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    Subject[PATCH 2/3] cfq-iosched: Fair cross-group preemption (implementation)
    Date
    Add a new function cfq_group_should_preempt() which uses the new
    service classes to decide if a task in one cgroup should preempt a
    task in another. We only allow it if the preempting task's vdisktime
    is "behind" the preempted task.

    Signed-off-by: Chad Talbott <ctalbott@google.com>
    ---
    block/cfq-iosched.c | 41 ++++++++++++++++++++++++++++++++++++++---
    1 files changed, 38 insertions(+), 3 deletions(-)

    diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    index 55e78b7..dfcce80 100644
    --- a/block/cfq-iosched.c
    +++ b/block/cfq-iosched.c
    @@ -1157,6 +1157,32 @@ void cfq_unlink_blkio_group(void *key, struct blkio_group *blkg)
    spin_unlock_irqrestore(cfqd->queue->queue_lock, flags);
    }

    +/*
    + * Return true if new_cfqg should preempt cfqg. A return value of
    + * false means "don't care." In that case, cfq has other heuristics
    + * to decide
    + */
    +static bool
    +cfq_group_should_preempt(struct cfq_queue *new_cfqq, struct cfq_queue *cfqq,
    + struct request *rq)
    +{
    + struct cfq_group *cfqg = cfqq->cfqg;
    + struct cfq_group *new_cfqg = new_cfqq->cfqg;
    + u64 grace_period;
    +
    + /* in-group preemption is handled elsewhere */
    + if (new_cfqg == cfqg)
    + return false;
    +
    + if (!(new_cfqg->class == BLKIO_RT_CLASS &&
    + cfqg->class == BLKIO_BE_CLASS))
    + return false;
    +
    + grace_period = cfq_scale_slice(cfqq->allocated_slice, cfqg);
    + return time_before64(new_cfqg->vdisktime,
    + cfqg->vdisktime + grace_period);
    +}
    +
    #else /* GROUP_IOSCHED */
    static struct cfq_group *cfq_get_cfqg(struct cfq_data *cfqd, int create)
    {
    @@ -1176,6 +1202,12 @@ cfq_link_cfqq_cfqg(struct cfq_queue *cfqq, struct cfq_group *cfqg) {
    static void cfq_release_cfq_groups(struct cfq_data *cfqd) {}
    static inline void cfq_put_cfqg(struct cfq_group *cfqg) {}

    +static bool
    +cfq_group_should_preempt(struct cfq_queue *new_cfqq, struct cfq_queue *cfqq,
    + struct request *rq)
    +{
    + return false;
    +}
    #endif /* GROUP_IOSCHED */

    /*
    @@ -3234,6 +3266,12 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
    if (!cfqq)
    return false;

    + if (cfq_group_should_preempt(new_cfqq, cfqq, rq))
    + return true;
    +
    + if (new_cfqq->cfqg != cfqq->cfqg)
    + return false;
    +
    if (cfq_class_idle(new_cfqq))
    return false;

    @@ -3253,9 +3291,6 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
    if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
    return true;

    - if (new_cfqq->cfqg != cfqq->cfqg)
    - return false;
    -
    if (cfq_slice_used(cfqq))
    return true;

    --
    1.7.3.1


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