Re: [net-next v2 3/4] eventpoll: Add epoll ioctl for epoll_params

[Willem de Bruijn]

Joe Damato wrote:
> On Wed, Jan 24, 2024 at 09:46:23PM -0500, Willem de Bruijn wrote:
> > Joe Damato wrote:
> > > Add an ioctl for getting and setting epoll_params. User programs can use
> > > this ioctl to get and set the busy poll usec time or packet budget
> > > params for a specific epoll context.
> > > 
> > > Signed-off-by: Joe Damato <jdamato@xxxxxxxxxx>
> > 
> > Please be sure to include the lists and people suggested by
> > `get_maintainer.pl -f fs/eventpoll.c`.
> 
> Thanks - I must have done something wrong when trying to get the maintainer
> list.
> 
> Should I resend this v2? Not sure what the appropriate thing to do is in
> this case. My apologies.

If you don't get additional feedback in a few days and still prefer
this option that might be an approach.

After reading the below thread, compare the different possible APIs
and either revise the code or perhaps add a small paragraph why you
think this is the preferred path.
 
> > Adding ioctls is generally discouraged.
> > 
> > As this affects the behavior of epoll_wait, should this just be a
> > flag to (a new variant of) epoll_wait?
> 
> I have no strong preference either way. It seems to me that adding a new
> system call is a fairly significant change vs adding an ioctl, but I am
> open to whatever is preferred by the maintainers.
> 
> I have no idea who would need to weigh-in to make this decision.
> 
> > Speaking from some experience with adding epoll_pwait2. I initially
> > there added a stateful change that would affect wait behavior. The
> > sensible feedback as the time was to just change the behavior of the
> > syscall it affected. Even if that requires a syscall (which is not
> > that different from an ioctl, if better defined).
> > 
> > The discussion in that thread may be informative to decide on API:
> > https://lwn.net/ml/linux-kernel/20201116161001.1606608-1-willemdebruijn.kernel@xxxxxxxxx/
> 
> Interesting thread, thanks for sending.
> 
> > Agreed on the overall principle that it is preferable to be able to
> > enable busypolling selectively. We already do for SO_BUSY_POLL and
> > sysctl busy_read.
> 
> Thanks for taking a look and providing feedback.
> 
> >
> > > ---
> > >  .../userspace-api/ioctl/ioctl-number.rst      |  1 +
> > >  fs/eventpoll.c                                | 47 +++++++++++++++++++
> > >  include/uapi/linux/eventpoll.h                | 12 +++++
> > >  3 files changed, 60 insertions(+)
> > > 
> > > diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> > > index 457e16f06e04..b33918232f78 100644
> > > --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
> > > +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
> > > @@ -309,6 +309,7 @@ Code  Seq#    Include File                                           Comments
> > >  0x89  0B-DF  linux/sockios.h
> > >  0x89  E0-EF  linux/sockios.h                                         SIOCPROTOPRIVATE range
> > >  0x89  F0-FF  linux/sockios.h                                         SIOCDEVPRIVATE range
> > > +0x8A  00-1F  linux/eventpoll.h
> > >  0x8B  all    linux/wireless.h
> > >  0x8C  00-3F                                                          WiNRADiO driver
> > >                                                                       <http://www.winradio.com.au/>
> > > diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> > > index 40bd97477b91..c1ee0fe01da1 100644
> > > --- a/fs/eventpoll.c
> > > +++ b/fs/eventpoll.c
> > > @@ -6,6 +6,8 @@
> > >   *  Davide Libenzi <davidel@xxxxxxxxxxxxxxx>
> > >   */
> > >  
> > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> > > +
> > >  #include <linux/init.h>
> > >  #include <linux/kernel.h>
> > >  #include <linux/sched/signal.h>
> > > @@ -869,6 +871,49 @@ static void ep_clear_and_put(struct eventpoll *ep)
> > >  		ep_free(ep);
> > >  }
> > >  
> > > +static long ep_eventpoll_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> > > +{
> > > +	int ret;
> > > +	struct eventpoll *ep;
> > > +	struct epoll_params epoll_params;
> > > +	void __user *uarg = (void __user *) arg;
> > > +
> > > +	if (!is_file_epoll(file))
> > > +		return -EINVAL;
> > > +
> > > +	ep = file->private_data;
> > > +
> > > +	switch (cmd) {
> > > +#ifdef CONFIG_NET_RX_BUSY_POLL
> > > +	case EPIOCSPARAMS:
> > > +		if (copy_from_user(&epoll_params, uarg, sizeof(epoll_params)))
> > > +			return -EFAULT;
> > > +
> > > +		if (epoll_params.busy_poll_budget > NAPI_POLL_WEIGHT)
> > > +			pr_err("busy poll budget %u exceeds suggested maximum %u\n",
> > > +					epoll_params.busy_poll_budget, NAPI_POLL_WEIGHT);
> > > +
> > > +		ep->busy_poll_usecs = epoll_params.busy_poll_usecs;
> > > +		ep->busy_poll_budget = epoll_params.busy_poll_budget;
> > > +		return 0;
> > > +
> > > +	case EPIOCGPARAMS:
> > > +		memset(&epoll_params, 0, sizeof(epoll_params));
> > > +		epoll_params.busy_poll_usecs = ep->busy_poll_usecs;
> > > +		epoll_params.busy_poll_budget = ep->busy_poll_budget;
> > > +		if (copy_to_user(uarg, &epoll_params, sizeof(epoll_params)))
> > > +			return -EFAULT;
> > > +
> > > +		return 0;
> > > +#endif
> > > +	default:
> > > +		ret = -EINVAL;
> > > +		break;
> > > +	}
> > > +
> > > +	return ret;
> > > +}
> > > +
> > >  static int ep_eventpoll_release(struct inode *inode, struct file *file)
> > >  {
> > >  	struct eventpoll *ep = file->private_data;
> > > @@ -975,6 +1020,8 @@ static const struct file_operations eventpoll_fops = {
> > >  	.release	= ep_eventpoll_release,
> > >  	.poll		= ep_eventpoll_poll,
> > >  	.llseek		= noop_llseek,
> > > +	.unlocked_ioctl	= ep_eventpoll_ioctl,
> > > +	.compat_ioctl   = compat_ptr_ioctl,
> > >  };
> > >  
> > >  /*
> > > diff --git a/include/uapi/linux/eventpoll.h b/include/uapi/linux/eventpoll.h
> > > index cfbcc4cc49ac..8eb0fdbce995 100644
> > > --- a/include/uapi/linux/eventpoll.h
> > > +++ b/include/uapi/linux/eventpoll.h
> > > @@ -85,4 +85,16 @@ struct epoll_event {
> > >  	__u64 data;
> > >  } EPOLL_PACKED;
> > >  
> > > +struct epoll_params {
> > > +	u64 busy_poll_usecs;
> > > +	u16 busy_poll_budget;
> > > +
> > > +	/* for future fields */
> > > +	u8 data[118];
> > > +} EPOLL_PACKED;
> > > +
> > > +#define EPOLL_IOC_TYPE 0x8A
> > > +#define EPIOCSPARAMS _IOW(EPOLL_IOC_TYPE, 0x01, struct epoll_params)
> > > +#define EPIOCGPARAMS _IOR(EPOLL_IOC_TYPE, 0x02, struct epoll_params)
> > > +
> > >  #endif /* _UAPI_LINUX_EVENTPOLL_H */
> > > -- 
> > > 2.25.1
> > > 
> > 
> >