Re: [PATCH] io_uring: don't mark S_ISBLK async work as unbounded

[Pavel Begunkov <asml.silence@gmail.com>]

On 04/04/2021 11:57, Hao Xu wrote:
> 在 2021/4/2 下午6:48, Pavel Begunkov 写道:
>> On 02/04/2021 11:32, Pavel Begunkov wrote:
>>> On 02/04/2021 09:52, Hao Xu wrote:
>>>> 在 2021/4/1 下午10:57, Jens Axboe 写道:
>>>>> S_ISBLK is marked as unbounded work for async preparation, because it
>>>>> doesn't match S_ISREG. That is incorrect, as any read/write to a block
>>>>> device is also a bounded operation. Fix it up and ensure that S_ISBLK
>>>>> isn't marked unbounded.
>>>>>
>>>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>>>
>>>> Hi Jens, I saw a (un)bounded work is for a (un)bounded worker to
>>>> execute. What is the difference between bounded and unbounded?
>>>
>>> Unbounded works are not bounded in execution time, i.e. they may take
>>> forever to complete. E.g. recv depends on the other end to send something,
>>> that not necessarily will ever happen.
>>
>> To elaborate a bit, one example of how it's used: because unbounded may
>> stay for long, it always spawns a new worker thread for each of them.
>>
>> If app submits SQEs as below, and send's are not actually sent for execution
>> but stashed somewhere internally in a list, e.g. waiting for a worker thread
>> to get free, it would just hang from the userspace perspective.
>>
>> recv(fd1), recv(fd1), send(fd1), send(fd1)
>>
> Hi Pavel, thank you for the patient explanation, I got the meaning of
>  bound/unbond now, but it seems there is no difference handling bounded and unbounded work in the current code?

It was used in io-wq, so see in io-wq.[c,h] files. Not sure about 5.12+, but
a quick look shows: 

wqe->acct[IO_WQ_ACCT_BOUND].max_workers = bounded;
...
wqe->acct[IO_WQ_ACCT_UNBOUND].max_workers =
			task_rlimit(current, RLIMIT_NPROC);



>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>> index 6d7a1b69712b..a16b7df934d1 100644
>>>>> --- a/fs/io_uring.c
>>>>> +++ b/fs/io_uring.c
>>>>> @@ -1213,7 +1213,7 @@ static void io_prep_async_work(struct io_kiocb *req)
>>>>>        if (req->flags & REQ_F_ISREG) {
>>>>>            if (def->hash_reg_file || (ctx->flags & IORING_SETUP_IOPOLL))
>>>>>                io_wq_hash_work(&req->work, file_inode(req->file));
>>>>> -    } else {
>>>>> +    } else if (!req->file || !S_ISBLK(file_inode(req->file)->i_mode)) {
>>>>>            if (def->unbound_nonreg_file)
>>>>>                req->work.flags |= IO_WQ_WORK_UNBOUND;
>>>>>        }
>>
> 

-- 
Pavel Begunkov