Re: [PATCH 13/23] KVM: PPC: Book3S HV: Accumulate timing information for real-mode code

[Paul Mackerras <paulus@samba.org>]

On Fri, Mar 20, 2015 at 12:15:15PM +0100, Alexander Graf wrote:
> 
> 
> On 20.03.15 10:39, Paul Mackerras wrote:
> > This reads the timebase at various points in the real-mode guest
> > entry/exit code and uses that to accumulate total, minimum and
> > maximum time spent in those parts of the code.  Currently these
> > times are accumulated per vcpu in 5 parts of the code:
> > 
> > * rm_entry - time taken from the start of kvmppc_hv_entry() until
> >   just before entering the guest.
> > * rm_intr - time from when we take a hypervisor interrupt in the
> >   guest until we either re-enter the guest or decide to exit to the
> >   host.  This includes time spent handling hcalls in real mode.
> > * rm_exit - time from when we decide to exit the guest until the
> >   return from kvmppc_hv_entry().
> > * guest - time spend in the guest
> > * cede - time spent napping in real mode due to an H_CEDE hcall
> >   while other threads in the same vcore are active.
> > 
> > These times are exposed in debugfs in a directory per vcpu that
> > contains a file called "timings".  This file contains one line for
> > each of the 5 timings above, with the name followed by a colon and
> > 4 numbers, which are the count (number of times the code has been
> > executed), the total time, the minimum time, and the maximum time,
> > all in nanoseconds.
> > 
> > Signed-off-by: Paul Mackerras <paulus@samba.org>
> 
> Have you measure the additional overhead this brings?

I haven't - in fact I did this patch so I could measure the overhead
or improvement from other changes I did, but it doesn't measure its
own overhead, of course.  I guess I need a workload that does a
defined number of guest entries and exits and measure how fast it runs
with and without the patch (maybe something like H_SET_MODE in a
loop).  I'll figure something out and post the results.  

Paul.