Re: Calculating real cpu usage of Xen domains correctly!

[Anthony Liguori <aliguori@us.ibm.com>]

Ian Pratt wrote:

>I would expect it to return the cumulative total CPU time in ns that the
>domain has received.
>  
>
I think what's happening is that cpu_time is being updated even if the 
domain is blocked. Looking in xen/common/schedule.c:__enter_scheduler(), 
the update to cpu_time is still being updated even if SCHED_OP(do_block, 
prev) is invoked. Honestly, I don't understand this code that well but I 
made the following change:

--- schedule.c~ 2005-02-21 22:19:31.000000000 -0600
+++ schedule.c 2005-02-23 19:44:52.000000000 -0600
@@ -389,13 +389,15 @@
{
/* This check is needed to avoid a race condition. */
if ( event_pending(prev) )
+ {
clear_bit(EDF_BLOCKED, &prev->ed_flags);
- else
+ prev->cpu_time += now - prev->lastschd;
+ } else
SCHED_OP(do_block, prev);
+ } else {
+ prev->cpu_time += now - prev->lastschd;
}

- prev->cpu_time += now - prev->lastschd;
-
/* get policy-specific decision on scheduling... */
next_slice = ops.do_schedule(now);

And cpu_time contained what it should contain. Before, if I ran a simple 
infinite loop in a domU (while true; do true; done), it would report 
dom0 and domU both using 99% of the CPU.

Now it shows domU using 99% of the CPU and dom0 using 1%. This is 
probably the wrong way to fix this problem but hopefully it makes the 
right solution obvious to someone who knows this code better.

Regards,
Anthony Liguori

Signed-off-by: Anthony Liguori

>If it doesn't I'd regard it as a bug.
>
>Seperately, we should make sure we store a total cumulative time that
>each CPU has been executing domains (other than idle).
>
>Ian
>
>  
>



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