linux-kernel - Re: [RFC][PATCH v2 4/7] taskstats: Add per task steal time accounting

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Message-ID: <20101116095101.5d86d1e5@mschwide.boeblingen.de.ibm.com>
Date:	Tue, 16 Nov 2010 09:51:01 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	Shailabh Nagar <nagar1234@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...e.hu>, Oleg Nesterov <oleg@...hat.com>,
	John stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	"jeremy.fitzhardinge" <jeremy.fitzhardinge@...rix.com>
Subject: Re: [RFC][PATCH v2 4/7] taskstats: Add per task steal time
 accounting

On Mon, 15 Nov 2010 19:08:44 +0100
Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Mon, 2010-11-15 at 18:59 +0100, Martin Schwidefsky wrote:
> > Steal time per task is at least good for performance problem analysis.
> > Sometimes knowing what is not the cause of a performance problem can help you
> > tremendously. If a task is slow and has no steal time, well then the hypervisor
> > is likely not the culprit. On the other hand if you do see lots of steal time
> > for a task while the rest of the system doesn't cause any steal time can tell
> > you something as well. That task might hit a specific function which causes
> > hypervisor overhead. The usefulness depends on the situation, it is another
> > data point which may or may not help you. 
> 
> If performance analysis is the only reason, why not add a tracepoint on
> vcpu enter that reports the duration the vcpu was out for and use perf
> to gather said data? It can tell you what process was running and what
> instruction it was at when the vcpu went away.
> 
> No need to add 40 bytes per task for that.

Which vcpu enter? We usually have z/VM as our hypervisor and want to be able
to do performance analysis with the data we gather inside the guest. There
is no vcpu enter we could put a tracepoint on. We would have to put tracepoints
on every possible interaction point with z/VM to get this data. To me it seems
a lot simpler to add the per-task steal time.

And if it is really the additional 40 bytes on x86 that bother you so much,
we could put them behind #ifdef CONFIG_VIRT_CPU_ACCOUNTING. There already
is one in the task_struct for prev_utime and prev_stime. 

-- 
blue skies,
   Martin.

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

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