wait_event()\ wait_event_interruptible()\ wait_event_interruptible_timeout() and wake_up()

[Valdis Kl=?utf-8?Q?=c4=93?=tnieks]

On Mon, 29 Jul 2019 22:48:57 +0530, Muni Sekhar said:
> Let us assume that multiple processes are waiting on wait_event()\
> wait_event_interruptible()\ wait_event_interruptible_timeout(), which
> process gets woken up on calling wake_up()??
>
> I presume wake_up() picks one process, but is there any algorithm to
> pick which process?

Hint:  If you have more than one process waiting, and they do the same thing
(think multiple copies of the same kthread), it probably doesn't matter.

If they do different things and which one gets picked matters for correctness,
you're doing it wrong and probably need some locking.

If they do different things and the results will be correct no matter which
order they're picked, but you want one to go first for latency/througput
considerations, you have a scheduling/priority issue and probably need to fix
it using the vast plethora of knobs and tools available for that purpose.

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