Re: [PATCH 1/2] sched: remove extraneous load manipulations

From: Gregory Haskins
Date: Mon Jul 21 2008 - 18:08:36 EST

Peter Zijlstra wrote:
On Thu, 2008-07-03 at 15:37 -0600, Gregory Haskins wrote:
commit 62fb185130e4d420f71a30ff59d8b16b74ef5d2b reverted some patches
in the scheduler, but it looks like it may have left a few redundant
calls to inc_load/dec_load remain in set_user_nice (since the
dequeue_task/enqueue_task take care of the load. This could result
in the load values being off since the load may change while dequeued.

I just checked out v2.6.25.10 but cannot see dequeue_task() do it.

Indeed. I think my eyes glazed over the dequeue vs deactivate, and enqueue vs activate. Good catch, and sorry for the noise. I was wrong..

Please ignore this patch.

-Greg


deactivate_task() otoh does do it.

static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
{
p->sched_class->dequeue_task(rq, p, sleep);
p->se.on_rq = 0;
}

vs

static void deactivate_task(struct rq *rq, struct task_struct *p, int sleep)
{
if (task_contributes_to_load(p))
rq->nr_uninterruptible++;

dequeue_task(rq, p, sleep);
dec_nr_running(p, rq);
}

where

static void dec_nr_running(struct task_struct *p, struct rq *rq)
{
rq->nr_running--;
dec_load(rq, p);
}

And since set_user_nice() actually changes the load we'd better not
forget to do this dec/inc load stuff.

So I'm thinking this patch would actually break stuff.

Signed-off-by: Gregory Haskins <ghaskins@xxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxx>
---

kernel/sched.c | 6 ++----
1 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 31f91d9..b046754 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4679,10 +4679,8 @@ void set_user_nice(struct task_struct *p, long nice)
goto out_unlock;
}
on_rq = p->se.on_rq;
- if (on_rq) {
+ if (on_rq)
dequeue_task(rq, p, 0);
- dec_load(rq, p);
- }
p->static_prio = NICE_TO_PRIO(nice);
set_load_weight(p);
@@ -4692,7 +4690,7 @@ void set_user_nice(struct task_struct *p, long nice)
if (on_rq) {
enqueue_task(rq, p, 0);
- inc_load(rq, p);
+
/*
* If the task increased its priority or is running and
* lowered its priority, then reschedule its CPU:


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