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