linux-kernel - Re: cpusets: randomize node rotor used in cpuset_mem_spread

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Message-Id: <20110415161831.12F8.A69D9226@jp.fujitsu.com>
Date:	Fri, 15 Apr 2011 16:18:45 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	kosaki.motohiro@...fujitsu.com,
	LKML <linux-kernel@...r.kernel.org>,
	Jack Steiner <steiner@....com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Paul Menage <menage@...gle.com>, Robin Holt <holt@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: cpusets: randomize node rotor used in cpuset_mem_spread_node()

Oops.
I should have look into !mempolicy part too.
I'm sorry.

> diff --git a/kernel/fork.c b/kernel/fork.c
> index e7548de..f59c686 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1119,6 +1119,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>   	}
>  	mpol_fix_fork_child_flag(p);
>  #endif
> +#ifdef CONFIG_CPUSETS
> +	p->cpuset_mem_spread_rotor = node_random(&p->mems_allowed);
> +	p->cpuset_slab_spread_rotor = node_random(&p->mems_allowed);
> +#endif

Michal, I think this should be

#ifdef CONFIG_CPUSETS
	if (cpuset_do_page_mem_spread())
		p->cpuset_mem_spread_rotor = node_random(&p->mems_allowed);
	if (cpuset_do_slab_mem_spread())
		p->cpuset_slab_spread_rotor = node_random(&p->mems_allowed);
#endif

because 99.999% people don't use cpuset's spread mem/slab feature and
get_random_int() isn't zero cost.

What do you think?



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