Re: [PATCH 1/1] kthread: disable preemption during complete()

[Tejun Heo <tj@kernel.org>]

(cc'ing Oleg and Peter)

On Wed, Jul 25, 2012 at 03:35:32PM -0700, Peter Boonstoppel wrote:
> After a kthread is created it signals the requester using complete()
> and enters TASK_UNINTERRUPTIBLE. However, since complete() wakes up
> the requesting thread this can cause a preemption. The preemption will
> not remove the task from the runqueue (for that schedule() has to be
> invoked directly).
> 
> This is a problem if directly after kthread creation you try to do a
> kthread_bind(), which will block in HZ steps until the thread is off
> the runqueue.
> 
> This patch disables preemption during complete(), since we call
> schedule() directly afterwards, so it will correctly enter
> TASK_UNINTERRUPTIBLE. This speeds up kthread creation/binding during
> cpu hotplug significantly.
> 
> Signed-off-by: Peter Boonstoppel <pboonstoppel@nvidia.com>
> ---
>  kernel/kthread.c |   11 +++++++++++
>  1 files changed, 11 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index b579af5..757d8dd 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -16,6 +16,7 @@
>  #include <linux/mutex.h>
>  #include <linux/slab.h>
>  #include <linux/freezer.h>
> +#include <linux/preempt.h>
>  #include <trace/events/sched.h>
>  
>  static DEFINE_SPINLOCK(kthread_create_lock);
> @@ -113,7 +114,17 @@ static int kthread(void *_create)
>  	/* OK, tell user we're spawned, wait for stop or wakeup */
>  	__set_current_state(TASK_UNINTERRUPTIBLE);
>  	create->result = current;
> +
> +	/*
> +	 * Disable preemption so we enter TASK_UNINTERRUPTIBLE after
> +	 * complete() instead of possibly being preempted. This speeds
> +	 * up clients that do a kthread_bind() directly after
> +	 * creation.
> +	 */
> +	preempt_disable();

Shouldn't this happen before setting current state to UNINTERRUPTIBLE?
What prevents preemption happening right above preempt_disable()?

>  	complete(&create->done);
> +	preempt_enable_no_resched();
> +
>  	schedule();

PeterZ, Oleg, can you guys please review this?

Thanks.

-- 
tejun