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

[Hao Xu <hao.xu@linux.dev>]

On 4/25/23 23:28, Pavel Begunkov wrote:
> On 4/25/23 15:42, 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.
>
> There were attempts like this one from Hao (CC'ed)
>
> https://lore.kernel.org/io-uring/20220627133541.15223-5-hao.xu@linux.dev/t/ 
>
>
> Not sure why it got stalled, but maybe Hao would be willing
> to pick it up again.


Hi folks, I'd like to pick it up again, but I just didn't get any reply 
at that time after sending

several versions of it...so before I restart that series, I'd like to 
ask Jens to comment the idea

of that patchset (fixed worker).


Thanks,

Hao