +
> + rc = cpuidle_xcede_blocked(latency_hint, latency_us,
> + xcede_records[i].responsive_to_irqs);
>
> - if (!xcede_records[i].responsive_to_irqs) {
> + if (rc) {
> + switch (rc) {
> + case ERR_NO_VALUE_ADD:
> + pr_info("cpuidle : Skipping XCEDE%d. No additional value-add\n",
> + latency_hint);
> + break;
> + case ERR_NO_EE_WAKEUP:
> pr_info("cpuidle : Skipping XCEDE%d. Not responsive to IRQs\n",
> latency_hint);
> + break;
> + }
> +
> continue;
> }
>
> - if (latency_us < min_latency_us)
> - min_latency_us = latency_us;
> snprintf(name, CPUIDLE_NAME_LEN, "XCEDE%d", latency_hint);
>
> /*


We need these heuristics to select/reject idle states exposed by
platform firmware to Linux primarily because not all states are really
useful to Linux on a given setup.

--Vaidy

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