linux-kernel - Re: [PATCH v3 09/22] sched: compute runnable load avg in cpu_load and cpu_avg_load_per

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Date:	Tue, 08 Jan 2013 22:27:05 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>, namhyung@...nel.org,
	Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	preeti@...ux.vnet.ibm.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 09/22] sched: compute runnable load avg in cpu_load
 and cpu_avg_load_per_task

On 01/07/2013 02:31 AM, Linus Torvalds wrote:
> On Sat, Jan 5, 2013 at 11:54 PM, Alex Shi <alex.shi@...el.com> wrote:
>>
>> I just looked into the aim9 benchmark, in this case it forks 2000 tasks,
>> after all tasks ready, aim9 give a signal than all tasks burst waking up
>> and run until all finished.
>> Since each of tasks are finished very quickly, a imbalanced empty cpu
>> may goes to sleep till a regular balancing give it some new tasks. That
>> causes the performance dropping. cause more idle entering.
> 
> Sounds like for AIM (and possibly for other really bursty loads), we
> might want to do some load-balancing at wakeup time by *just* looking
> at the number of running tasks, rather than at the load average. Hmm?

Millions thanks for your suggestions! :)

It's worth to try use instant load -- nr_running in waking balancing, I
will try this. but in this case, I tried to print sleeping tasks by
print_task() in sched/debug.c. Find the 2000 tasks were forked on just 2
LCPUs which in different cpu sockets whenever with/without this load avg
patch.

So, I am wondering if it's worth to consider the sleeping tasks' load in
fork/wake balancing. Does anyone consider this in history?

===
print_task(struct seq_file *m, struct rq *rq, struct task_struct *p)
 {
        if (rq->curr == p)
                SEQ_printf(m, "R");
+       else if (!p->on_rq)
+               SEQ_printf(m, "S");
        else
                SEQ_printf(m, " ");
...
@@ -166,13 +170,14 @@ static void print_rq(struct seq_file *m, struct rq
*rq, int rq_cpu)
        read_lock_irqsave(&tasklist_lock, flags);

        do_each_thread(g, p) {
-               if (!p->on_rq || task_cpu(p) != rq_cpu)
+               if (task_cpu(p) != rq_cpu)
                        continue;
===

> 
> The load average is fundamentally always going to run behind a bit,
> and while you want to use it for long-term balancing, a short-term you
> might want to do just a "if we have a huge amount of runnable
> processes, do a load balancing *now*". Where "huge amount" should
> probably be relative to the long-term load balancing (ie comparing the
> number of runnable processes on this CPU right *now* with the load
> average over the last second or so would show a clear spike, and a
> reason for quick action).

Many thanks for suggestion!
Will try it. :)
> 
>            Linus
> 
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