Re: [PATCH tip/core/rcu 4/9] sched: add is_idle_task() to handle invalidated uses of idle_cpu()

[Josh Triplett <josh@joshtriplett.org>]

On Tue, Nov 15, 2011 at 12:28:00PM -0800, Paul E. McKenney wrote:
> From: Paul E. McKenney <paul.mckenney@linaro.org>
> 
> Commit 908a3283 (Fix idle_cpu()) invalidated some uses of idle_cpu(),
> which used to say whether or not the CPU was running the idle task,
> but now instead says whether or not the CPU is running the idle task
> in the absence of pending wakeups.  Although this new implementation
> gives a better answer to the question "is this CPU idle?", it also
> invalidates other uses that were made of idle_cpu().
> 
> This commit therefore introduces a new is_idle_task() API member
> that determines whether or not the specified task is one of the
> idle tasks, allowing open-coded "->pid == 0" sequences to be replaced
> by something more meaningful.
> 
> Suggested-by: Josh Triplett <josh@joshtriplett.org>
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  include/linux/sched.h |    1 +
>  kernel/sched.c        |    9 +++++++++
>  2 files changed, 10 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 68daf4f..4b1077b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2071,6 +2071,7 @@ extern int sched_setscheduler(struct task_struct *, int,
>  extern int sched_setscheduler_nocheck(struct task_struct *, int,
>  				      const struct sched_param *);
>  extern struct task_struct *idle_task(int cpu);
> +extern int is_idle_task(struct task_struct *p);

The kernel has a bool type; please use it here rather than int, to make
the return value more obvious.

Also, any particular reason not to make this a static inline?

- Josh Triplett