linux-kernel - Re: [PATCH v2 2/3] sched/fair: Move hot load

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Message-ID: <xm26io4f4dsd.fsf@sword-of-the-dawn.mtv.corp.google.com>
Date:	Thu, 03 Dec 2015 09:56:02 -0800
From:	bsegall@...gle.com
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Waiman Long <Waiman.Long@....com>, Ingo Molnar <mingo@...hat.com>,
	linux-kernel@...r.kernel.org, Yuyang Du <yuyang.du@...el.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline

Peter Zijlstra <peterz@...radead.org> writes:

> I made this:
>
> ---
> Subject: sched/fair: Move hot load_avg into its own cacheline
> From: Waiman Long <Waiman.Long@....com>
> Date: Wed, 2 Dec 2015 13:41:49 -0500
>
[...]
> @@ -7402,11 +7405,12 @@ void __init sched_init(void)
>  #endif /* CONFIG_RT_GROUP_SCHED */
>  
>  #ifdef CONFIG_CGROUP_SCHED
> +	task_group_cache = KMEM_CACHE(task_group, 0);
> +
>  	list_add(&root_task_group.list, &task_groups);
>  	INIT_LIST_HEAD(&root_task_group.children);
>  	INIT_LIST_HEAD(&root_task_group.siblings);
>  	autogroup_init(&init_task);
> -
>  #endif /* CONFIG_CGROUP_SCHED */
>  
>  	for_each_possible_cpu(i) {
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -248,7 +248,12 @@ struct task_group {
>  	unsigned long shares;
>  
>  #ifdef	CONFIG_SMP
> -	atomic_long_t load_avg;
> +	/*
> +	 * load_avg can be heavily contended at clock tick time, so put
> +	 * it in its own cacheline separated from the fields above which
> +	 * will also be accessed at each tick.
> +	 */
> +	atomic_long_t load_avg ____cacheline_aligned;
>  #endif
>  #endif
>  

This loses the cacheline-alignment for task_group, is that ok?
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