Re: [REPOST-v2] sched: Prevent wakeup to enter critical section needlessly

[Ivo Sieben]

Hi

2012/11/19 Oleg Nesterov <oleg@xxxxxxxxxx>:
>
> I am wondering if it makes sense unconditionally. A lot of callers do
>
>         if (waitqueue_active(q))
>                 wake_up(...);
>
> this patch makes the optimization above pointless and adds mb().
>
>
> But I won't argue.
>
> Oleg.
>

This patch solved an issue for me that I had with the TTY line
discipline idle handling:
Testing on a PREEMPT_RT system with TTY serial communication. Each
time the TTY line discipline is dereferenced the Idle handling wait
queue is woken up (see function put_ldisc in /drivers/tty/tty_ldisc.c)
However line discipline idle handling is not used very often so the
wait queue is empty most of the time. But still the wake_up() function
enters the critical section guarded by spin locks. This causes
additional scheduling overhead when a lower priority thread has
control of that same lock.

The /drivers/tty/tty_ldisc.c did not use the waitqueue_active() call
to check if the waitqueue was filled.... maybe I should solve this
problem the other way around: and make tty_ldisc.c first do the
waitqueue_active() call?

Regards,
Ivo
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