Re: [Qemu-devel] Linux kernel polling for QEMU

[Fam Zheng <famz@redhat.com>]

On Tue, 11/29 16:24, Andrew Jones wrote:
> On Tue, Nov 29, 2016 at 10:17:46PM +0800, Fam Zheng wrote:
> > On Tue, 11/29 14:27, Paolo Bonzini wrote:
> > > 
> > > 
> > > On 29/11/2016 14:24, Fam Zheng wrote:
> > > > On Tue, 11/29 12:17, Paolo Bonzini wrote:
> > > >>
> > > >>
> > > >> On 29/11/2016 11:32, Fam Zheng wrote:
> > > >>>
> > > >>> The kernel change will be a new prctl operation (should it be a different
> > > >>> syscall to extend?) to register a new type of eventfd called "idle eventfd":
> > > >>>
> > > >>>     prctl(PR_ADD_IDLE_EVENTFD, int eventfd);
> > > >>>     prctl(PR_DEL_IDLE_EVENTFD, int eventfd);
> > > >>>
> > > >>> It will be notified by kernel each time when the thread's local core has no
> > > >>> runnable threads (i.e., entering idle state).
> > > >>>
> > > >>> QEMU can then add this eventfd to its event loop when it has events to poll, and
> > > >>> watch virtqueue/linux-aio memory from userspace in the fd handlers.  Effectiely,
> > > >>> if a ppoll() would have blocked because there are no new events, it could now
> > > >>> return immediately because of idle_eventfd events, and do the idle polling.
> > > >>
> > > >> This has two issues:
> > > >>
> > > >> * it only reports the leading edge of single_task_running().  Is it also
> > > >> useful to stop polling on the trailing edge?
> > > > 
> > > > QEMU can clear the eventfd right after event firing so I don't think it is
> > > > necessary.
> > > 
> > > Yes, but how would QEMU know that the eventfd has fired?  It would be
> > > very expensive to read the eventfd on each iteration of polling.
> > 
> > The idea is to ppoll() the eventfd together with other fds (ioeventfd and
> > linux-aio etc.), and in the handler, call event_notifier_test_and_clear()
> > followed by a polling loop for some period.
> > 
> > Fam
> > 
> > > 
> > > Paolo
> > > 
> > > >> * it still needs a system call before polling is entered.  Ideally, QEMU
> > > >> could run without any system call while in polling mode.
> > > >>
> > > >> Another possibility is to add a system call for single_task_running().
> > > >> It should be simple enough that you can implement it in the vDSO and
> > > >> avoid a context switch.  There are convenient hooking points in
> > > >> add_nr_running and sub_nr_running.
> > > > 
> > > > That sounds good!
> > > > 
> > > > Fam
> > > > 
> >
> 
> While we have a ppoll audience, another issue with the current polling
> is that we can block with an infinite timeout set (-1), and it can
> actually end up being infinite, i.e. vcpus will never run again. I'm
> able to exhibit this with kvm-unit-tests.

I don't understand, why does ppoll() block vcpus? They are in different threads.
Could you elaborate the kvm-unit-test case?

> For these rare cases where
> no other timeout has been selected, shouldn't we have a default timeout?
> Anyone want to pick a number? I have a baseless compulsion to use 10 ms...

Timeout set as -1 means there is abosolutely no event to expect, waking up after
10 ms is a waste. It's a bug if guest hangs in this case.

Fam