Subject: Re: [PATCH net-next v5 1/3] eventpoll: support busy poll per epoll instance

From: Joe Damato <[email-masked]>
Date: Mon, 5 Feb 2024 10:25:55 -0800

On Wed, Jan 31, 2024 at 06:08:03PM +0000, Joe Damato wrote:
> Allow busy polling on a per-epoll context basis. The per-epoll context
> usec timeout value is preferred, but the pre-existing system wide sysctl
> value is still supported if it specified.
> 
> Note that this change uses an xor: either per epoll instance busy polling
> is enabled on the epoll instance or system wide epoll is enabled. Enabling
> both is disallowed.

I just realized that I updated the code below to use a an or (||) instead
of xor (^), but forgot to update the commit message.

I can fix this and send a v6.

> 
> Signed-off-by: Joe Damato <[email-masked]>
> ---
>  fs/eventpoll.c | 49 +++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 45 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 3534d36a1474..ce75189d46df 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -227,6 +227,8 @@ struct eventpoll {
>  #ifdef CONFIG_NET_RX_BUSY_POLL
>  	/* used to track busy poll napi_id */
>  	unsigned int napi_id;
> +	/* busy poll timeout */
> +	u64 busy_poll_usecs;
>  #endif
>  
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> @@ -386,12 +388,44 @@ static inline int ep_events_available(struct eventpoll *ep)
>  		READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;
>  }
>  
> +/**
> + * busy_loop_ep_timeout - check if busy poll has timed out. The timeout value
> + * from the epoll instance ep is preferred, but if it is not set fallback to
> + * the system-wide global via busy_loop_timeout.
> + *
> + * @start_time: The start time used to compute the remaining time until timeout.
> + * @ep: Pointer to the eventpoll context.
> + *
> + * Return: true if the timeout has expired, false otherwise.
> + */
> +static inline bool busy_loop_ep_timeout(unsigned long start_time, struct eventpoll *ep)
> +{
> +#ifdef CONFIG_NET_RX_BUSY_POLL
> +	unsigned long bp_usec = READ_ONCE(ep->busy_poll_usecs);
> +
> +	if (bp_usec) {
> +		unsigned long end_time = start_time + bp_usec;
> +		unsigned long now = busy_loop_current_time();
> +
> +		return time_after(now, end_time);
> +	} else {
> +		return busy_loop_timeout(start_time);
> +	}
> +#endif
> +	return true;
> +}
> +
>  #ifdef CONFIG_NET_RX_BUSY_POLL
> +static bool ep_busy_loop_on(struct eventpoll *ep)
> +{
> +	return !!ep->busy_poll_usecs || net_busy_loop_on();
> +}
> +
>  static bool ep_busy_loop_end(void *p, unsigned long start_time)
>  {
>  	struct eventpoll *ep = p;
>  
> -	return ep_events_available(ep) || busy_loop_timeout(start_time);
> +	return ep_events_available(ep) || busy_loop_ep_timeout(start_time, ep);
>  }
>  
>  /*
> @@ -404,7 +438,7 @@ static bool ep_busy_loop(struct eventpoll *ep, int nonblock)
>  {
>  	unsigned int napi_id = READ_ONCE(ep->napi_id);
>  
> -	if ((napi_id >= MIN_NAPI_ID) && net_busy_loop_on()) {
> +	if ((napi_id >= MIN_NAPI_ID) && ep_busy_loop_on(ep)) {
>  		napi_busy_loop(napi_id, nonblock ? NULL : ep_busy_loop_end, ep, false,
>  			       BUSY_POLL_BUDGET);
>  		if (ep_events_available(ep))
> @@ -430,7 +464,8 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
>  	struct socket *sock;
>  	struct sock *sk;
>  
> -	if (!net_busy_loop_on())
> +	ep = epi->ep;
> +	if (!ep_busy_loop_on(ep))
>  		return;
>  
>  	sock = sock_from_file(epi->ffd.file);
> @@ -442,7 +477,6 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
>  		return;
>  
>  	napi_id = READ_ONCE(sk->sk_napi_id);
> -	ep = epi->ep;
>  
>  	/* Non-NAPI IDs can be rejected
>  	 *	or
> @@ -466,6 +500,10 @@ static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
>  {
>  }
>  
> +static inline bool ep_busy_loop_on(struct eventpoll *ep)
> +{
> +	return false;
> +}
>  #endif /* CONFIG_NET_RX_BUSY_POLL */
>  
>  /*
> @@ -2058,6 +2096,9 @@ static int do_epoll_create(int flags)
>  		error = PTR_ERR(file);
>  		goto out_free_fd;
>  	}
> +#ifdef CONFIG_NET_RX_BUSY_POLL
> +	ep->busy_poll_usecs = 0;
> +#endif
>  	ep->file = file;
>  	fd_install(fd, file);
>  	return fd;
> -- 
> 2.25.1
>