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    SubjectRe: [PATCH v5] sched: Avoid unnecessary work in reweight_entity
    Hi, Peter

    This is the v5, any I found there will be the case that weight == se->load.weight
    in reweight_entity(very often according to the log), so I add the check point.

    And I try to use perf to see whether it can help to prove the improvement, but get:

    invalid or unsupported event: 'sched:sched_switch'

    when use command "perf sched record".

    And also someone told me that even use perf sched may not be able to prove anything
    because what I do may only help improve little performance which can not be easily
    detected :(

    So may be we can prove this patch's performance improvement logically, the summary is:

    1. reduce times to check "se->on" from 2 to 1.
    2. reduce times to check "parent_entity(se)" from 2 to 1.
    3. reduce times to check "entity_is_task(se)" from 2 to 1.
    4. reduce times to set "cfs_rq->load.inv_weight" from 4 to 2.
    5. reduce times to set "rq_of(cfs_rq)->load.inv_weight" from 2 to 1.
    6. no need to use "list_add(&se->group_node, &cfs_rq->tasks);" any more.
    7. no need to use "list_del_init(&se->group_node);" any more.
    8. no need to do "cfs_rq->nr_running++" and "cfs_rq->nr_running--" any more.

    above summary is already in previous mail, any for v5, we get one more:

    9. do nothing if the new weight is same to the old weight.

    And as reight_entity is invoked very often, I think this patch can do some help to the
    performance, although there are no numbers, we can prove it logically :)

    Best regards,
    Michael Wang

    On 02/17/2012 02:03 PM, Michael Wang wrote:

    > From: Michael Wang <wangyun@linux.vnet.ibm.com>
    >
    > In original code, using account_entity_dequeue and account_entity_enqueue
    > as a pair will do some work unnecessary, such like remove node from list
    > and add it back again immediately.
    >
    > And because there are no really enqueue and dequeue happen here, it is not
    > suitable to use account_entity_dequeue and account_entity_enqueue here.
    >
    > This patch combine the work of them in order to save time.
    >
    > since v1:
    > Add more description.
    > Mark add_cfs_task_weight as inline.
    > correct useless and wrong code.
    > Add check point for the case there is no change on weight.
    >
    > Signed-off-by: Michael Wang <wangyun@linux.vnet.ibm.com>
    > ---
    > kernel/sched/fair.c | 16 ++++++++++------
    > 1 files changed, 10 insertions(+), 6 deletions(-)
    >
    > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > index 7c6414f..20aa355 100644
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -777,7 +777,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
    > */
    >
    > #if defined CONFIG_SMP && defined CONFIG_FAIR_GROUP_SCHED
    > -static void
    > +static inline void
    > add_cfs_task_weight(struct cfs_rq *cfs_rq, unsigned long weight)
    > {
    > cfs_rq->task_weight += weight;
    > @@ -938,20 +938,24 @@ static inline void update_entity_shares_tick(struct cfs_rq *cfs_rq)
    > {
    > }
    > # endif /* CONFIG_SMP */
    > +
    > static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
    > unsigned long weight)
    > {
    > + if (weight == se->load.weight)
    > + return;
    > +
    > if (se->on_rq) {
    > /* commit outstanding execution time */
    > if (cfs_rq->curr == se)
    > update_curr(cfs_rq);
    > - account_entity_dequeue(cfs_rq, se);
    > + update_load_add(&cfs_rq->load, weight - se->load.weight);
    > + if (!parent_entity(se))
    > + update_load_add(&rq_of(cfs_rq)->load, weight - se->load.weight);
    > + if (entity_is_task(se))
    > + add_cfs_task_weight(cfs_rq, weight - se->load.weight);
    > }
    > -
    > update_load_set(&se->load, weight);
    > -
    > - if (se->on_rq)
    > - account_entity_enqueue(cfs_rq, se);
    > }
    >
    > static void update_cfs_shares(struct cfs_rq *cfs_rq)




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