Re: [PATCH V2] block/throttle: Add IO throttled information in blkio.throttle.

From: Tao Ma
Date: Tue Sep 04 2012 - 10:12:52 EST


On 09/04/2012 09:35 PM, Vivek Goyal wrote:
> On Fri, Aug 31, 2012 at 01:15:09PM +0800, Tao Ma wrote:
>
> [..]
>> diff --git a/block/blk-throttle.c b/block/blk-throttle.c
>> index 1588c2d..9317d71 100644
>> --- a/block/blk-throttle.c
>> +++ b/block/blk-throttle.c
>> @@ -46,6 +46,8 @@ struct tg_stats_cpu {
>> struct blkg_rwstat service_bytes;
>> /* total IOs serviced, post merge */
>> struct blkg_rwstat serviced;
>> + /* total IOs queued, not submitted to the underlying device. */
>> + struct blkg_rwstat io_queued;
>> };
>
> Couple of questions.
>
> - blkg_rwstat is "unsigned" and io_queued can go negative too (Because
> throttled bio can very well be dispatched from other cpu from a worker
> thread). So is it a good idea to represent a negative number with
> unsingned type?
>
> - As this stat is per cpu, a reader might very well see negative (or a
> huge unsigned value) as number of io_queued. Not sure if that is acceptable.
> How would user space come to know whether it is a valid value or not. I
> thought per cpu stats are good for continuously increasing values but
> not necessarily for values which can increase as well as decrease.
You are right. So I should just use throtl_grp->nr_queued to display the
total numbers of ios being throttled and I guess a rcu_read_lock should
be enough for me to access that data.

Thanks for the review.

Thanks
Tao
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