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


--
Regards
Gui Jianfeng



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