Re: [PATCH 0 of 6] PowerPC KVM patches for 2.6.29

[Hollis Blanchard <hollisb@us.ibm.com>]

On Wed, 2008-12-03 at 13:22 -0600, Hollis Blanchard wrote:
> On Wed, 2008-12-03 at 19:06 +0200, Avi Kivity wrote:
> > Hollis Blanchard wrote:
> > >
> > >> I'm not thrilled about the private exit timing statistics gathering, 
> > >> hopefully it can be morphed into the more general framework.
> > >>     
> > >
> > > Is there anything in particular you have in mind? I think it could be
> > > generally useful, but since x86 has hardware support for performance
> > > monitoring, oprofile will already give you more accurate information. Of
> > > course, I don't think you could extract standard deviation from an
> > > oprofile report, and that has been very useful for us because it can
> > > tell us e.g. 99% of instruction emulation is handled in the minimum
> > > amount of time, but 1% takes hundreds of ms.
> > >   
> > 
> > kvmtrace is basically a bunch of trace_marker()s sprinkled around the 
> > code.  the marker infrastructure allow you to attach a callback to the 
> > markers, where you can do the accounting.  The nice thing it can be 
> > switched off at runtime, being replaced by a nop so 
> > compiled-in-but-disabled overhead is very low.
> 
> However, we grab timestamps extremely early and late in the exit
> handlers, in contexts from which it is not safe to call C code. This is
> really important because we need to be able to measure the time spent in
> the interrupt handler assembly. For x86 that may be confined to a simple
> inline asm statement, but the code in
> arch/powerpc/kvm/booke_interrupts.S is non-trivial and worth measuring.

Well, I suppose we could save the timestamp to the vcpu, and call
kvmtrace code later using that data. I'll stop now and let Christian
comment when he's back. :)

-- 
Hollis Blanchard
IBM Linux Technology Center