Re: [PATCH][retry 1] Add support for Pause Filtering to AMD SVM

Avi Kivity
2009-05-11T15:05:23Z

Peter Zijlstra wrote:
> On Mon, 2009-05-11 at 17:24 +0300, Avi Kivity wrote:
>
>
>>> I.e. this is a somewhat poor solution as far as scheduling goes. But
>>> i'm wondering what the CPU side does. Can REP-NOP really take
>>> thousands of cycles? If yes, under what circumstances?
>>>
>>>
>> The guest is running rep-nop in a loop while trying to acquire a
>> spinlock. The hardware detects this (most likely, repeated rep-nop with
>> the same rip) and exits. We can program the loop count; obviously if
>> we're spinning for only a short while it's better to keep spinning while
>> hoping the lock will be released soon.
>>
>> The idea is to detect that the guest is not making forward progress and
>> yield. If I could tell the scheduler, you may charge me a couple of
>> milliseconds, I promise not to sue, that would be ideal. Other tasks
>> can become eligible, hopefully the task holding the spinlock, and by the
>> time we're scheduled back the long running task will have finished and
>> released the lock.
>>
>> For newer Linux as a guest we're better off paravirtualizing this, so we
>> can tell the host which vcpu holds the lock; in this case kvm will want
>> to say, take a couple milliseconds off my account and transfer it to
>> this task (so called directed yield). However there's no reason to
>> paravirtualize all cpu_relax() calls.
>>
>
> So we're now officially giving up on (soft) realtime virtualization?
>
>

Wouldn't realtime guests be in a realtime scheduling class? That ought
to ignore this time_yield() (or however it is eventually named).

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