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

[Jens Axboe <axboe@kernel.dk>]

On 1/4/23 1:45?PM, Pavel Begunkov wrote:
> On 1/4/23 20:34, Jens Axboe wrote:
>> 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.
> 
> Looks it's 24B, and wait_queue_entry is uncomfortable 40B.

(also see followup email). Yes, it's only 24 bytes indeed.

>> 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.
> 
> Than it's not about hotness and caches but rather memory
> consumption due to padding, which is still a good argument.

Right, it's nice to not keep io_ring_ctx bigger than it needs to be. And
if moved out-of-line, then it'd pack better and we would not "waste"
another cacheline on adding this wait_queue_head for polling.

-- 
Jens Axboe