Subject: Re: [RFC PATCH v2 2/4] cpuidle: (POWER) cpuidle driver for pSeries

From: Benjamin Herrenschmidt <[email-masked]>
Date: Mon, 28 Nov 2011 10:03:28 +1100

On Thu, 2011-11-17 at 16:58 +0530, Deepthi Dharwar wrote:
> This patch implements a backhand cpuidle driver for pSeries
> based on pseries_dedicated_idle_loop and pseries_shared_idle_loop
> routines.  The driver is built only if CONFIG_CPU_IDLE is set. This
> cpuidle driver uses global registration of idle states and
> not per-cpu.

 .../...

> +#define MAX_IDLE_STATE_COUNT	2
> +
> +static int max_cstate = MAX_IDLE_STATE_COUNT - 1;

Why "cstate" ? This isn't an x86 :-)

> +static struct cpuidle_device __percpu *pseries_idle_cpuidle_devices;

Shorter name please. pseries_cpuidle_devs is fine.

> +static struct cpuidle_state *cpuidle_state_table;
> +
> +void update_smt_snooze_delay(int snooze)
> +{
> +	struct cpuidle_driver *drv = cpuidle_get_driver();
> +	if (drv)
> +		drv->states[0].target_residency = snooze;
> +}
> +
> +static inline void idle_loop_prolog(unsigned long *in_purr, ktime_t *kt_before)
> +{
> +
> +	*kt_before = ktime_get_real();
> +	*in_purr = mfspr(SPRN_PURR);
> +	/*
> +	 * Indicate to the HV that we are idle. Now would be
> +	 * a good time to find other work to dispatch.
> +	 */
> +	get_lppaca()->idle = 1;
> +	get_lppaca()->donate_dedicated_cpu = 1;
> +}

I notice that you call this on shared processors as well. The old ocde
used to not set donate_dedicated_cpu in that case. I assume that's not a
big deal and that the HV will just ignore it in the shared processor
case but please add a comment after you've verified it.

> +static inline  s64 idle_loop_epilog(unsigned long in_purr, ktime_t kt_before)
> +{
> +	get_lppaca()->wait_state_cycles += mfspr(SPRN_PURR) - in_purr;
> +	get_lppaca()->donate_dedicated_cpu = 0;
> +	get_lppaca()->idle = 0;
> +
> +	return ktime_to_us(ktime_sub(ktime_get_real(), kt_before));
> +}
> +
> +
> +static int snooze_loop(struct cpuidle_device *dev,
> +			struct cpuidle_driver *drv,
> +			int index)
> +{
> +	unsigned long in_purr;
> +	ktime_t kt_before;
> +
> +	idle_loop_prolog(&in_purr, &kt_before);
> +
> +	local_irq_enable();
> +	set_thread_flag(TIF_POLLING_NRFLAG);
> +	while (!need_resched()) {
> +		ppc64_runlatch_off();
> +		HMT_low();
> +		HMT_very_low();
> +	}
> +	HMT_medium();
> +	clear_thread_flag(TIF_POLLING_NRFLAG);
> +	smp_mb();
> +	local_irq_disable();
> +
> +	dev->last_residency =
> +		(int)idle_loop_epilog(in_purr, kt_before);
> +	return index;
> +}

So your snooze loop has no timeout, is that handled by the cpuidle
driver using some kind of timer ? That sounds a lot less efficient than
passing a max delay to the snooze loop to handle getting into a deeper
state after a bit of snoozing rather than interrupting etc...

