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" <ebied...@xmission.com>
> > Signed-off-by: Luis R. Rodriguez <mcg...@kernel.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
