linux-kernel - Re: [PATCH 5/6] blkcg: move io_service_bytes and io_serviced stats into blkcg

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Message-ID: <20150626160142.GA24554@redhat.com>
Date:	Fri, 26 Jun 2015 12:01:42 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, avanzini.arianna@...il.com,
	kernel-team@...com
Subject: Re: [PATCH 5/6] blkcg: move io_service_bytes and io_serviced stats
 into blkcg_gq

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
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