Subject: Re: [PATCH net-next v6 1/4] eventpoll: support busy poll per epoll instance

From: Joe Damato <[email-masked]>
Date: Thu, 8 Feb 2024 10:06:02 -0800

On Thu, Feb 08, 2024 at 06:46:25PM +0100, Eric Dumazet wrote:
> On Mon, Feb 5, 2024 at 10:05 PM Joe Damato <[email-masked]> 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.
> >
> > 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
> 
> It seems this local helper is only called from code compiled when
> CONFIG_NET_RX_BUSY_POLL
> is set.
> 
> Not sure why you need an #ifdef here.

Thanks, you are right.

I'll move this down to be within CONFIG_NET_RX_BUSY_POLL and get rid of the
#ifdef for the v7.

Thanks for your review.
 
> > +       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
> >