[patch] sched: improve tick time missed wakeup preempt protection

[Mike Galbraith]

sched: improve tick time missed wakeup preempt protection

f685ceac provides protection from tasks just missing wakeup preemption, and then
having to wait a full slice. However, it offers this protection to tasks which
have no business receiving the benefit, namely SCHED_BATCH and SCHED_IDLE. It
also treats all tasks equally, which obviously isn't true. Exclude tasks of
other than SCHED_NORMAL class, and scale minimum runtime before a tick time
preemption by the difference in task weights, after which, we can just use the
standard wakeup preempt vruntime delta test, sysctl_sched_wakeup_granularity.

Signed-off-by: Mike Galbraith <efa...@gmx.de>
Cc: Ingo Molnar <mi...@elte.hu>
Cc: Peter Zijlstra <a.p.zi...@chello.nl>
LKML-Reference: <new-submission>

---
kernel/sched_fair.c | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)

Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -830,17 +830,23 @@ check_preempt_tick(struct cfs_rq *cfs_rq
* narrow margin doesn't have to wait for a full slice.
* This also mitigates buddy induced latencies under load.
*/
- if (!sched_feat(WAKEUP_PREEMPT))
+ if (!sched_feat(WAKEUP_PREEMPT) || cfs_rq->nr_running < 2)
return;
-
- if (delta_exec < sysctl_sched_min_granularity)
- return;
-
- if (cfs_rq->nr_running > 1) {
+ else {
struct sched_entity *se = __pick_next_entity(cfs_rq);
+ unsigned long min = sysctl_sched_min_granularity;
s64 delta = curr->vruntime - se->vruntime;

- if (delta > ideal_runtime)
+ if (task_of(se)->policy != SCHED_NORMAL)
+ return;
+ if (delta < 0)
+ return;
+ if (curr->load.weight != se->load.weight)
+ min = calc_delta_mine(min, curr->load.weight, &se->load);
+ if (delta_exec < min)
+ return;
+
+ if (delta > sysctl_sched_wakeup_granularity)
resched_task(rq_of(cfs_rq)->curr);
}
}

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[Peter Zijlstra]

You can loose the else, the if branch does an unconditional return,
there's no other way to get below there than 'else' ;-)

[Mike Galbraith]

On Sun, 2009-11-22 at 14:16 +0100, Peter Zijlstra wrote:

> You can loose the else, the if branch does an unconditional return,
> there's no other way to get below there than 'else' ;-)

Ok. Can't plug tail into a function, doesn't fit on a line, so..

sched: improve tick time missed wakeup preempt protection

f685ceac provides protection from tasks just missing wakeup preemption, and then
having to wait a full slice. However, it offers this protection to tasks which
have no business receiving the benefit, namely SCHED_BATCH and SCHED_IDLE. It
also treats all tasks equally, which obviously isn't true. Exclude tasks of
other than SCHED_NORMAL class, and scale minimum runtime before a tick time
preemption by the difference in task weights, after which, we can just use the
standard wakeup preempt vruntime delta test, sysctl_sched_wakeup_granularity.

Signed-off-by: Mike Galbraith <efa...@gmx.de>
Cc: Ingo Molnar <mi...@elte.hu>
Cc: Peter Zijlstra <a.p.zi...@chello.nl>
LKML-Reference: <new-submission>

---
kernel/sched_fair.c | 27 +++++++++++++++++----------
1 file changed, 17 insertions(+), 10 deletions(-)

Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c

@@ -811,7 +811,10 @@ dequeue_entity(struct cfs_rq *cfs_rq, st
static void
check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
{
+ struct sched_entity *next;
unsigned long ideal_runtime, delta_exec;

+ unsigned long min = sysctl_sched_min_granularity;

+ s64 delta;

ideal_runtime = sched_slice(cfs_rq, curr);
delta_exec = curr->sum_exec_runtime - curr->prev_sum_exec_runtime;
@@ -825,24 +828,28 @@ check_preempt_tick(struct cfs_rq *cfs_rq
return;

}

+ if (!sched_feat(WAKEUP_PREEMPT) || cfs_rq->nr_running < 2)

+ return;
+
/*
* Ensure that a task that missed wakeup preemption by a

* narrow margin doesn't have to wait for a full slice.
* This also mitigates buddy induced latencies under load.
*/
- if (!sched_feat(WAKEUP_PREEMPT))

- return;
+ next = __pick_next_entity(cfs_rq);
+ delta = curr->vruntime - next->vruntime;

- if (delta_exec < sysctl_sched_min_granularity)
+ if (task_of(next)->policy != SCHED_NORMAL)

+ return;
+ if (delta < 0)
+ return;

+ if (curr->load.weight != next->load.weight)
+ min = calc_delta_mine(min, curr->load.weight, &next->load);
+ if (delta_exec < min)
return;

- if (cfs_rq->nr_running > 1) {

- struct sched_entity *se = __pick_next_entity(cfs_rq);
- s64 delta = curr->vruntime - se->vruntime;
-
- if (delta > ideal_runtime)
- resched_task(rq_of(cfs_rq)->curr);
- }
+ if (delta > sysctl_sched_wakeup_granularity)
+ resched_task(rq_of(cfs_rq)->curr);
}

static void