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Re: [Qemu-devel] Linux kernel polling for QEMU
From: |
Andrew Jones |
Subject: |
Re: [Qemu-devel] Linux kernel polling for QEMU |
Date: |
Tue, 29 Nov 2016 16:24:28 +0100 |
User-agent: |
Mutt/1.6.0.1 (2016-04-01) |
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. 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...
Thanks,
drew
- Re: [Qemu-devel] Linux kernel polling for QEMU, (continued)
- Re: [Qemu-devel] Linux kernel polling for QEMU, Christian Borntraeger, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Fam Zheng, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Paolo Bonzini, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Fam Zheng, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Paolo Bonzini, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Fam Zheng, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU,
Andrew Jones <=
- Re: [Qemu-devel] Linux kernel polling for QEMU, Fam Zheng, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Andrew Jones, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Paolo Bonzini, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Andrew Jones, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Peter Maydell, 2016/11/30
- Re: [Qemu-devel] Linux kernel polling for QEMU, Andrew Jones, 2016/11/30
- Re: [Qemu-devel] Linux kernel polling for QEMU, Peter Maydell, 2016/11/30
- Re: [Qemu-devel] Linux kernel polling for QEMU, Paolo Bonzini, 2016/11/30
- Re: [Qemu-devel] Linux kernel polling for QEMU, Paolo Bonzini, 2016/11/29
- Re: [Qemu-devel] Linux kernel polling for QEMU, Stefan Hajnoczi, 2016/11/29