Subject: Re: [PATCH RFC] net: lls epoll support

From: Eliezer Tamir <[email-masked]>
Date: Tue, 25 Jun 2013 18:34:36 +0300

On 25/06/2013 17:26, yaniv saar wrote:
> On Wed, Jun 19, 2013 at 2:42 PM, Eliezer Tamir
> <[email-masked]> wrote:
>>
>> [this patch needs the poll patch to be applied first]
>> with sockperf doing epoll on 1000 sockets I see an avg latency of 6us
>>
>
> hi eliezer,
>
> please consider the following solution for epoll that is based on
> polling dev+queue.
> instead of looping over the socket as in LLS, maintain in eventpool
> struct a list of device+queues (qdlist).

Thanks for looking into this.
I'm currently working on a solution that has a lot similar to what you 
are proposing.

We don't need a new id mechanism, we already have the napi_id.
The nice thing about the napi_id is that the only locking it needs
is an rcu_read_lock when dereferencing.

we don't need to remember the ll_usec value of each socket because
the patch for select/poll (currently waiting for review) added
a separate sysctl value for poll.

I would like to find a way for the user to specify how long to busy
wait, directly from the system call, but I was not able to find
a simple way of adding this without a change to the system call
prototype.

we do however need to track when a socket's napi_id changes.
But for that we can hook into sk_mark_ll().

so here is a list of proposed changes:

1. add a linked list of unique napi_id's to struct eventpoll.
each id will have a collision list of sockets that have the same id.
-a hash is gratuitous, we expect the unique list to have 0 to 2
  elements in most cases.

2. when a new socket is added, if its id is new it gets added to the 
unique list, otherwise to the collision list of that id.

3. when a socket is removed, if it's on the unique list, replace it
with the first on its former collision list.

4. add callback mechanism to sk_mark_ll() which will be activated when
  the mark changes, update the lists.
(a socket may be polled by more than one epoll so be careful)

5. add  and remove to/from the lists in ep_insert and ep_remove
  respectively. check if we need to do something for ep_modify().

6. add an ep_poll helper that will round robin polling on the
files in the unique list.

7. init everything from epoll_create.

locking:

napi_id's are great since they don't need locking except for an
rcu_read_lock when polling on one.

the lists need a spinlock for adding/removing, maybe they
can use ep->lock.

callback registration/removal needs to use the same mechanism that
ep_add / ep_remove use to protect themselves from the rest of epoll.