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    SubjectRe: [RFC -v2 PATCH 2/3] sched: add yield_to function
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    On Fri, 2010-12-17 at 07:57 +0100, Mike Galbraith wrote:
    > On Thu, 2010-12-16 at 14:49 -0500, Rik van Riel wrote:

    > > >> +static void yield_to_fair(struct rq *rq, struct task_struct *p)
    > > >> +{
    > > >> + struct sched_entity *se =&p->se;
    > > >> + struct cfs_rq *cfs_rq = cfs_rq_of(se);
    > > >> + u64 remain = slice_remain(current);
    > > >> +
    > > >> + dequeue_task(rq, p, 0);
    > > >> + se->vruntime -= remain;
    > > >> + if (se->vruntime< cfs_rq->min_vruntime)
    > > >> + se->vruntime = cfs_rq->min_vruntime;
    > > >
    > > > This has an excellent chance of moving the recipient rightward.. and the
    > > > yielding task didn't yield anything. This may achieve the desired
    > > > result or may just create a nasty latency spike... but it makes no
    > > > arithmetic sense.
    > >
    > > Good point, the current task calls yield() in the function
    > > that calls yield_to_fair, but I seem to have lost the code
    > > that penalizes the current task's runtime...
    > >
    > > I'll reinstate that.
    >
    > See comment in parentheses above :)

    BTW, with this vruntime donation thingy, what prevents a task from
    forking off accomplices who do nothing but wait for a wakeup and
    yield_to(exploit)?

    Even swapping vruntimes in the same cfs_rq is dangerous as hell, because
    one party is going backward.

    -Mike



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