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. > > * 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
