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

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Message-ID: <20100720095546.2f899e04@mschwide.boeblingen.de.ibm.com>
Date:	Tue, 20 Jul 2010 09:55:46 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	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

On Mon, 19 Jul 2010 16:57:11 -0700
Venkatesh Pallipadi <venki@...gle.com> wrote:

> Currently, the softirq and hardirq time reporting is only done at the
> CPU level. 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. Also, as the
> accoounting is already done at the CPU level, reporting the same at
> the task level does not add any significant computational overhead
> other than task level storage (patch 1).

I never understood why the softirq and hardirq time gets accounted to a
task at all. Why is it that the poor task that is running gets charged
with the cpu time of an interrupt that has nothing to do with the task?
I consider this to be a bug, and now this gets formalized in the
taskstats interface? Imho not a good idea.

> The softirq/hardirq statistics commonly done based on tick based sampling.
> Though some archs have CONFIG_VIRT_CPU_ACCOUNTING based fine granularity
> accounting. Having similar mechanism to get fine granularity accounting
> on x86 will be a major challenge, given the state of TSC reliability
> on various platforms and also the overhead it may add in common paths
> like syscall entry exit.
> 
> An alternative is to have a generic (sched_clock based) and configurable
> fine-granularity accounting of si and hi time which can be reported
> over the /proc/<pid>/stat API (patch 2).

To get fine granular accounting for interrupts you need to do a
sched_clock call on irq entry and another one on irq exit. Isn't that
too expensive on a x86 system? (I do think this is a good idea but
still there is the worry about the overhead).

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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