linux-kernel - Re: [PATCH 23/24] cfq-iosched: collect stats from dead cfqgs

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Date:	Wed, 2 Jan 2013 11:44:15 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	lizefan@...wei.com, axboe@...nel.dk,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, ctalbott@...gle.com, rni@...gle.com
Subject: Re: [PATCH 23/24] cfq-iosched: collect stats from dead cfqgs

On Wed, Jan 02, 2013 at 11:30:10AM -0500, Tejun Heo wrote:
> Hey, Vivek.
> 
> On Wed, Jan 02, 2013 at 11:24:15AM -0500, Vivek Goyal wrote:
> > On Fri, Dec 28, 2012 at 12:35:45PM -0800, Tejun Heo wrote:
> > > To support hierarchical stats, it's necessary to remember stats from
> > > dead children.  Add cfqg->dead_stats and make a dying cfqg transfer
> > > its stats to the parent's dead-stats.
> > 
> > Hi Tejun,
> > 
> > Why not directly transfer stats to cfqg->stats. IOW, what's the advantage
> > of maintaining dead_stats separately.
> 
> Backward compatibility?  The existing stat cgroupfs files expect to
> see non-recursive stats.

Oh yes. Missed that. So dead_stats makes sense.

[..]
> > In fact if we have a realiable way of resetting status then online/offline
> > infrastructure might not be required? I think per cpu stats will be a
> > problem though and that's why we probably require logic to online/offline
> > the group?
> 
> Hmmm?  What do you mean?

I mean if we had a reliable way of resetting stats after transferring
then we would not need to keep a track of whether group is online/offline.
We could add everything and adding zero will not change anything. In fact
it will also take care of residual IO (IO which happened after transfer
of stats).

Or I missed the real reason of why do we have group online/offline
infrastructure.

Thanks
Vivek
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