Re: ppc32: Weird process scheduling behaviour with 2.6.24-rc
From: Peter Zijlstra <hidden>
Date: 2008-01-28 08:56:49
On Sun, 2008-01-27 at 17:13 +0100, Michel Dänzer wrote:
In summary, there are two separate problems with similar symptoms, which
had me confused at times:
* With CONFIG_FAIR_USER_SCHED disabled, there are severe
interactivity hickups with a niced CPU hog and top running. This
started with commit 810e95ccd58d91369191aa4ecc9e6d4a10d8d0c8. The revert at the bottom causes the wakeup granularity to shrink for + nice and to grow for - nice. That is, it becomes easier to preempt a + nice task, and harder to preempt a - nice task. I think we originally had that; didn't comment it, forgot the reason changed it because the units didn't match. Another reason might have been the more difficult preemption of - nice tasks. That might - niced tasks to cause horrible latencies - Ingo, any recollection? Are you perhaps running with a very low HZ (HZ=100)? (If wakeup preemption fails, tick preemption will take over). Also, could you try lowering: /proc/sys/kernel/sched_wakeup_granularity_ns
* With CONFIG_FAIR_USER_SCHED enabled, X becomes basically
unusable with a niced CPU hog, with or without top running. I
don't know when this started, possibly when this option was
first introduced.Srivatsa found an issue that might explain the very bad behaviour under group scheduling. But I gather you're not at all interested in this feature?
quoted hunk ↗ jump to hunk
FWIW, the patch below (which reverts commit 810e95ccd58d91369191aa4ecc9e6d4a10d8d0c8) restores 2.6.24 interactivity to the same level as 2.6.23 here with CONFIG_FAIR_USER_SCHED disabled (my previous report to the contrary was with CONFIG_FAIR_USER_SCHED enabled because I didn't yet realize the difference it makes), but I don't know if that's the real fix.diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index da7c061..a7cc22a 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c@@ -843,7 +843,6 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p) struct task_struct *curr = rq->curr; struct cfs_rq *cfs_rq = task_cfs_rq(curr); struct sched_entity *se = &curr->se, *pse = &p->se; - unsigned long gran; if (unlikely(rt_prio(p->prio))) { update_rq_clock(rq);@@ -866,11 +865,8 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p) pse = parent_entity(pse); } - gran = sysctl_sched_wakeup_granularity; - if (unlikely(se->load.weight != NICE_0_LOAD)) - gran = calc_delta_fair(gran, &se->load); - if (pse->vruntime + gran < se->vruntime) + if (pse->vruntime + sysctl_sched_wakeup_granularity < se->vruntime) resched_task(curr); }