linux-kernel - Re: [RFC v2 1/2] swait: add idle variants which don't contribute to load average

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Date:   Fri, 16 Jun 2017 08:47:32 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     peterz@...radead.org, paulmck@...ux.vnet.ibm.com, oleg@...hat.com,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        paul.gortmaker@...driver.com, ebiederm@...ssion.com,
        dmitry.torokhov@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 1/2] swait: add idle variants which don't contribute to
 load average

On Thu, Jun 15, 2017 at 11:48:19AM -0700, Luis R. Rodriguez wrote:
> There are cases where folks are using an interruptible swait when
> using kthreads. This is rather confusing given you'd expect
> interruptible waits to be -- interruptible, but kthreads are not
> interruptible ! The reason for such practice though is to avoid
> having these kthreads contribute to the system load average.
> 
> When systems are idle some kthreads may spend a lot of time blocking if
> using swait_event_timeout(). This would contribute to the system load
> average. On systems without preemption this would mean the load average
> of an idle system is bumped to 2 instead of 0. On systems with PREEMPT=y
> this would mean the load average of an idle system is bumped to 3
> instead of 0.
> 
> This adds proper API using TASK_IDLE to make such goals explicit and
> avoid confusion.
> 
> Suggested-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org>
> ---
>  include/linux/swait.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/include/linux/swait.h b/include/linux/swait.h
> index 2c700694d50a..105c70e23286 100644
> --- a/include/linux/swait.h
> +++ b/include/linux/swait.h
> @@ -194,4 +194,29 @@ do {									\
>  	__ret;								\
>  })
>  
> +#define __swait_event_idle(wq, condition)				\
> +	___swait_event(wq, condition, TASK_IDLE, 0, schedule())
> +
> +#define swait_event_idle(wq, condition)					\

Better to have some comments before to describe the purpose of this API?
Something like:

/**
 * swait_event_idle - wait uninterruptibly without system load contribution
 * @wq: the waitqueue to wait on
 * @condition: a C expression for the event to wait for
 *
 * The process is put to sleep (TASK_IDLE) until the
 * @condition evaluates to true or a signal is received.
 * The @condition is checked each time the waitqueue @wq is woken up.
 *
 * This function is mostly used when a kthread waits some condition and
 * doesn't want to contribute to system load
 */

so is for swait_event_idle_timeout().

Thoughts?

Regards,
Boqun

> +({									\
> +	int __ret = 0;							\
> +	if (!(condition))						\
> +		__ret = __swait_event_idle(wq, condition);		\
> +	__ret;								\
> +})
> +
> +#define __swait_event_idle_timeout(wq, condition, timeout)		\
> +	___swait_event(wq, ___wait_cond_timeout(condition),		\
> +		       TASK_IDLE, timeout,				\
> +		       __ret = schedule_timeout(__ret))
> +
> +#define swait_event_idle_timeout(wq, condition, timeout)		\
> +({									\
> +	long __ret = timeout;						\
> +	if (!___wait_cond_timeout(condition))				\
> +		__ret = __swait_event_idle_timeout(wq,			\
> +						   condition, timeout);	\
> +	__ret;								\
> +})
> +
>  #endif /* _LINUX_SWAIT_H */
> -- 
> 2.11.0
> 

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