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

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Date:	Tue, 16 Nov 2010 13:16:08 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
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>,
	Avi Kivity <avi@...hat.com>
Subject: Re: [RFC][PATCH v2 4/7] taskstats: Add per task steal time
 accounting

On Tue, 2010-11-16 at 09:51 +0100, Martin Schwidefsky wrote:
> 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.

Oh, you guys don't have a hypercall wrapper to exploit? Because from
what I heard from the kvm/xen/lguest people I gathered they could in
fact do something like I proposed.

In fact, kvm seems to already have these tracepoints: kvm_exit/kvm_entry
and it has a separate excplicit hypercall tracepoint as well:
kvm_hypercall.

Except that the per-task steal time gives you lot less detail, being
able to profile on vcpu exit/enter gives you a much more powerfull
performance tool. Aside from being able to measure the steal-time it
allows you to instantly find hypercalls (both explicit as well as
implicit), so you can also measure the hypercall induced steal-time as
well.

> 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. 

Making it configurable would definitely help the embedded people, not
sure about VIRT_CPU_ACCOUNTING though, I bet the x86 virt weird^Wpeople
would like it too -- if only to strive for feature parity if nothing
else :/

Its just that I'm not at all convinced its the best approach to solve
the problem posed, and once its committed we're stuck with it due to
ABI.

We should be very careful not to die a death of thousand cuts with all
this accounting madness, there's way too many weird-ass process
accounting junk that adds ABI constraints as it is.

I think its definitely worth investing extra time to implement these
tracepoints if at all possible on your architecture before committing
yourself to something like this.

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