Re: [Xen-devel] [PATCH] Add a timer mode that disables pending missed ticks

[Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>]

Keir,

Attached is a fix for pt_process_missed_ticks().
Without this fix, systems run very erratically and some
guests panic on boot complaining that timer interrupts
are not working. As you can imagine.

Also, I have some longer term measurements of the
accuracy of the sync and async methods.
The hardware is an eight cpu AMD box. Two eight
vcpu guests, rh4u4-64 and sles9sp3-64.
All vcpus running loads.

Method   Test duration          Clock errors

SYNC       56000 sec         6.4, 6.7 sec   (.012%)
ASYNC      52000 sec         13, 19 sec     (.036%)

More testing should be done to validate the significance
of this difference.

regards,
Dave


Dave Winchell wrote:

> Keir Fraser wrote:
>
> > On 3/11/07 21:17, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:
> >
> >  
> >
> > > Thanks for applying the fixes in the last submit.
> > > In moving the test for no_missed_tick_accounting into
> > > pt_process_missed_ticks()
> > > you forgot to add the scheduling part.
> > >   
> >
> > Actually it was deliberate, but clearly it was one code 
> > simplification too
> > far: thanks for spotting it! I'll go the async route, but we do need 
> > to set
> > pending_intr_nr to 1. We can't leave that out -- the point of the async
> > route is to send a tick to the vcpu immediately, since it hasn't had 
> > one for
> > more than a tick period. If we wait for the timeout to do that then 
> > we have
> > to wait a whole extra period after the vcpu is re-scheduled.
> >
> > Attached is my proposed patch. I think it's quite neat. :-)
> >  
> It looks good to me.
>
> thanks,
> Dave
>
> > -- Keir
> >
> >  

diff -r dfe9c0c10a2c xen/arch/x86/hvm/vpt.c
--- a/xen/arch/x86/hvm/vpt.c    Mon Nov 05 13:23:55 2007 +0000
+++ b/xen/arch/x86/hvm/vpt.c    Wed Nov 07 08:55:45 2007 -0500
@@ -59,7 +59,7 @@ static void pt_process_missed_ticks(stru
     if ( mode_is(pt->vcpu->domain, no_missed_tick_accounting) )
     {
         pt->pending_intr_nr = 1;
-        pt->scheduled = now + pt->scheduled;
+        pt->scheduled = now + pt->period;
     }
     else
     {

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