Re: [PATCH 10/19] io-conroller: Prepare elevator layer for single queue schedulers

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Jun 12, 2009 at 08:37:25AM +0800, Gui Jianfeng wrote:
> Vivek Goyal wrote:
> > On Thu, Jun 11, 2009 at 04:10:55PM +0800, Gui Jianfeng wrote:
> >> Vivek Goyal wrote:
> >> ...
> >>>  
> >>>  /*
> >>> @@ -1296,6 +1302,13 @@ void io_group_chain_link(struct request_queue *q, void *key,
> >>>  		iog = io_cgroup_lookup_group(iocg, key);
> >>>  		io_group_set_parent(prev, iog);
> >>>  	}
> >>> +
> >>> +	if (unlikely(efqd->only_root_group))
> >>> +		/*
> >>> +		 * TODO: Take care of force expiry of existing queue before
> >>> +		 * new queue is queued.
> >>> +		 */
> >>> +		efqd->only_root_group = 0;
> >>   Hi Vivek,
> >>
> >>   This flag isn't set back when all child groups go away. Am i missing something?
> >>   BTW, why not just determine "only root group" by cgroup itself? Although there might be 
> >>   some impact if cgroup is built but no request goes into it. but i think this isn't a big
> >>   deal. How about the following patch?
> >>
> > 
> > Hi Gui,
> > 
> > Determining if there are any children present or not from cgroup sounds like
> > a good idea. Just that cost of the operation now has increased. I am not
> > sure how significant that is. But for the time being we can stick to your
> > implementation. 
> 
>   I don't introduce any extra locking here, so i guess the cost is very limited.
> 
> > 
> > One question inline below.
> > 
> >> Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
> >> ---
> >>  block/elevator-fq.c |   21 ++++++++++-----------
> >>  block/elevator-fq.h |    1 -
> >>  2 files changed, 10 insertions(+), 12 deletions(-)
> >>
> >> diff --git a/block/elevator-fq.c b/block/elevator-fq.c
> >> index a516dce..f33155c 100644
> >> --- a/block/elevator-fq.c
> >> +++ b/block/elevator-fq.c
> >> @@ -76,7 +76,6 @@ void elv_del_ioq_busy(struct elevator_queue *e, struct io_queue *ioq,
> >>  void elv_activate_ioq(struct io_queue *ioq, int add_front);
> >>  void elv_deactivate_ioq(struct elv_fq_data *efqd, struct io_queue *ioq,
> >>  					int requeue);
> >> -
> >>  static int bfq_update_next_active(struct io_sched_data *sd)
> >>  {
> >>  	struct io_group *iog;
> >> @@ -1131,6 +1130,14 @@ struct io_cgroup io_root_cgroup = {
> >>  	.ioprio_class = IO_DEFAULT_GRP_CLASS,
> >>  };
> >>  
> >> +static int is_only_root_group(void)
> >> +{
> >> +	if (list_empty(&io_root_cgroup.css.cgroup->children))
> >> +		return 1;
> >> +
> > 
> > Do we need some kind of locking here to make sure cgroup->children list is not
> > being modified?
> 
>   Even if the children list is modified, i think this is not a big problem, and just
>   get a mis-judgement for one time. Anyway, children list changing is rarely happens.
>   For this corner case, IMHO, there's no need to introduce the cgroup lock, for this 
>   lock costs too much.
> 

Ok. Thanks. I will include these changes in next posting.

Vivek