linux-kernel - Re: [patch] restore sched_exec load balance heuristics

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Date:	Mon, 10 Nov 2008 10:29:37 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ken Chen <kenchen@...gle.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [patch] restore sched_exec load balance heuristics

* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

>  void sched_exec(void)
>  {
>  	int new_cpu, this_cpu = get_cpu();
> -	new_cpu = sched_balance_self(this_cpu, SD_BALANCE_EXEC);
> +	struct task_group *tg;
> +	long weight, eload;
> +
> +	tg = task_group(current);
> +	weight = current->se.load.weight;
> +	eload = -effective_load(tg, this_cpu, -weight, -weight);
> +
> +	new_cpu = sched_balance_self(this_cpu, SD_BALANCE_EXEC, eload);

okay, i think this will work.

it feels somewhat backwards though on a conceptual level.

There's nothing particularly special about exec-balancing: the load 
picture is in equilibrium - it is in essence a rebalancing pass done 
not in the scheduler tick but in a special place in the middle of 
exec() where the old-task / new-task cross section is at a minimum 
level.

_fork_ balancing is what is special: there we'll get a new context so 
we have to take the new load into account. It's a bit like wakeup 
balancing. (just done before the new task is truly woken up)

OTOH, triggering the regular busy-balance at exec() time isnt totally 
straightforward either: the 'old' task is the current task so it 
cannot be balanced away. We have to trigger all the active-migration 
logic - which again makes exec() balancing special.

So maybe this patch is the best solution after all. Ken, does it do 
the trick for your workload, when applied against v2.6.28-rc4?

You might even try to confirm that your testcase still works fine even 
if you elevate the load average with +1.0 on every cpu by starting 
infinite CPU eater loops on every CPU, via this bash oneliner:

  for ((i=0;i<2;i++)); do while :; do :; done & done

(change the '2' to '4' if you test this on a quad, not on a dual-core 
box)

the desired behavior would be for your "exec hopper" testcase to not 
hop between cpus, but to stick the same CPU most of the time.

	Ingo
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