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

[Dave Winchell <dwinchell@virtualiron.com>]

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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@virtualiron.com> 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
>>
>>  
>>
>


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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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