Re: [PATCH 4/4] io_uring: mark opcodes that always need io-wq punt

Pavel Begunkov
2023-04-25T15:47:41Z

On 4/25/23 16:25, Jens Axboe wrote:
> On 4/25/23 9:07?AM, Ming Lei wrote:
>> On Tue, Apr 25, 2023 at 08:50:33AM -0600, Jens Axboe wrote:
>>> On 4/25/23 8:42?AM, Ming Lei wrote:
>>>> On Tue, Apr 25, 2023 at 07:31:10AM -0600, Jens Axboe wrote:
>>>>> On 4/24/23 8:50?PM, Ming Lei wrote:
>>>>>> On Mon, Apr 24, 2023 at 08:18:02PM -0600, Jens Axboe wrote:
>>>>>>> On 4/24/23 8:13?PM, Ming Lei wrote:
>>>>>>>> On Mon, Apr 24, 2023 at 08:08:09PM -0600, Jens Axboe wrote:
>>>>>>>>> On 4/24/23 6:57?PM, Ming Lei wrote:
>>>>>>>>>> On Mon, Apr 24, 2023 at 09:24:33AM -0600, Jens Axboe wrote:
>>>>>>>>>>> On 4/24/23 1:30?AM, Ming Lei wrote:
>>>>>>>>>>>> On Thu, Apr 20, 2023 at 12:31:35PM -0600, Jens Axboe wrote:
>>>>>>>>>>>>> Add an opdef bit for them, and set it for the opcodes where we always
>>>>>>>>>>>>> need io-wq punt. With that done, exclude them from the file_can_poll()
>>>>>>>>>>>>> check in terms of whether or not we need to punt them if any of the
>>>>>>>>>>>>> NO_OFFLOAD flags are set.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>>>>>>>>>> ---
>>>>>>>>>>>>> io_uring/io_uring.c | 2 +-
>>>>>>>>>>>>> io_uring/opdef.c | 22 ++++++++++++++++++++--
>>>>>>>>>>>>> io_uring/opdef.h | 2 ++
>>>>>>>>>>>>> 3 files changed, 23 insertions(+), 3 deletions(-)
>>>>>>>>>>>>>
>>>>>>>>>>>>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>>>>>>>>>>>>> index fee3e461e149..420cfd35ebc6 100644
>>>>>>>>>>>>> --- a/io_uring/io_uring.c
>>>>>>>>>>>>> +++ b/io_uring/io_uring.c
>>>>>>>>>>>>> @@ -1948,7 +1948,7 @@ static int io_issue_sqe(struct io_kiocb *req, unsigned int issue_flags)
>>>>>>>>>>>>> return -EBADF;
>>>>>>>>>>>>>
>>>>>>>>>>>>> if (issue_flags & IO_URING_F_NO_OFFLOAD &&
>>>>>>>>>>>>> - (!req->file || !file_can_poll(req->file)))
>>>>>>>>>>>>> + (!req->file || !file_can_poll(req->file) || def->always_iowq))
>>>>>>>>>>>>> issue_flags &= ~IO_URING_F_NONBLOCK;
>>>>>>>>>>>>
>>>>>>>>>>>> I guess the check should be !def->always_iowq?
>>>>>>>>>>>
>>>>>>>>>>> How so? Nobody that takes pollable files should/is setting
>>>>>>>>>>> ->always_iowq. If we can poll the file, we should not force inline
>>>>>>>>>>> submission. Basically the ones setting ->always_iowq always do -EAGAIN
>>>>>>>>>>> returns if nonblock == true.
>>>>>>>>>>
>>>>>>>>>> I meant IO_URING_F_NONBLOCK is cleared here for ->always_iowq, and
>>>>>>>>>> these OPs won't return -EAGAIN, then run in the current task context
>>>>>>>>>> directly.
>>>>>>>>>
>>>>>>>>> Right, of IO_URING_F_NO_OFFLOAD is set, which is entirely the point of
>>>>>>>>> it :-)
>>>>>>>>
>>>>>>>> But ->always_iowq isn't actually _always_ since fallocate/fsync/... are
>>>>>>>> not punted to iowq in case of IO_URING_F_NO_OFFLOAD, looks the naming of
>>>>>>>> ->always_iowq is a bit confusing?
>>>>>>>
>>>>>>> Yeah naming isn't that great, I can see how that's bit confusing. I'll
>>>>>>> be happy to take suggestions on what would make it clearer.
>>>>>>
>>>>>> Except for the naming, I am also wondering why these ->always_iowq OPs
>>>>>> aren't punted to iowq in case of IO_URING_F_NO_OFFLOAD, given it
>>>>>> shouldn't improve performance by doing so because these OPs are supposed
>>>>>> to be slow and always slept, not like others(buffered writes, ...),
>>>>>> can you provide one hint about not offloading these OPs? Or is it just that
>>>>>> NO_OFFLOAD needs to not offload every OPs?
>>>>>
>>>>> The whole point of NO_OFFLOAD is that items that would normally be
>>>>> passed to io-wq are just run inline. This provides a way to reap the
>>>>> benefits of batched submissions and syscall reductions. Some opcodes
>>>>> will just never be async, and io-wq offloads are not very fast. Some of
>>>>
>>>> Yeah, seems io-wq is much slower than inline issue, maybe it needs
>>>> to be looked into, and it is easy to run into io-wq for IOSQE_IO_LINK.
>>>
>>> Indeed, depending on what is being linked, you may see io-wq activity
>>> which is not ideal.
>>
>> That is why I prefer to fused command for ublk zero copy, because the
>> registering buffer approach suggested by Pavel and Ziyang has to link
>> register buffer OP with the actual IO OP, and it is observed that
>> IOPS drops to 1/2 in 4k random io test with registered buffer approach.
>
> It'd be worth looking into if we can avoid io-wq for link execution, as
> that'd be a nice win overall too. IIRC, there's no reason why it can't
> be done like initial issue rather than just a lazy punt to io-wq.

I might've missed a part of the discussion, but links are _usually_
executed by task_work, e.g.

io_submit_flush_completions() -> io_queue_next() -> io_req_task_queue()

There is one optimisation where if we're already in io-wq, it'll try
to serve the next linked request by the same io-wq worker with no
overhead on requeueing, but otherwise it'll only get there if
the request can't be executed nowait / async as per usual rules.

The tw execution part might be further optimised, it can be executed
almost in place instead of queueing a tw. It saved quite a lot of CPU
when I tried it with BPF requests.

--
Pavel Begunkov