Re: [PATCH 3/6] io_uring: add support for NO_OFFLOAD

[Jens Axboe <axboe@kernel.dk>]

On 4/19/23 7:01?PM, Pavel Begunkov wrote:
> On 4/19/23 17:25, Jens Axboe wrote:
>> Some applications don't necessarily care about io_uring not blocking for
>> request issue, they simply want to use io_uring for batched submission
>> of IO. However, io_uring will always do non-blocking issues, and for
>> some request types, there's simply no support for doing non-blocking
>> issue and hence they get punted to io-wq unconditionally. If the
>> application doesn't care about issue potentially blocking, this causes
>> a performance slowdown as thread offload is not nearly as efficient as
>> inline issue.
>>
>> Add support for configuring the ring with IORING_SETUP_NO_OFFLOAD, and
>> add an IORING_ENTER_NO_OFFLOAD flag to io_uring_enter(2). If either one
>> of these is set, then io_uring will ignore the non-block issue attempt
>> for any file which we cannot poll for readiness. The simplified io_uring
>> issue model looks as follows:
>>
>> 1) Non-blocking issue is attempted for IO. If successful, we're done for
>>     now.
>>
>> 2) Case 1 failed. Now we have two options
>>        a) We can poll the file. We arm poll, and we're done for now
>>        until that triggers.
>>         b) File cannot be polled, we punt to io-wq which then does a
>>        blocking attempt.
>>
>> If either of the NO_OFFLOAD flags are set, we should never hit case
>> 2b. Instead, case 1 would issue the IO without the non-blocking flag
>> being set and perform an inline completion.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>>   include/linux/io_uring_types.h |  3 +++
>>   include/uapi/linux/io_uring.h  |  7 +++++++
>>   io_uring/io_uring.c            | 26 ++++++++++++++++++++------
>>   io_uring/io_uring.h            |  2 +-
>>   io_uring/sqpoll.c              |  3 ++-
>>   5 files changed, 33 insertions(+), 8 deletions(-)
>>
>> diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
>> index 4dd54d2173e1..c54f3fb7ab1a 100644
>> --- a/include/linux/io_uring_types.h
>> +++ b/include/linux/io_uring_types.h
>> @@ -403,6 +403,7 @@ enum {
>>       REQ_F_APOLL_MULTISHOT_BIT,
>>       REQ_F_CLEAR_POLLIN_BIT,
>>       REQ_F_HASH_LOCKED_BIT,
>> +    REQ_F_NO_OFFLOAD_BIT,
>>       /* keep async read/write and isreg together and in order */
>>       REQ_F_SUPPORT_NOWAIT_BIT,
>>       REQ_F_ISREG_BIT,
>> @@ -475,6 +476,8 @@ enum {
>>       REQ_F_CLEAR_POLLIN    = BIT_ULL(REQ_F_CLEAR_POLLIN_BIT),
>>       /* hashed into ->cancel_hash_locked, protected by ->uring_lock */
>>       REQ_F_HASH_LOCKED    = BIT_ULL(REQ_F_HASH_LOCKED_BIT),
>> +    /* don't offload to io-wq, issue blocking if needed */
>> +    REQ_F_NO_OFFLOAD    = BIT_ULL(REQ_F_NO_OFFLOAD_BIT),
>>   };
>>     typedef void (*io_req_tw_func_t)(struct io_kiocb *req, struct io_tw_state *ts);
>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
>> index 0716cb17e436..ea903a677ce9 100644
>> --- a/include/uapi/linux/io_uring.h
>> +++ b/include/uapi/linux/io_uring.h
>> @@ -173,6 +173,12 @@ enum {
>>    */
>>   #define IORING_SETUP_DEFER_TASKRUN    (1U << 13)
>>   +/*
>> + * Don't attempt non-blocking issue on file types that would otherwise
>> + * punt to io-wq if they cannot be completed non-blocking.
>> + */
>> +#define IORING_SETUP_NO_OFFLOAD        (1U << 14)
>> +
>>   enum io_uring_op {
>>       IORING_OP_NOP,
>>       IORING_OP_READV,
>> @@ -443,6 +449,7 @@ struct io_cqring_offsets {
>>   #define IORING_ENTER_SQ_WAIT        (1U << 2)
>>   #define IORING_ENTER_EXT_ARG        (1U << 3)
>>   #define IORING_ENTER_REGISTERED_RING    (1U << 4)
>> +#define IORING_ENTER_NO_OFFLOAD        (1U << 5)
>>     /*
>>    * Passed in for io_uring_setup(2). Copied back with updated info on success
>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
>> index 9568b5e4cf87..04770b06de16 100644
>> --- a/io_uring/io_uring.c
>> +++ b/io_uring/io_uring.c
>> @@ -1947,6 +1947,10 @@ static int io_issue_sqe(struct io_kiocb *req, unsigned int issue_flags)
>>       if (unlikely(!io_assign_file(req, def, issue_flags)))
>>           return -EBADF;
>>   +    if (req->flags & REQ_F_NO_OFFLOAD &&
>> +        (!req->file || !file_can_poll(req->file)))
>> +        issue_flags &= ~IO_URING_F_NONBLOCK;
>> +
>>       if (unlikely((req->flags & REQ_F_CREDS) && req->creds != current_cred()))
>>           creds = override_creds(req->creds);
>>   @@ -2337,7 +2341,7 @@ static __cold int io_submit_fail_init(const struct io_uring_sqe *sqe,
>>   }
>>     static inline int io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
>> -             const struct io_uring_sqe *sqe)
>> +             const struct io_uring_sqe *sqe, bool no_offload)
>>       __must_hold(&ctx->uring_lock)
>>   {
>>       struct io_submit_link *link = &ctx->submit_state.link;
>> @@ -2385,6 +2389,9 @@ static inline int io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
>>           return 0;
>>       }
>>   +    if (no_offload)
>> +        req->flags |= REQ_F_NO_OFFLOAD;
> 
> Shouldn't it be a part of the initial "in syscall" submission
> but not extended to tw? I'd say it should, otherwise it risks
> making !DEFER_TASKRUN totally unpredictable. E.g. any syscall
> can try to execute tw and get stuck waiting in there.

Yeah, it should probably be ignore outside of off io_uring_enter(2)
submissions. If we do that, we could drop the flag too (and the flags
extension).

-- 
Jens Axboe