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SubjectRe: [PATCH V3] cpuidle: Handle tick_broadcast_enter() failure gracefully
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On Friday, May 08, 2015 04:18:02 PM Rafael J. Wysocki wrote:
> 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

I should have said "shallower", sorry about that.

The state chosen by the governor satisfies certain latency requirements and we
can't violate those by choosing a deeper state here.

But the patch I sent actually did the right thing. :-)

> 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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--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.


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