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Message-Id: <1231910014.610.14.camel@marge.simson.net>
Date: Wed, 14 Jan 2009 06:13:34 +0100
From: Mike Galbraith <efault@....de>
To: Brian Rogers <brian@...w.org>
Cc: Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [BUG] How to get real-time priority using idle priority
On Tue, 2009-01-13 at 03:58 +0100, Mike Galbraith wrote:
> On Mon, 2009-01-12 at 17:05 -0800, Brian Rogers wrote:
> > I'll try Mike's "more complete" patch on top of 2.6.29-rc1 and see what
> > that does.
>
> Don't bother. I just tried a SCHED_IDLE make -j8 and had character
> repeats while typing. Must be another spot.
Hrmph, what an annoying problem. The below works pretty well, but
_still_ has latency problems in some circumstances.
The more I look at this, the more I think these guys _really_ want to be
a separate class. The problem is the incredible rate of min_vruntime
advancement creating absurdly huge spreads.
Hm, maybe I could advance min_vruntime at nice 0 when these guys are
running, only advance their vruntime at warp 512, but that seems awfully
hackish. If they were a separate class, they could use the full nice
spectrum instead of being merely mega-nice.
diff --git a/kernel/sched.c b/kernel/sched.c
index deb5ac8..5221515 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1888,8 +1888,29 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
schedstat_inc(p, se.nr_forced2_migrations);
}
#endif
- p->se.vruntime -= old_cfsrq->min_vruntime -
- new_cfsrq->min_vruntime;
+ if (old_cpu != new_cpu) {
+ u64 vruntime, min_vruntime;
+ s64 delta = p->se.vruntime - old_cfsrq->min_vruntime;
+
+ /*
+ * min_vruntimes may be advancing at wildly different
+ * rates, so we must scale the delta accordingly.
+ */
+ if (new_cfsrq->load.weight != old_cfsrq->load.weight) {
+ int negative = delta < 0;
+
+ delta = negative ? -delta : delta;
+ delta = calc_delta_mine(delta,
+ new_cfsrq->load.weight, &old_cfsrq->load);
+ delta = negative ? -delta : delta;
+ }
+ vruntime = new_cfsrq->min_vruntime + delta;
+ min_vruntime = new_cfsrq->min_vruntime - sysctl_sched_latency;
+ delta = (s64)(vruntime - min_vruntime);
+ if (delta > 0)
+ min_vruntime = vruntime;
+ p->se.vruntime = min_vruntime;
+ }
__set_task_cpu(p, new_cpu);
}
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 8e1352c..b18658f 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -673,7 +673,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
if (!initial) {
/* sleeps upto a single latency don't count. */
- if (sched_feat(NEW_FAIR_SLEEPERS)) {
+ if (sched_feat(NEW_FAIR_SLEEPERS) &&
+ task_of(se)->policy != SCHED_IDLE) {
unsigned long thresh = sysctl_sched_latency;
/*
@@ -793,6 +794,10 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
}
#endif
se->prev_sum_exec_runtime = se->sum_exec_runtime;
+
+ /* Idle tasks reach parity as soon as they get a chance to run. */
+ if (unlikely(task_of(se)->policy == SCHED_IDLE))
+ se->vruntime = cfs_rq->min_vruntime;
}
static int
@@ -1340,14 +1345,18 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
static void set_last_buddy(struct sched_entity *se)
{
- for_each_sched_entity(se)
- cfs_rq_of(se)->last = se;
+ for_each_sched_entity(se) {
+ if (likely(task_of(se)->policy != SCHED_IDLE))
+ cfs_rq_of(se)->last = se;
+ }
}
static void set_next_buddy(struct sched_entity *se)
{
- for_each_sched_entity(se)
- cfs_rq_of(se)->next = se;
+ for_each_sched_entity(se) {
+ if (likely(task_of(se)->policy != SCHED_IDLE))
+ cfs_rq_of(se)->next = se;
+ }
}
/*
@@ -1393,12 +1402,18 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int sync)
return;
/*
- * Batch tasks do not preempt (their preemption is driven by
+ * Batch and idle tasks do not preempt (their preemption is driven by
* the tick):
*/
- if (unlikely(p->policy == SCHED_BATCH))
+ if (unlikely(p->policy != SCHED_NORMAL))
return;
+ /* Idle tasks are by definition preempted by everybody. */
+ if (unlikely(curr->policy == SCHED_IDLE)) {
+ resched_task(curr);
+ return;
+ }
+
if (!sched_feat(WAKEUP_PREEMPT))
return;
--
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