Messages in this thread Patch in this message | | | Subject | Re: [BUG] How to get real-time priority using idle priority | From | Mike Galbraith <> | Date | Mon, 12 Jan 2009 14:03:53 +0100 |
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On Mon, 2009-01-12 at 06:09 +0100, Mike Galbraith wrote: > On Sun, 2009-01-11 at 02:58 -0800, Brian Rogers wrote: > > The attached program gives itself idle priority, then forks off two > > child processes that execute a busy loop. The result is that sometimes > > all other processes stop and the whole system freezes until this program > > exits. An affected system will respond to pings, but X freezes, the > > cursor won't move, SSH sessions won't respond or echo characters back, > > and not even a text console will budge. Hitting Alt-SysRq-N twice can > > sometimes unfreeze the system, or you can just wait for the program to exit. > > > > This bug is in 2.6.29-rc1. I have also observed this bug in 2.6.28 on > > two dual-core systems, an Athlon X2 desktop and a Core 2 Duo laptop. > > Both are running a 64-bit system. Using i386 and amd64 Ubuntu Jaunty > > daily builds with a 2.6.28 kernel, I found I could reproduce the problem > > with the 64-bit kernel, but not the 32-bit kernel. Since that might just > > be due to a difference in the kernel configurations, I'm attaching the > > kernel configuration on which I know this problem can be triggered. > > > > It may take a couple-dozen runs of this program for the freeze to occur. > > Just hit control-C and re-run the program until it happens. When it does > > freeze, the effect is immediate, so there's no chance you'll interrupt > > the program too soon. > > Hi, > > I haven't been able to reproduce complete hangs, but with your proggy > adapted to my quad, _and_ the addition of a SCHED_NORMAL hog, I can > reproduce some very bad interactivity, including massive character > repeats while attempting to type, and general "lurchiness" (_bad_ wakeup > latency). I'll poke it with a sharp stick or two.
Would you mind trying the below?
Impact: fix latency issues when SCHED_IDLE tasks are queued.
Exclude SCHED_IDLE tasks from wakeup preemption, ensure that same will always be wakeup preempted, and exclude them from being buddies so they will only be selected via their vruntime.
Signed-off-by: Mike Galbraith <efault@gmx.de>
diff --git a/kernel/sched.c b/kernel/sched.c index deb5ac8..5582065 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -1888,8 +1888,7 @@ 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; + p->se.vruntime = new_cfsrq->min_vruntime; __set_task_cpu(p, new_cpu); } diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 8e1352c..500ed14 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -1340,14 +1340,18 @@ 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 +1397,18 @@ 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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