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    SubjectRe: [PATCH 1/2] sched/wait: add round robin wakeup mode
    On Mon, Feb 09, 2015 at 11:06:17PM -0500, Jason Baron wrote:
    > On 02/09/2015 04:50 PM, Peter Zijlstra wrote:
    > > On Mon, Feb 09, 2015 at 08:05:57PM +0000, Jason Baron wrote:
    > >> diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
    > >> index 852143a..17d1039 100644
    > >> --- a/kernel/sched/wait.c
    > >> +++ b/kernel/sched/wait.c
    > >> @@ -71,8 +71,11 @@ static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
    > >> unsigned flags = curr->flags;
    > >>
    > >> if (curr->func(curr, mode, wake_flags, key) &&
    > >> - (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
    > >> + (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive) {
    > >> + if (flags & WQ_FLAG_ROUND_ROBIN)
    > >> + list_move_tail(&curr->task_list, &q->task_list);
    > >> break;
    > >> + }
    > >> }
    > >> }
    > > I think you meant to write something like:
    > >
    > > if (curr->func(curr, mode, wake_flags, key) &&
    > > (flags & WQ_FLAG_EXCLUSIVE)) {
    > > if (flag & WQ_FLAG_ROUND_ROBIN)
    > > list_move_tail(&curr->task_list, &q->task_list);
    > > if (!--nr_exclusive)
    > > break;
    > > }
    > >
    > > Otherwise can only work for nr_exclusive==1.
    >
    > Indeed. I'm also wondering if its worth avoiding the list_move_tail()
    > for the case where nr_exclusive is initially 0. IE the wake all case,
    > where we are just going to end up doing a bunch of list_move_tail()
    > calls, but end up in the same state.

    After writing this email, it occurred to me that you could probably do
    this with a custom wake function.

    Where autoremove_wake_function() does a list_del_init() you could do a
    rotate_wake_function() that does list_move_tail().

    That would avoid the entire WQ flag muckery.


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