On 01/04/2011 04:17 PM, Anthony Liguori wrote:
On 01/03/2011 04:01 AM, Avi Kivity wrote:
On 01/03/2011 11:46 AM, Jan Kiszka wrote:
Hi,
at least in kvm mode, the qemu_fair_mutex seems to have lost its
function of balancing qemu_global_mutex access between the io-thread
and
vcpus. It's now only taken by the latter, isn't it?
This and the fact that qemu-kvm does not use this kind of lock made me
wonder what its role is and if it is still relevant in practice. I'd
like to unify the execution models of qemu-kvm and qemu, and this lock
is the most obvious difference (there are surely more subtle ones as
well...).
IIRC it was used for tcg, which has a problem that kvm doesn't have:
a tcg vcpu needs to hold qemu_mutex when it runs, which means there
will always be contention on qemu_mutex. In the absence of fairness,
the tcg thread could dominate qemu_mutex and starve the iothread.
No, it's actually the opposite IIRC.
TCG relies on the following behavior. A guest VCPU runs until 1) it
encounters a HLT instruction 2) an event occurs that forces the TCG
execution to break.
(2) really means that the TCG thread receives a signal. Usually, this
is the periodic timer signal.
What about a completion? an I/O completes, the I/O thread wakes up,
needs to acquire the global lock (and force tcg off it) inject and
interrupt, and go back to sleep.
When the TCG thread, it needs to let the IO thread run for at least
one iteration. Coordinating the execution of the IO thread such that
it's guaranteed to run at least once and then having it drop the qemu
mutex long enough for the TCG thread to acquire it is the purpose of
the qemu_fair_mutex.
That doesn't compute - the iothread doesn't hog the global lock (it
sleeps most of the time, and drops the lock while sleeping), so the
iothread cannot starve out tcg. On the other hand, tcg does hog the
global lock, so it needs to be made to give it up so the iothread can
run, for example my completion example.
I think the abstraction we need here is a priority lock, with higher
priority given to the iothread. A lock() operation that takes
precedence would atomically signal the current owner to drop the lock.
Under kvm we'd run a normal mutex, so the it wouldn't need to take the
extra mutex.
--
error compiling committee.c: too many arguments to function
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