linux-kernel - Re: [PATCH 0/4] Finer granularity and task/cgroup irq time accounting

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Message-ID: <20100824124726.GQ4684@balbir.in.ibm.com>
Date:	Tue, 24 Aug 2010 18:17:26 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Venkatesh Pallipadi <venki@...gle.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Menage <menage@...gle.com>, linux-kernel@...r.kernel.org,
	Paul Turner <pjt@...gle.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH 0/4] Finer granularity and task/cgroup irq time accounting

* Peter Zijlstra <peterz@...radead.org> [2010-08-24 13:53:55]:

> On Tue, 2010-08-24 at 17:08 +0530, Balbir Singh wrote:
> > 
> > The point is for containers it is more likely to give the right answer
> > and so on. Yes, the results are not 100% accurate. 
> 
> Consider one group heavily dirtying pages, it stuffs the IO queues full
> and gets blocked on IO completion. Since the CPU is then free to
> schedule something else we start running things from another group,
> those IO completions will come in while we run other group and get
> accounted to other group -- FAIL.
> 
> s/group/task/ etc..
> 
> That just really doesn't work, accounting async work, esp stuff that is
> not under software control it very tricky indeed.

Yes, we don't have sufficient context to charge the correct context. I
think openvz has some technology there, we will too when we have I/O
cgroups at a cgroup level, but the instances of such operations are
too many to accurately identify them all.

> 
> So what are you wanting to do, and why. Do you really need accounting
> madness?

I think Venki gave the answer in the posting

"There are usecases where reporting this time against task
or task groups or cgroups will be useful for user/administrator
in terms of resource planning and utilization charging"

I don't have any specific use cases, I was just reviewing the patchset
and trying to understand how to solve the problem.


-- 
	Three Cheers,
	Balbir
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