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Message-ID: <65b1cb7f73a6a_250560294bd@willemb.c.googlers.com.notmuch>
Date: Wed, 24 Jan 2024 21:46:23 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Joe Damato <jdamato@...tly.com>,
netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Cc: chuck.lever@...cle.com,
jlayton@...nel.org,
linux-api@...r.kernel.org,
brauner@...nel.org,
edumazet@...gle.com,
davem@...emloft.net,
alexander.duyck@...il.com,
sridhar.samudrala@...el.com,
kuba@...nel.org,
weiwan@...gle.com,
Joe Damato <jdamato@...tly.com>
Subject: Re: [net-next v2 3/4] eventpoll: Add epoll ioctl for epoll_params
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@...tly.com>
Please be sure to include the lists and people suggested by
`get_maintainer.pl -f fs/eventpoll.c`.
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?
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@gmail.com/
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.
> ---
> .../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@...ilserver.org>
> */
>
> +#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
>
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