Re: [PATCH RFC] sched: introduce add_wait_queue_exclusive_head

[Peng Tao]

On Tue, Mar 18, 2014 at 11:47 PM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 03/18, Peter Zijlstra wrote:
>>
>> I think we can avoid the entire function if we add
>> WQ_FLAG_LIFO and make prepare_to_wait_event() DTRT.
>
> Agreed, this looks very natural.
>
> prepare_to_wait_event(WQ_FLAG_LIFO) should probably skip all "flags == 0"
> entries before list_add(). Given that it is supposed that the users won't
> mix exclusive and !exclusive, perhaps the additional list_for_each() won't
> hurt?
>
So please allow me to summary and define the semantics of wait/wake_up
w.r.t. this.

prepare_to_wait_event(0): task is added at the head of the queue.

prepare_to_wait_event(WQ_FLAG_LIFO): task is added at the head of
exclusive queue head

prepare_to_wait_event(WQ_FLAG_EXCLUSIVE): task is added at the tail of the queue

Maybe we should rename the flags to WQ_FLAG_EXCLUSIVE_FIFO and
WQ_FLAG_EXCLUSIVE_LIFO?

>> Then we only need to teach ___wait() about this; and I suppose we could
>> make .exclusive=-1 be the LIFO flag.
>
> Or we can change ___wait_event()
>
>         -       if (exclusive)                                                  \
>         -               __wait.flags = WQ_FLAG_EXCLUSIVE;                       \
>         -       else                                                            \
>         -               __wait.flags = 0;                                       \
>         +       __wait.flags = exclusive;                                       \
>
> and obviously change the callers. Perhaps this argument should be renamed
> then.
>
Current __wait.flags allows possible extension to the existing
interface. If we change it to __wait.flags = exclusive, we drop the
future extensibility. Is it acceptable?

Thanks,
Tao

> But I am fine either way, just I like the idea to extend the current
> interface.
>
> Oleg.
>
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