linux-kernel - Re: [PATCH] sched/fair: Restore env status before goto redo in load

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Message-ID: <20150327153023.GG23123@twins.programming.kicks-ass.net>
Date:	Fri, 27 Mar 2015 16:30:23 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Xunlei Pang <xlpang@....com>
Cc:	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Juri Lelli <juri.lelli@...il.com>,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: Re: [PATCH] sched/fair: Restore env status before goto redo in
 load_balance()

On Wed, Mar 18, 2015 at 02:31:02PM +0800, Xunlei Pang wrote:
> From: Xunlei Pang <pang.xunlei@...aro.org>
> 
> In load_balance(), some members of lb_env will be assigned with
> new values in LBF_DST_PINNED case. But lb_env::flags may still
> retain LBF_ALL_PINNED if no proper tasks were found afterwards
> due to another balance, task affinity changing, etc, which can
> really happen because busiest rq lock has already been released.

Sure..

> This is wrong, for example with env.dst_cpu assigned new_dst_cpu
> when going back to "redo" label, it may cause should_we_balance()
> to return false which is unreasonable.

Why? You've got a very unlikely, very hard case, its unlikely that
anything we do will substantially improve the situation, but you make
the code uglier for it.

> This patch restores proper status of env before "goto redo", and
> improves "out_all_pinned" and "out_one_pinned" labels.

That doesn't even begin to explain half of what the patch does.

> @@ -6977,12 +6978,19 @@ more_balance:
>  		/* All tasks on this runqueue were pinned by CPU affinity */
>  		if (unlikely(env.flags & LBF_ALL_PINNED)) {
>  			cpumask_clear_cpu(cpu_of(busiest), cpus);
> -			if (!cpumask_empty(cpus)) {
> -				env.loop = 0;
> -				env.loop_break = sched_nr_migrate_break;
> -				goto redo;
> +			if (env.new_dst_cpu != -1) {

I really don't get this, how can this not be?

> +				env.new_dst_cpu = -1;
> +				cpumask_or(cpus, cpus,
> +					sched_group_cpus(sd->groups));
> +				cpumask_and(cpus, cpus, cpu_active_mask);

More unexplained magic, why is this right?

The rest of the patch isn't much better.
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