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? * 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. Paolo
