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SubjectRe: [patch 0/7] sched: change nohz idle load balancing logic to push model
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On Fri, 2010-05-21 at 17:09 -0700, Suresh Siddha wrote:

> From: Venkatesh Pallipadi <venki@google.com>
> Subject: sched: Change nohz ilb logic from pull to push model
>
> In the new push model, all idle CPUs indeed go into nohz mode. There is
> still the concept of idle load balancer (performing the load balancing
> on behalf of all the idle cpu's in the system). Busy CPU kicks the nohz
> balancer when any of the nohz CPUs need idle load balancing.
> The kickee CPU does the idle load balancing on behalf of all idle CPUs
> instead of the normal idle balance.
>
> This addresses the below two problems with the current nohz ilb logic:
> * the idle load balancer continued to have periodic ticks during idle and
> wokeup frequently, even though it did not have any rebalancing to do on
> behalf of any of the idle CPUs.
> * On x86 and CPUs that have APIC timer stoppage on idle CPUs, this
> periodic wakeup can result in a periodic additional interrupt on a CPU
> doing the timer broadcast.
>
> Also currently we are migrating the unpinned timers from an idle to the cpu
> doing idle load balancing (when all the cpus in the system are idle,
> there is no idle load balancing cpu and timers get added to the same idle cpu
> where the request was made. So the existing optimization works only on semi idle
> system).
>
> And In semi idle system, we no longer have periodic ticks on the idle load
> balancer CPU. Using that cpu will add more delays to the timers than intended
> (as that cpu's timer base may not be uptodate wrt jiffies etc). This was
> causing mysterious slowdowns during boot etc.
>
> For now, in the semi idle case, use the nearest busy cpu for migrating timers
> from an idle cpu. This is good for power-savings anyway.
>
> Signed-off-by: Venkatesh Pallipadi <venki@google.com>
> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
> ---

I sorted the conflict with Venki's update_cpu_load() patch as below.

Thanks!

---
Index: linux-2.6/kernel/sched_fair.c
===================================================================
--- linux-2.6.orig/kernel/sched_fair.c
+++ linux-2.6/kernel/sched_fair.c
@@ -3446,6 +3446,9 @@ static void nohz_idle_balance(int this_c
break;
}

+ raw_spin_lock_irq(&rq->lock);
+ update_cpu_load(rq);
+ raw_spin_unlock_irq(&rq->lock);
rebalance_domains(balance_cpu, CPU_IDLE);

rq = cpu_rq(balance_cpu);
@@ -3518,40 +3521,12 @@ static void run_rebalance_domains(struct

rebalance_domains(this_cpu, idle);

-#ifdef CONFIG_NO_HZ
/*
- * If this cpu is the owner for idle load balancing, then do the
+ * If this cpu has a pending nohz_balance_kick, then do the
* balancing on behalf of the other idle cpus whose ticks are
* stopped.
*/
- if (this_rq->idle_at_tick &&
- atomic_read(&nohz.load_balancer) == this_cpu) {
- struct rq *rq;
- int balance_cpu;
-
- for_each_cpu(balance_cpu, nohz.cpu_mask) {
- if (balance_cpu == this_cpu)
- continue;
-
- /*
- * If this cpu gets work to do, stop the load balancing
- * work being done for other cpus. Next load
- * balancing owner will pick it up.
- */
- if (need_resched())
- break;
-
- rq = cpu_rq(balance_cpu);
- raw_spin_lock_irq(&rq->lock);
- update_cpu_load(rq);
- raw_spin_unlock_irq(&rq->lock);
- rebalance_domains(balance_cpu, CPU_IDLE);
-
- if (time_after(this_rq->next_balance, rq->next_balance))
- this_rq->next_balance = rq->next_balance;
- }
- }
-#endif
+ nohz_idle_balance(this_cpu, idle);
}

static inline int on_null_domain(int cpu)


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