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

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Message-ID: <20120904133528.GB13768@redhat.com>
Date:	Tue, 4 Sep 2012 09:35:29 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tao Ma <tm@....ma>
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 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.

Thanks
Vivek
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