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Date:   Mon, 20 Jul 2020 12:04:41 +0530
From:   Vaidyanathan Srinivasan <svaidy@linux.ibm.com>
To:     "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
Cc:     Nicholas Piggin <npiggin@gmail.com>,
        Anton Blanchard <anton@ozlabs.org>,
        Nathan Lynch <nathanl@linux.ibm.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Michael Neuling <mikey@neuling.org>, linuxppc-dev@ozlabs.org,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH 5/5] cpuidle-pseries: Block Extended CEDE(1) which adds
 no additional value.
Message-ID: <20200720063441.GF4000@drishya.in.ibm.com>
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* 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