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SubjectRe: High priority threads causing severe CPU load imbalances
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From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: High priority threads causing severe CPU load imbalances
Date: Thu, 08 Apr 2010 18:15:44 +0200

> On Tue, 2010-04-06 at 22:05 +0530, Suresh Jayaraman wrote:
>> Perhaps there is a chance that with more CPUs, different number of high
>> priority threads the problem could get worser as I mentioned above..?
>
> One thing that could be happening (triggered by what Igawa-san said,
> although his case is more complicated by involving the cgroup stuff) is
> that f_b_g() ends up selecting a group that contains these niced tasks
> and then f_b_q() will not find a suitable source queue because all of
> them will have but a single runnable task on it and hence we simply
> bail.
>
> We'd somehow have to teach update_*_lb_stats() not to consider groups
> where nr_running <= nr_cpus. I don't currently have a patch for that,
> but I think that is the direction you might need to look in.

I made a patch for my understanding the load_balance()'s behavior.
This patch reduced CPU load imbalances but not perfect.
---
Cpu0 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu1 : 90.1%us, 0.0%sy, 0.0%ni, 9.9%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu2 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu3 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu4 : 98.7%us, 0.3%sy, 0.0%ni, 1.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu5 : 96.1%us, 1.0%sy, 0.0%ni, 3.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Cpu6 : 99.0%us, 0.7%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.3%si, 0.0%st
Cpu7 :100.0%us, 0.0%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 8032460k total, 807628k used, 7224832k free, 30692k buffers
Swap: 0k total, 0k used, 0k free, 347308k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ P COMMAND
9872 root 20 0 66128 632 268 R 99 0.0 0:13.69 4 bash
9876 root 20 0 66128 632 268 R 99 0.0 0:10.31 2 bash
9877 root 20 0 66128 632 268 R 99 0.0 0:10.79 3 bash
9871 root 20 0 66128 632 268 R 99 0.0 0:13.70 0 bash
9873 root 20 0 66128 632 268 R 99 0.0 0:13.68 1 bash
9874 root 20 0 66128 632 268 R 98 0.0 0:10.00 6 bash
9875 root 20 0 66128 632 268 R 92 0.0 0:11.22 4 bash
9878 root 20 0 66128 632 268 R 91 0.0 0:10.03 7 bash
---
Also, this patch caused ping-pong load balances..
This patch is regards the sched_group as a idle sched_group
if local sched_group's cpu is CPU_IDLE.

But the state is not stable because active_load_balance() runs at this situation IIUC.


I'll investigate more.

===
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 5a5ea2c..806be90 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -2418,6 +2418,7 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
int i;
unsigned int balance_cpu = -1, first_idle_cpu = 0;
unsigned long avg_load_per_task = 0;
+ int idle_group = 0;

if (local_group)
balance_cpu = group_first_cpu(group);
@@ -2440,6 +2441,12 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
}

load = target_load(i, load_idx);
+ /* This group is idle if it has a idle cpu. */
+ if (idle == CPU_IDLE) {
+ idle_group = 1;
+ sgs->group_load = 0;
+ sgs->sum_weighted_load = 0;
+ }
} else {
load = source_load(i, load_idx);
if (load > max_cpu_load)
@@ -2451,6 +2458,10 @@ static inline void update_sg_lb_stats(struct sched_domain *sd,
sgs->group_load += load;
sgs->sum_nr_running += rq->nr_running;
sgs->sum_weighted_load += weighted_cpuload(i);
+ if (!idle_group) {
+ sgs->group_load += load;
+ sgs->sum_weighted_load += weighted_cpuload(i);
+ }

}

===

Thanks.
--
Masayuki Igawa


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