linux-kernel - Re: [PATCH 07/18] io-controller: Export disk time used and nr sectors dipatched through cgroups

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Message-ID: <20090513145127.GB7696@redhat.com>
Date:	Wed, 13 May 2009 10:51:27 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Gui Jianfeng <guijianfeng@...fujitsu.com>
Cc:	nauman@...gle.com, dpshah@...gle.com, lizf@...fujitsu.com,
	mikew@...gle.com, fchecconi@...il.com, paolo.valente@...more.it,
	jens.axboe@...cle.com, ryov@...inux.co.jp, fernando@....ntt.co.jp,
	s-uchida@...jp.nec.com, taka@...inux.co.jp, jmoyer@...hat.com,
	dhaval@...ux.vnet.ibm.com, balbir@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, righi.andrea@...il.com,
	agk@...hat.com, dm-devel@...hat.com, snitzer@...hat.com,
	m-ikeda@...jp.nec.com, akpm@...ux-foundation.org
Subject: Re: [PATCH 07/18] io-controller: Export disk time used and nr
	sectors dipatched through cgroups

On Wed, May 13, 2009 at 10:39:07AM +0800, Gui Jianfeng wrote:
> Vivek Goyal wrote:
> ...
> >  
> > +/*
> > + * traverse through all the io_groups associated with this cgroup and calculate
> > + * the aggr disk time received by all the groups on respective disks.
> > + */
> > +static u64 calculate_aggr_disk_time(struct io_cgroup *iocg)
> > +{
> > +	struct io_group *iog;
> > +	struct hlist_node *n;
> > +	u64 disk_time = 0;
> > +
> > +	rcu_read_lock();
> 
>   This function is in slow-path, so no need to call rcu_read_lock(), just need to ensure
>   that the caller already holds the iocg->lock.
> 

Or can we get rid of requirement of iocg_lock here and just read the io
group data under rcu read lock? Actually I am wondering why do we require
an iocg_lock here. We are not modifying the rcu protected list. We are
just traversing through it and reading the data.

Thanks
Vivek

> > +	hlist_for_each_entry_rcu(iog, n, &iocg->group_data, group_node) {
> > +		/*
> > +		 * There might be groups which are not functional and
> > +		 * waiting to be reclaimed upon cgoup deletion.
> > +		 */
> > +		if (rcu_dereference(iog->key))
> > +			disk_time += iog->entity.total_service;
> > +	}
> > +	rcu_read_unlock();
> > +
> > +	return disk_time;
> > +}
> > +
> 
> -- 
> Regards
> Gui Jianfeng
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