Re: [Qemu-devel] virtio-scsi-pci iothread spins at 100%

[Ed Swierk]

On Wed, Feb 8, 2017 at 8:33 AM, Ed Swierk <address@hidden> wrote:
> Recently I noticed that when I configure a virtio-scsi-pci device
> using an iothread, as soon as the guest virtio-scsi driver loads, the
> iothread spins at 100%:
>
>   -object iothread,id=iothread1 -device virtio-scsi-pci,iothread=iothread1
>
> This occurs whether or not a disk is attached, with either
> poll-max-ns=0 or poll-max-ns=32768, and with Linux 3.13, 4.1 and 4.4
> guests. The iothread stops spinning as soon as the guest driver is
> unloaded.
>
> I bisected the issue to commit 684e508c23d28af8d6ed2c62738a0f60447c8274:
>
>   aio: add .io_poll_begin/end() callbacks
>
> It doesn't seem to affect performance, but obviously consuming CPU
> cycles when there's no disk attached is undesirable. Is this an
> expected side effect of implementing iothread polling? Is there a way
> to revert to the old non-polling behavior?

FWIW, changing

  return run_poll_handlers_once(ctx);

to

  return false;

in try_poll_mode() in aio-posix.c makes the iothread stop spinning,
but I don't know what damage this will cause.

--Ed