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

Avi Kivity
2009-05-11T14:25:17Z

Ingo Molnar wrote:
>
>>> +static int pause_interception(struct vcpu_svm *svm, struct kvm_run
>>> *kvm_run)
>>> +{
>>> + /* Simple yield */
>>> + vcpu_put(&svm->vcpu);
>>> + schedule();
>>> + vcpu_load(&svm->vcpu);
>>> + return 1;
>>> +
>>>
>> Ingo, will this do anything under CFS, or will CFS note that
>> nothing has changed in the accounting and reschedule us
>> immediately?
>>
>
> The scheduler will yield to another task only if the current task
> has become ineligible. I.e schedule() is largely a NOP on
> TASK_RUNNING tasks (i.e. here).
>

Especially on preemptible kernels, where the schedule() would have
already happened if it could cause anything, IIUC.

> 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.

--
error compiling committee.c: too many arguments to function