Re: [PATCH v9 net-next 2/7] net: add low latency socket poll

[Eliezer Tamir]

On 05/06/2013 18:59, Eric Dumazet wrote:
> On Wed, 2013-06-05 at 18:46 +0300, Eliezer Tamir wrote:
> > On 05/06/2013 18:39, Eric Dumazet wrote:
> > > On Wed, 2013-06-05 at 18:30 +0300, Eliezer Tamir wrote:
> > > > On 05/06/2013 18:21, Eric Dumazet wrote:
> >
> > > > > It would also make sense to give end_time as a parameter, so that the
> > > > > polling() code could really give  a end_time for the whole duration of
> > > > > poll().
> > > > >
> > > > > (You then should test can_poll_ll(end_time) _before_ call to
> > > > > ndo_ll_poll())
> > > >
> > > > how would you handle a nonblocking operation in that case?
> > > > I guess if we have a socket option, then we don't need to handle none
> > > > blocking any diffrent, since the user specified exactly how much time to
> > > > waste polling. right?
> > >
> > > If the thread already spent 50us in the poll() system call, it for sure
> > > should not call any ndo_ll_poll(). This makes no more sense at this
> > > point.
> >
> > what about a non-blocking read from a socket?
> > Right now we assume this means poll only once since the application will
> > repeat as needed.
> >
> > maybe add a "once" parameter that will cause sk_poll_ll() to ignore end
> > time and only try once?
>
> extern bool __sk_poll_ll(struct sock *sk, cycles_t end);
>
> static inline bool sk_poll_ll(struct sock *sk, bool nonblock)
> {
> 	return __sk_poll_ll(sk, nonblock, ll_end_time());
> }
>
> In the poll() code, we should call ll_end_time() once, even if we poll
> 1000 fds.

Right now we have three uses for sk_poll_ll

1. blocking read - In this case we loop until:
  a !skb_queue_empty(&sk->sk_receive_queue)
or
  b !can_poll_ll(end_time)

2. non-blocking read - only try once, ignoring end time.

3. poll/select - for each socket we only try once (nonblock==1),
 we loop in poll/select until we are lucky or run out of time.

For 1 we want to loop inside sk_poll_ll() but for 3 we loop in poll/select.

So it seems all we need is for sk_poll_ll() to not call ll_end_time() if 
nonblock is set.

( something like cycles_t end_time = nonblock ? 0 : ll_end_time(); )

Or we could move out looping in all cases to the calling function.
Does this mean we should push out rcu_read_lock_bh() into the caller
 as well?

-Eliezer
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