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

From: Eliezer Tamir <[email-masked]>
Date: Thu, 06 Jun 2013 15:50:42 +0300

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