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

[Vivek Goyal <vgoyal redhat com>]

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