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SubjectRe: [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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