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

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Message-ID: <20130102163010.GC11220@mtj.dyndns.org>
Date:	Wed, 2 Jan 2013 11:30:10 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
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

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.

> > +	cfqg_stats_merge(&parent->dead_stats, &cfqg->stats);
> > +	cfqg_stats_merge(&parent->dead_stats, &cfqg->dead_stats);
> > +	cfqg_stats_reset(&cfqg->stats);
> > +	cfqg_stats_reset(&cfqg->dead_stats);
> 
> Anyway group will be marked offline and later freed. So resetting stats
> might not be required.

Yeah, it isn't strictly necessary.  I tried to transfer the residual
IOs between offline and release first so the resetting.  Kinda like
them there tho.

> 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?

Thanks.

-- 
tejun
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