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

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Date:   Tue, 20 Jun 2017 23:32:48 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     "Luis R. Rodriguez" <mcgrof@...nel.org>, 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 Fri, Jun 16, 2017 at 08:47:32AM +0800, Boqun Feng wrote:
> 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.

Except we get no signals.

>  * 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().

Will add for both calls and elaborate on the return value for the timeout case.

  Luis