On Wed, Nov 30, 2016 at 06:50:09PM +0800, Fam Zheng wrote: > On Wed, 11/30 09:38, Stefan Hajnoczi wrote: > > On Wed, Nov 30, 2016 at 01:42:14PM +0800, Fam Zheng wrote: > > > On Tue, 11/29 20:43, Stefan Hajnoczi wrote: > > > > On Tue, Nov 29, 2016 at 1:24 PM, Fam Zheng <f...@redhat.com> wrote: > > > > > On Tue, 11/29 12:17, Paolo Bonzini wrote: > > > > >> On 29/11/2016 11:32, Fam Zheng wrote: > > > > >> * 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! > > > > > > > > With this solution QEMU can either poll virtqueues or the host kernel > > > > can poll NIC and storage controller descriptor rings, but not both at > > > > the same time in one thread. This is one of the reasons why I think > > > > exploring polling in the kernel makes more sense. > > > > > > That's true. I have one question though: controller rings are in a > > > different > > > layer in the kernel, I wonder what the syscall interface looks like to ask > > > kernel to poll both hardware rings and memory locations in the same loop? > > > It's > > > not obvious to me after reading your eventfd patch. > > > > Current descriptor ring polling in select(2)/poll(2) is supported for > > network sockets. Take a look at the POLL_BUSY_LOOP flag in > > fs/select.c:do_poll(). If the .poll() callback sets the flag then it > > indicates that the fd supports busy loop polling. > > > > The way this is implemented for network sockets is that the socket looks > > up the napi index and is able to use the NIC driver to poll the rx ring. > > Then it checks whether the socket's receive queue contains data after > > the rx ring was processed. > > > > The virtio_net.ko driver supports this interface, for example. See > > drivers/net/virtio_net.c:virtnet_busy_poll(). > > > > Busy loop polling isn't supported for block I/O yet. There is currently > > a completely independent code path for O_DIRECT synchronous I/O where > > NVMe can poll for request completion. But it doesn't work together with > > asynchronous I/O (e.g. Linux AIO using eventfd with select(2)/poll(2)). > > This makes perfect sense now, thanks for the pointers! > > > > > > > The disadvantage of the kernel approach is that you must make the > > > > ppoll(2)/epoll_wait(2) syscall even for polling, and you probably need > > > > to do eventfd reads afterwards so the minimum event loop iteration > > > > latency is higher than doing polling in userspace. > > > > > > And userspace drivers powered by dpdk or vfio will still want to do > > > polling in > > > userspace anyway, we may want to take that into account as well. > > > > vfio supports interrupts so it can definitely be integrated with > > adaptive kernel polling (i.e. poll for a little while and then wait for > > an interrupt if there was no event). > > > > Does dpdk ever use interrupts? > > Yes, interrupt mode is supported there. For example see the intx/msix init > code > in drivers/net/ixgbe/ixgbe_ethdev.c:ixgbe_dev_start().
If the application wants to poll 100% of the time then userspace polling is the right solution. Userspace polling also makes sense when all event sources can be polled from userspace - it's faster than using kernel polling. But I think in adaptive polling + wait use cases or when there are a mixture of userspace and kernel event sources to poll, then it makes sense to use kernel polling. In QEMU we have the latter so I think we need to contribute to kernel polling. Stefan
signature.asc
Description: PGP signature
