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

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Message-ID: <50460C61.6050705@tao.ma>
Date:	Tue, 04 Sep 2012 22:12:49 +0800
From:	Tao Ma <tm@....ma>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH V2] block/throttle: Add IO throttled information in blkio.throttle.

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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