Re: [Xen-devel] Calculating real cpu usage of Xen domains correctly!

[John L Griffin <jlg@xxxxxxxxxx>]

Oops, I indeed misread your patch -- my mistake.

However, I'm concerned that we're missing something bigger.  This is my 
understanding of what the BLOCKED flag (and the surrounding code) means:

1. The guest OS calls HYPERVISOR_block() (thus setting the BLOCKED flag) 
whenever it wants to yield the processor because it's waiting for an 
event.

2. This blocking can happen anytime -- including after the guest OS has 
been running for quite some time.

3. All the "event_pending(prev)" check in __enter_scheduler() is for is to 
say "whoops, an event arrived in the time between when the guest OS 
blocked & right now, so I should clear the BLOCKED flag."  This is so the 
domain can be rescheduled at the scheduler's earliest discretion (possibly 
immediately).

If these are true, then the original code was correct -- "prev->cpu_time" 
should be updated during any call to the __enter_scheduler() function, 
regardless of the state of the BLOCKED flag.

Which makes me wonder if something is seriously misbehaving to cause the 
weird CPU usage totals you're seeing -- like a yield()ed or block()ed 
domain improperly getting rescheduled immediately, or an improper 
modification of the prev->lastschd counter, or the "if (prev == next)" 
optimization [later in __enter_scheduler()] leaves out some crucial 
accounting, or...?

JLG



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