Re: [PATCH V2] cpuidle: Handle tick_broadcast_enter() failure gracefully

[Rafael J. Wysocki]

On Thursday, May 07, 2015 11:17:21 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 are we not
> aware 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 asking the cpuidle
> governor to choose 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@xxxxxxxxxxxxxxxxxx>
> ---
> Changes from V1: https://lkml.org/lkml/2015/5/7/24
> Rebased on the latest linux-pm/bleeding-edge
> 
>  drivers/cpuidle/cpuidle.c          |   21 +++++++++++++++++----
>  drivers/cpuidle/governors/ladder.c |   13 ++++++++++---
>  drivers/cpuidle/governors/menu.c   |    6 +++++-
>  include/linux/cpuidle.h            |    6 +++---
>  include/linux/sched.h              |   16 ++++++++++++++++
>  kernel/sched/core.c                |   17 +++++++++++++++++
>  kernel/sched/fair.c                |    2 +-
>  kernel/sched/idle.c                |    8 +-------
>  kernel/sched/sched.h               |   24 ------------------------
>  9 files changed, 70 insertions(+), 43 deletions(-)
> 
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index 8c24f95..b7e86f4 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"
> @@ -168,10 +169,17 @@ 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 = cpuidle_select(drv, dev, !broadcast);

No, you can't do that.

This code path may be used by suspend-to-idle and that should not call
cpuidle_select().

What's needed here seems to be a fallback mechanism like "choose the
deepest state shallower than X and such that it won't stop the tick".
You don't really need to run a full governor for that.


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