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Message-ID: <20120725224044.GC32378@google.com>
Date: Wed, 25 Jul 2012 15:40:44 -0700
From: Tejun Heo <tj@...nel.org>
To: Peter Boonstoppel <pboonstoppel@...dia.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Paul Gortmaker <paul.gortmaker@...driver.com>,
Henrique de Moraes Holschuh <ibm-acpi@....eng.br>,
Andy Walls <awalls@...metrocast.net>,
Diwakar Tundlam <dtundlam@...dia.com>,
Peter Zijlstra <peterz@...radead.org>,
Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1/1] kthread: disable preemption during complete()
(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@...dia.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
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