[dm-devel] Re: [PATCH] io-controller: Add io group reference handling for request

[Andrea Righi <righi andrea gmail com>]

On Fri, May 15, 2009 at 01:15:24PM +0800, Gui Jianfeng wrote:
> Vivek Goyal wrote:
> ...
> >  }
> > @@ -1462,20 +1462,27 @@ struct io_cgroup *get_iocg_from_bio(stru
> >  /*
> >   * Find the io group bio belongs to.
> >   * If "create" is set, io group is created if it is not already present.
> > + * If "curr" is set, io group is information is searched for current
> > + * task and not with the help of bio.
> > + *
> > + * FIXME: Can we assume that if bio is NULL then lookup group for current
> > + * task and not create extra function parameter ?
> >   *
> > - * Note: There is a narrow window of race where a group is being freed
> > - * by cgroup deletion path and some rq has slipped through in this group.
> > - * Fix it.
> >   */
> > -struct io_group *io_get_io_group_bio(struct request_queue *q, struct bio *bio,
> > -					int create)
> > +struct io_group *io_get_io_group(struct request_queue *q, struct bio *bio,
> > +					int create, int curr)
> 
>   Hi Vivek,
> 
>   IIUC we can get rid of curr, and just determine iog from bio. If bio is not NULL,
>   get iog from bio, otherwise get it from current task.

Consider also that get_cgroup_from_bio() is much more slow than
task_cgroup() and need to lock/unlock_page_cgroup() in
get_blkio_cgroup_id(), while task_cgroup() is rcu protected.

BTW another optimization could be to use the blkio-cgroup functionality
only for dirty pages and cut out some blkio_set_owner(). For all the
other cases IO always occurs in the same context of the current task,
and you can use task_cgroup().

However, this is true only for page cache pages, for IO generated by
anonymous pages (swap) you still need the page tracking functionality
both for reads and writes.

-Andrea

> 
> >  {
> >  	struct cgroup *cgroup;
> >  	struct io_group *iog;
> >  	struct elv_fq_data *efqd = &q->elevator->efqd;
> >  
> >  	rcu_read_lock();
> > -	cgroup = get_cgroup_from_bio(bio);
> > +
> > +	if (curr)
> > +		cgroup = task_cgroup(current, io_subsys_id);
> > +	else
> > +		cgroup = get_cgroup_from_bio(bio);
> > +
> >  	if (!cgroup) {
> >  		if (create)
> >  			iog = efqd->root_group;
> > @@ -1500,7 +1507,7 @@ out:
> >  	rcu_read_unlock();
> >  	return iog;
> >  }
> 
> -- 
> Regards
> Gui Jianfeng
>