Re: [PATCH liburing 3/5] Add helpers to set and get eventfd notification status

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, May 15, 2020 at 07:11:11PM +0200, Stefano Garzarella wrote:
> On Fri, May 15, 2020 at 10:53:50AM -0600, Jens Axboe wrote:
> > On 5/15/20 10:43 AM, Stefano Garzarella wrote:
> > > This patch adds the new IORING_CQ_EVENTFD_DISABLED flag. It can be
> > > used to disable/enable notifications from the kernel when a
> > > request is completed and queued to the CQ ring.
> > > 
> > > We also add two helpers function to check if the notifications are
> > > enabled and to enable/disable them.
> > > 
> > > If the kernel doesn't provide CQ ring flags, the notifications are
> > > always enabled if an eventfd is registered.
> > > 
> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > ---
> > >  src/include/liburing.h          | 30 ++++++++++++++++++++++++++++++
> > >  src/include/liburing/io_uring.h |  7 +++++++
> > >  2 files changed, 37 insertions(+)
> > > 
> > > diff --git a/src/include/liburing.h b/src/include/liburing.h
> > > index ea596f6..fe03547 100644
> > > --- a/src/include/liburing.h
> > > +++ b/src/include/liburing.h
> > > @@ -9,7 +9,9 @@ extern "C" {
> > >  #include <sys/socket.h>
> > >  #include <sys/uio.h>
> > >  #include <sys/stat.h>
> > > +#include <errno.h>
> > >  #include <signal.h>
> > > +#include <stdbool.h>
> > >  #include <inttypes.h>
> > >  #include <time.h>
> > >  #include "liburing/compat.h"
> > > @@ -445,6 +447,34 @@ static inline unsigned io_uring_cq_ready(struct io_uring *ring)
> > >  	return io_uring_smp_load_acquire(ring->cq.ktail) - *ring->cq.khead;
> > >  }
> > >  
> > > +static inline int io_uring_cq_eventfd_enable(struct io_uring *ring,
> > > +					     bool enabled)
> > > +{
> > > +	uint32_t flags;
> > > +
> > > +	if (!ring->cq.kflags)
> > > +		return -ENOTSUP;
> > > +
> > > +	flags = *ring->cq.kflags;
> > > +
> > > +	if (enabled)
> > > +		flags &= ~IORING_CQ_EVENTFD_DISABLED;
> > > +	else
> > > +		flags |= IORING_CQ_EVENTFD_DISABLED;
> > > +
> > > +	IO_URING_WRITE_ONCE(*ring->cq.kflags, flags);
> > > +
> > > +	return 0;
> > > +}
> > 
> > The -ENOTSUP seems a bit odd, I wonder if we should even flag that as an
> > error.
> 
> Do you think it's better to ignore the enabling/disabling if we don't have
> the flag field available?
> 
> > 
> > The function should probably also be io_uring_cq_eventfd_toggle() or
> > something like that, as it does both enable and disable.
> > 
> > Either that, or have two functions, and enable and disable.
> 
> Okay, I'll change it in io_uring_cq_eventfd_toggle().
> 
> > 
> > The bigger question is probably how to handle kernels that don't
> > have this feature. It'll succeed, but we'll still post events. Maybe
> > the kernel side should have a feature flag that we can test?
> 
> I thought about that, and initially I added a
> IORING_FEAT_EVENTFD_DISABLE, but then I realized that we are adding
> the CQ 'flags' field together with the eventfd disabling feature.
> 
> So I supposed that if 'p->cq_off.flags' is not zero, than the kernel
> supports CQ flags and also the IORING_CQ_EVENTFD_DISABLED bit.
> 
> Do you think that's okay, or should we add IORING_FEAT_EVENTFD_DISABLE
> (or something similar)?

Hi Jens,
I'm changing io_uring_cq_eventfd_enable() to io_uring_cq_eventfd_toggle().

Any advice on the error and eventual feature flag?

Thank you very much,
Stefano