Re: [RFC v2 09/13] io_uring: separate wq for ring polling

[Jens Axboe <axboe@kernel.dk>]

On 1/4/23 1:28?PM, Pavel Begunkov wrote:
> On 1/4/23 18:08, Jens Axboe wrote:
>> On 1/2/23 8:04?PM, Pavel Begunkov wrote:
>>> Don't use ->cq_wait for ring polling but add a separate wait queue for
>>> it. We need it for following patches.
>>>
>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>> ---
>>>   include/linux/io_uring_types.h | 1 +
>>>   io_uring/io_uring.c            | 3 ++-
>>>   io_uring/io_uring.h            | 9 +++++++++
>>>   3 files changed, 12 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
>>> index dcd8a563ab52..cbcd3aaddd9d 100644
>>> --- a/include/linux/io_uring_types.h
>>> +++ b/include/linux/io_uring_types.h
>>> @@ -286,6 +286,7 @@ struct io_ring_ctx {
>>>           unsigned        cq_entries;
>>>           struct io_ev_fd    __rcu    *io_ev_fd;
>>>           struct wait_queue_head    cq_wait;
>>> +        struct wait_queue_head    poll_wq;
>>>           unsigned        cq_extra;
>>>       } ____cacheline_aligned_in_smp;
>>>   
>>
>> Should we move poll_wq somewhere else, more out of the way?
> 
> If we care about polling perf and cache collisions with
> cq_wait, yeah we can. In any case it's a good idea to at
> least move it after cq_extra.
> 
>> Would need to gate the check a flag or something.
> 
> Not sure I follow

I guess I could've been a bit more verbose... If we consider poll on the
io_uring rather uncommon, then moving the poll_wq outside of the hotter
cq_wait cacheline(s) would make sense. Each wait_queue_head is more than
a cacheline. Then we could have a flag in a spot that's hot anyway
whether to check it or not, eg in that same section as cq_wait.

Looking at the layout right now, we're at 116 bytes for that section, or
two cachelines with 12 bytes to spare. If we add poll_wq, then we'll be
at 196 bytes, which is 4 bytes over the next cacheline. So it'd
essentially double the size of that section. If we moved it outside of
the aligned sections, then it'd pack better.

-- 
Jens Axboe