Re: [PATCH 5/6] blkcg: move io_service_bytes and io_serviced stats into blkcg_gq

[Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Thu, Jun 25, 2015 at 05:38:56PM -0400, Tejun Heo wrote:
> Currently, both cfq-iosched and blk-throttle keep track of
> io_service_bytes and io_serviced stats.  While keeping track of them
> separately may be useful during development, it doesn't make much
> sense otherwise.  Also, blkcg stats are collected at different points
> and ignoring blk_do_io_stat() which can lead to subtly different
> results which are more confusing than helpful.
> 
> This patch adds ->stat_bytes and ->stat_ios to blkg (blkcg_gq),
> removes the counterparts from cfq-iosched and blk-throttle and let
> them print from the common blkg counters.
> 
> The outputs are still filtered by whether the policy has
> blkg_policy_data on a given blkg, so cfq's output won't show up if it
> has never been used for a given blkg.  The only times when the outputs
> would differ significantly are when policies are attached on the fly
> or elevators are switched back and forth.  Those are quite exceptional
> operations and I don't think they warrant keeping separate counters.
> 

Hi Tejun,

I have some questions and concerns.

- Previously throttling policy was calculating number of IOs in terms of
  number of bios while CFQ was calculating it in terms of number of
  requests. I think this will be a behavior change now? IIUC, now even
  throttling policy will report number of requests and number of bios.

- Looks like completion and stat is now lined to request. I am wondering
  what will happen to bio based targets. One can put a throttling policy
  on any of the stacked dm devices. I suspect we will be broken there?

- Can you please also update the blkio-controller.txt. Specifically
  blkio.throttle.io_serviced.

Thanks
Vivek