Re: VST and Sched Load Balance
From: Srivatsa Vaddagiri (vatsa_at_in.ibm.com)
Date: 05/05/05
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Date: Thu, 5 May 2005 20:09:58 +0530 To: Nick Piggin <nickpiggin@yahoo.com.au>, mingo@elte.hu
On Thu, Apr 07, 2005 at 11:07:55PM +1000, Nick Piggin wrote:
> Srivatsa Vaddagiri wrote:
>
> >I think a potential area which VST may need to address is
> >scheduler load balance. If idle CPUs stop taking local timer ticks for
> >some time, then during that period it could cause the various runqueues to
> >go out of balance, since the idle CPUs will no longer pull tasks from
> >non-idle CPUs.
> >
>
> Yep.
>
> >Do we care about this imbalance? Especially considering that most
> >implementations will let the idle CPUs sleep only for some max duration
> >(~900 ms in case of x86).
> >
>
> I think we do care, yes. It could be pretty harmful to sleep for
> even a few 10s of ms on a regular basis for some workloads. Although
> I guess many of those will be covered by try_to_wake_up events...
>
> Not sure in practice, I would imagine it will hurt some multiprocessor
> workloads.
I am looking at the recent changes in load balance and I see that load
balance on fork has been introduced (SD_BALANCE_FORK). I think this changes
the whole scenario.
Considering the fact that there was already balance on wake_up and the
fact that the scheduler checks for imbalance before running the idle task
(load_balance_newidle), I don't know if sleeping idle CPUs can cause a
load imbalance (fork/wakeup happening on other CPUs will probably push
tasks to it and wake it up anyway? exits can change the balance, but probably
is not relevant here?)
Except for a small fact: if the CPU sleeps w/o taking rebalance_ticks,
its cpu_load[] can become incorrect over a period.
I noticed that load_balance_newidle uses newidle_idx to gauge the current cpu's
load. As a result, it can see non-zero load for the idle cpu. Because of this
it can decide to not pull tasks.
The rationale here (of using non-zero load): is it to try and let the
cpu become idle? Somehow, this doesn't make sense, because in the very next
rebalance_tick (assuming that the idle cpu does not sleep), it will start using
the idle_idx, which will cause the load to show up as zero and can cause the
idle CPU to pull some tasks.
Have I missed something here?
Anyway, if the idle cpu were to sleep instead, the next rebalance_tick will
not happen and it will not pull the tasks to restore load balance.
If my above understanding is correct, I see two potential solutions for this:
A. Have load_balance_newidle use zero load for current cpu while
checking for busiest cpu.
B. Or, if we want to retain load_balance_newidle the way it is, have
the idle thread call back scheduler to zero the load and retry
load balance, _when_ it decides that it wants to sleep (there
are conditions under which a idle cpu may not want to sleep. for ex:
the next timer is only a tick, 1ms, away).
In either case, if the load balance still fails to pull any tasks, then it means
there is really no imbalance. Tasks that will be added into the system later
(fork/wake_up) will be balanced across the CPUs because of the load-balance
code that runs during those events.
A possible patch for B follows below:
---
linux-2.6.12-rc3-mm2-vatsa/include/linux/sched.h | 1
linux-2.6.12-rc3-mm2-vatsa/kernel/sched.c | 38 +++++++++++++++++++++++
2 files changed, 39 insertions(+)
diff -puN kernel/sched.c~sched-nohz kernel/sched.c
--- linux-2.6.12-rc3-mm2/kernel/sched.c~sched-nohz 2005-05-04 18:23:30.000000000 +0530
+++ linux-2.6.12-rc3-mm2-vatsa/kernel/sched.c 2005-05-05 11:37:12.000000000 +0530
@@ -2214,6 +2214,44 @@ static inline void idle_balance(int this
}
}
+#ifdef CONFIG_NO_IDLE_HZ
+/*
+ * Try hard to pull tasks. Called by idle task before it sleeps shutting off
+ * local timer ticks. This clears the various load counters and tries to pull
+ * tasks. If it cannot, then it means that there is really no imbalance at this
+ * point. Any imbalance that arises in future (because of fork/wake_up) will be
+ * handled by the load balance that happens during those events.
+ *
+ * Returns 1 if tasks were pulled over, 0 otherwise.
+ */
+int idle_balance_retry(void)
+{
+ int j, moved = 0, this_cpu = smp_processor_id();
+ struct sched_domain *sd;
+ runqueue_t *this_rq = this_rq();
+ unsigned long flags;
+
+ spin_lock_irqsave(&this_rq->lock, flags);
+
+ for (j = 0; j < 3; j++)
+ this_rq->cpu_load[j] = 0;
+
+ for_each_domain(this_cpu, sd) {
+ if (sd->flags & SD_BALANCE_NEWIDLE) {
+ if (load_balance_newidle(this_cpu, this_rq, sd)) {
+ /* We've pulled tasks over so stop searching */
+ moved = 1;
+ break;
+ }
+ }
+ }
+
+ spin_unlock_irqrestore(&this_rq->lock, flags);
+
+ return moved;
+}
+#endif
+
/*
* active_load_balance is run by migration threads. It pushes running tasks
* off the busiest CPU onto idle CPUs. It requires at least 1 task to be
diff -puN include/linux/sched.h~sched-nohz include/linux/sched.h
--- linux-2.6.12-rc3-mm2/include/linux/sched.h~sched-nohz 2005-05-04 18:23:30.000000000 +0530
+++ linux-2.6.12-rc3-mm2-vatsa/include/linux/sched.h 2005-05-04 18:23:37.000000000 +0530
@@ -897,6 +897,7 @@ extern int task_curr(const task_t *p);
extern int idle_cpu(int cpu);
extern int sched_setscheduler(struct task_struct *, int, struct sched_param *);
extern task_t *idle_task(int cpu);
+extern int idle_balance_retry(void);
void yield(void);
_
--
Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017
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