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SubjectRe: [PATCH V3] cpuidle: Handle tick_broadcast_enter() failure gracefully
Date
On Friday, May 08, 2015 01:05:32 PM Preeti U Murthy wrote:
> When a CPU has to enter an idle state where tick stops, it makes a call
> to tick_broadcast_enter(). The call will fail if this CPU is the
> broadcast CPU. Today, under such a circumstance, the arch cpuidle code
> handles this CPU. This is not convincing because not only do we not
> know what the arch cpuidle code does, but we also do not account for the
> idle state residency time and usage of such a CPU.
>
> This scenario can be handled better by simply choosing an idle state
> where in ticks do not stop. To accommodate this change move the setting
> of runqueue idle state from the core to the cpuidle driver, else the
> rq->idle_state will be set wrong.
>
> Signed-off-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
> ---
> Changes from V2: https://lkml.org/lkml/2015/5/7/78
> Introduce a function in cpuidle core to select an idle state where ticks do not
> stop rather than going through the governors.
>
> Changes from V1: https://lkml.org/lkml/2015/5/7/24
> Rebased on the latest linux-pm/bleeding-edge branch
>
> drivers/cpuidle/cpuidle.c | 45 +++++++++++++++++++++++++++++++++++++++++++--
> include/linux/sched.h | 16 ++++++++++++++++
> kernel/sched/core.c | 17 +++++++++++++++++
> kernel/sched/fair.c | 2 +-
> kernel/sched/idle.c | 6 ------
> kernel/sched/sched.h | 24 ------------------------
> 6 files changed, 77 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 8c24f95..d1af760 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -21,6 +21,7 @@
> #include <linux/module.h>
> #include <linux/suspend.h>
> #include <linux/tick.h>
> +#include <linux/sched.h>
> #include <trace/events/power.h>
>
> #include "cpuidle.h"
> @@ -146,6 +147,36 @@ int cpuidle_enter_freeze(struct cpuidle_driver *drv, struct cpuidle_device *dev)
> return index;
> }
>
> +/*
> + * find_tick_valid_state - select a state where tick does not stop
> + * @dev: cpuidle device for this cpu
> + * @drv: cpuidle driver for this cpu
> + */
> +static int find_tick_valid_state(struct cpuidle_device *dev,
> + struct cpuidle_driver *drv)
> +{
> + int i, ret = -1;
> +
> + for (i = CPUIDLE_DRIVER_STATE_START; i < drv->state_count; i++) {
> + struct cpuidle_state *s = &drv->states[i];
> + struct cpuidle_state_usage *su = &dev->states_usage[i];
> +
> + /*
> + * We do not explicitly check for latency requirement
> + * since it is safe to assume that only shallower idle
> + * states will have the CPUIDLE_FLAG_TIMER_STOP bit
> + * cleared and they will invariably meet the latency
> + * requirement.
> + */
> + if (s->disabled || su->disable ||
> + (s->flags & CPUIDLE_FLAG_TIMER_STOP))
> + continue;
> +
> + ret = i;
> + }
> + return ret;
> +}
> +
> /**
> * cpuidle_enter_state - enter the state and update stats
> * @dev: cpuidle device for this cpu
> @@ -168,10 +199,17 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
> * CPU as a broadcast timer, this call may fail if it is not available.
> */
> if (broadcast && tick_broadcast_enter()) {
> - default_idle_call();
> - return -EBUSY;
> + index = find_tick_valid_state(dev, drv);

Well, the new state needs to be deeper than the old one or you may violate the
governor's choice and this doesn't guarantee that.

Also I don't quite see a reason to duplicate the find_deepest_state() functionality
here.

> + if (index < 0) {
> + default_idle_call();
> + return -EBUSY;
> + }
> + target_state = &drv->states[index];
> }
>
> + /* Take note of the planned idle state. */
> + idle_set_state(smp_processor_id(), target_state);

And I wouldn't do this either.

The behavior here is pretty much as though the driver demoted the state chosen
by the governor and we don't call idle_set_state() again in those cases.

> +
> trace_cpu_idle_rcuidle(index, dev->cpu);
> time_start = ktime_get();

Overall, something like the patch below (untested) should work I suppose?

---
drivers/cpuidle/cpuidle.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)

Index: linux-pm/drivers/cpuidle/cpuidle.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/cpuidle.c
+++ linux-pm/drivers/cpuidle/cpuidle.c
@@ -73,17 +73,19 @@ int cpuidle_play_dead(void)
}

static int find_deepest_state(struct cpuidle_driver *drv,
- struct cpuidle_device *dev, bool freeze)
+ struct cpuidle_device *dev, bool freeze,
+ int limit, unsigned int flags_to_avoid)
{
unsigned int latency_req = 0;
int i, ret = freeze ? -1 : CPUIDLE_DRIVER_STATE_START - 1;

- for (i = CPUIDLE_DRIVER_STATE_START; i < drv->state_count; i++) {
+ for (i = CPUIDLE_DRIVER_STATE_START; i < limit; i++) {
struct cpuidle_state *s = &drv->states[i];
struct cpuidle_state_usage *su = &dev->states_usage[i];

if (s->disabled || su->disable || s->exit_latency <= latency_req
- || (freeze && !s->enter_freeze))
+ || (freeze && !s->enter_freeze)
+ || (s->flags & flags_to_avoid))
continue;

latency_req = s->exit_latency;
@@ -100,7 +102,7 @@ static int find_deepest_state(struct cpu
int cpuidle_find_deepest_state(struct cpuidle_driver *drv,
struct cpuidle_device *dev)
{
- return find_deepest_state(drv, dev, false);
+ return find_deepest_state(drv, dev, false, drv->state_count, 0);
}

static void enter_freeze_proper(struct cpuidle_driver *drv,
@@ -139,7 +141,7 @@ int cpuidle_enter_freeze(struct cpuidle_
* that interrupts won't be enabled when it exits and allows the tick to
* be frozen safely.
*/
- index = find_deepest_state(drv, dev, true);
+ index = find_deepest_state(drv, dev, true, drv->state_count, 0);
if (index >= 0)
enter_freeze_proper(drv, dev, index);

@@ -168,8 +170,13 @@ int cpuidle_enter_state(struct cpuidle_d
* CPU as a broadcast timer, this call may fail if it is not available.
*/
if (broadcast && tick_broadcast_enter()) {
- default_idle_call();
- return -EBUSY;
+ index = find_deepest_state(drv, dev, false, index,
+ CPUIDLE_FLAG_TIMER_STOP);
+ if (index < 0) {
+ default_idle_call();
+ return -EBUSY;
+ }
+ target_state = &drv->states[index];
}

trace_cpu_idle_rcuidle(index, dev->cpu);


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