> +static int dedicated_cede_loop(struct cpuidle_device *dev,
> +				struct cpuidle_driver *drv,
> +				int index)
> +{
> +	unsigned long in_purr;
> +	ktime_t kt_before;
> +
> +	idle_loop_prolog(&in_purr, &kt_before);
> +
> +	ppc64_runlatch_off();
> +	HMT_medium();
> +	cede_processor();
> +
> +	dev->last_residency =
> +		(int)idle_loop_epilog(in_purr, kt_before);
> +	return index;
> +}
> +
> +static int shared_cede_loop(struct cpuidle_device *dev,
> +			struct cpuidle_driver *drv,
> +			int index)
> +{
> +	unsigned long in_purr;
> +	ktime_t kt_before;
> +
> +	idle_loop_prolog(&in_purr, &kt_before);
> +
> +	/*
> +	 * Yield the processor to the hypervisor.  We return if
> +	 * an external interrupt occurs (which are driven prior
> +	 * to returning here) or if a prod occurs from another
> +	 * processor. When returning here, external interrupts
> +	 * are enabled.
> +	 */
> +	cede_processor();
> +
> +	dev->last_residency =
> +		(int)idle_loop_epilog(in_purr, kt_before);
> +	return index;
> +}
> +
> +/*
> + * States for dedicated partition case.
> + */
> +static struct cpuidle_state dedicated_states[MAX_IDLE_STATE_COUNT] = {
> +	{ /* Snooze */
> +		.name = "snooze",
> +		.desc = "snooze",
> +		.flags = CPUIDLE_FLAG_TIME_VALID,
> +		.exit_latency = 0,
> +		.target_residency = 0,
> +		.enter = &snooze_loop },
> +	{ /* CEDE */
> +		.name = "CEDE",
> +		.desc = "CEDE",
> +		.flags = CPUIDLE_FLAG_TIME_VALID,
> +		.exit_latency = 1,
> +		.target_residency = 10,
> +		.enter = &dedicated_cede_loop },
> +};
> +
> +/*
> + * States for shared partition case.
> + */
> +static struct cpuidle_state shared_states[MAX_IDLE_STATE_COUNT] = {
> +	{ /* Shared Cede */
> +		.name = "Shared Cede",
> +		.desc = "Shared Cede",
> +		.flags = CPUIDLE_FLAG_TIME_VALID,
> +		.exit_latency = 0,
> +		.target_residency = 0,
> +		.enter = &shared_cede_loop },
> +};
> +
> +int pseries_notify_cpuidle_add_cpu(int cpu)
> +{
> +	struct cpuidle_device *dev =
> +			per_cpu_ptr(pseries_idle_cpuidle_devices, cpu);
> +	if (dev && cpuidle_get_driver()) {
> +		cpuidle_disable_device(dev);
> +		cpuidle_enable_device(dev);
> +	}
> +	return 0;
> +}
> +
> +/*
> + * pseries_idle_cpuidle_driver_init()
> + */
> +static int pseries_idle_cpuidle_driver_init(void)
> +{

Drop the "idle", those names are too long.

> +	int cstate;

And use something else than 'cstate', we are among civilized people
here ;-)

> +	struct cpuidle_driver *drv = &pseries_idle_driver;
> +
> +	drv->state_count = 0;
> +
> +	for (cstate = 0; cstate < MAX_IDLE_STATE_COUNT; ++cstate) {
> +
> +		if (cstate > max_cstate)
> +			break;
> +
> +		/* is the state not enabled? */
> +		if (cpuidle_state_table[cstate].enter == NULL)
> +			continue;
> +
> +		drv->states[drv->state_count] =	/* structure copy */
> +			cpuidle_state_table[cstate];
> +
> +		if (cpuidle_state_table == dedicated_states)
> +			drv->states[drv->state_count].target_residency =
> +				__get_cpu_var(smt_snooze_delay);
> +
> +		drv->state_count += 1;
> +	}
> +
> +	return 0;
> +}
> +
> +/* pseries_idle_devices_uninit(void)
> + * unregister cpuidle devices and de-allocate memory
> + */
> +static void pseries_idle_devices_uninit(void)
> +{
> +	int i;
> +	struct cpuidle_device *dev;
> +
> +	for_each_possible_cpu(i) {
> +		dev = per_cpu_ptr(pseries_idle_cpuidle_devices, i);
> +		cpuidle_unregister_device(dev);
> +	}
> +
> +	free_percpu(pseries_idle_cpuidle_devices);
> +	return;
> +}
> +
> +/* pseries_idle_devices_init()
> + * allocate, initialize and register cpuidle device
> + */
> +static int pseries_idle_devices_init(void)
> +{
> +	int i;
> +	struct cpuidle_driver *drv = &pseries_idle_driver;
> +	struct cpuidle_device *dev;
> +
> +	pseries_idle_cpuidle_devices = alloc_percpu(struct cpuidle_device);
> +	if (pseries_idle_cpuidle_devices == NULL)
> +		return -ENOMEM;
> +
> +	for_each_possible_cpu(i) {
> +		dev = per_cpu_ptr(pseries_idle_cpuidle_devices, i);
> +		dev->state_count = drv->state_count;
> +		dev->cpu = i;
> +		if (cpuidle_register_device(dev)) {
> +			printk(KERN_DEBUG \
> +				"cpuidle_register_device %d failed!\n", i);
> +			return -EIO;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +/*
> + * pseries_idle_probe()
> + * Choose state table for shared versus dedicated partition
> + */
> +static int pseries_idle_probe(void)
> +{
> +	if (max_cstate == 0) {
> +		printk(KERN_DEBUG "pseries processor idle disabled.\n");
> +		return -EPERM;
> +	}
> +
> +	if (!firmware_has_feature(FW_FEATURE_SPLPAR)) {
> +		printk(KERN_DEBUG "Using default idle\n");
> +		return -ENODEV;
> +	}

Don't even printk here, this will happen on all !pseries machines and
the debug output isn't useful. Also move the check of max-cstate to
after the splpar test.

Overall, I quite like these patches, my comments are all pretty minor,
hopefully the next round should be the one.

Cheers,
Ben.