Re: [Qemu-devel] User space vs kernel space instructions distribution.

[Christopher Covington <cov@codeaurora.org>]

On 07/14/2015 04:45 AM, Peter Maydell wrote:
> On 14 July 2015 at 09:32, Shlomo Pongratz <shlomopongratz@gmail.com> wrote:
>> Hi,
>>
>> I'm running aarm64 QEMU and I'm counting the number of instructions which
>> "belong" to user space vs kernel space. My measurements shows that 99
>> percent of instructions are in kernel space.

How are you counting? Instrumenting QEMU?

>> I've used both the address of the instructions and the EL just to be sure. I
>> also added an option to disable block chaining just to make sure that all
>> the instructions in every TB  is counted.
>> When examining some kernel's instructions against the objdump of the kernel
>> I've noticed that most of them are in interrupts/timers area.
>>
>> Does this make sense?
>> Did someone also encountered this phenomenon?
> 
> Depends entirely on your workload, obviously. If the system only
> boots then most instructions will be in kernel space. If the system
> is only sitting idle then it'll just be executing the kernel space
> idle loop. If you're measuring solely the section of time where
> a userspace program is doing real work with the CPU and you're
> still seeing a 99% figure then the obvious conclusion would be that
> your measurement approach is wrong...
> 
> If your measurement instrumentation is intrusive and is significantly
> slowing down QEMU then you'll naturally find that the guest spends
> more time in timer interrupt handling, because the timer interrupts
> come in in real time, and you've just effectively reduced the speed
> of your CPU, so it can get less useful work done between timer
> interrupts.

I find such behavior undesirable. As best I understand, -icount exists to
provide an alternative, although it may have bugs. I've tinkered with -icount
a little, but I have yet to come up with useful tests to verify its correct
behavior. If anyone has suggestions for how to test it, I'd be eager to hear.

Chris

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