Re: [PATCH 5/6] io_uring: implement multishot mode for accept

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

On 9/4/21 11:39 PM, Pavel Begunkov wrote:
> On 9/3/21 12:00 PM, Hao Xu wrote:
>> Refactor io_accept() to support multishot mode.
> 
> Multishot with the fixed/direct mode sounds weird (considering that
> the slot index is specified by userspace), let's forbid them.
> 
> io_accept_prep() {
> 	if (accept->file_slot && (flags & MULTISHOT))
> 		return -EINVAL;
> 	...
> }

Ah, never mind, it's already there in 6/6

> 
>> theoretical analysis:
>>   1) when connections come in fast
>>     - singleshot:
>>               add accept sqe(userpsace) --> accept inline
>>                               ^                 |
>>                               |-----------------|
>>     - multishot:
>>              add accept sqe(userspace) --> accept inline
>>                                               ^     |
>>                                               |--*--|
>>
>>     we do accept repeatedly in * place until get EAGAIN
>>
>>   2) when connections come in at a low pressure
>>     similar thing like 1), we reduce a lot of userspace-kernel context
>>     switch and useless vfs_poll()
>>
>> tests:
>> Did some tests, which goes in this way:
>>
>>   server    client(multiple)
>>   accept    connect
>>   read      write
>>   write     read
>>   close     close
>>
>> Basically, raise up a number of clients(on same machine with server) to
>> connect to the server, and then write some data to it, the server will
>> write those data back to the client after it receives them, and then
>> close the connection after write return. Then the client will read the
>> data and then close the connection. Here I test 10000 clients connect
>> one server, data size 128 bytes. And each client has a go routine for
>> it, so they come to the server in short time.
>> test 20 times before/after this patchset, time spent:(unit cycle, which
>> is the return value of clock())
>> before:
>>   1930136+1940725+1907981+1947601+1923812+1928226+1911087+1905897+1941075
>>   +1934374+1906614+1912504+1949110+1908790+1909951+1941672+1969525+1934984
>>   +1934226+1914385)/20.0 = 1927633.75
>> after:
>>   1858905+1917104+1895455+1963963+1892706+1889208+1874175+1904753+1874112
>>   +1874985+1882706+1884642+1864694+1906508+1916150+1924250+1869060+1889506
>>   +1871324+1940803)/20.0 = 1894750.45
>>
>> (1927633.75 - 1894750.45) / 1927633.75 = 1.65%
>>
>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>> ---
>>
>> not sure if we should cancel it when io_cqring_fill_event() reurn false
>>
>>  fs/io_uring.c | 34 +++++++++++++++++++++++++++++-----
>>  1 file changed, 29 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index dae7044e0c24..eb81d37dce78 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -4885,16 +4885,18 @@ static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>  
>>  static int io_accept(struct io_kiocb *req, unsigned int issue_flags)
>>  {
>> +	struct io_ring_ctx *ctx = req->ctx;
>>  	struct io_accept *accept = &req->accept;
>>  	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
>>  	unsigned int file_flags = force_nonblock ? O_NONBLOCK : 0;
>>  	bool fixed = !!accept->file_slot;
>>  	struct file *file;
>> -	int ret, fd;
>> +	int ret, ret2 = 0, fd;
>>  
>>  	if (req->file->f_flags & O_NONBLOCK)
>>  		req->flags |= REQ_F_NOWAIT;
>>  
>> +retry:
>>  	if (!fixed) {
>>  		fd = __get_unused_fd_flags(accept->flags, accept->nofile);
>>  		if (unlikely(fd < 0))
>> @@ -4906,20 +4908,42 @@ static int io_accept(struct io_kiocb *req, unsigned int issue_flags)
>>  		if (!fixed)
>>  			put_unused_fd(fd);
>>  		ret = PTR_ERR(file);
>> -		if (ret == -EAGAIN && force_nonblock)
>> -			return -EAGAIN;
>> +		if (ret == -EAGAIN && force_nonblock) {
>> +			if ((req->flags & (REQ_F_APOLL_MULTISHOT | REQ_F_POLLED)) ==
>> +			    (REQ_F_APOLL_MULTISHOT | REQ_F_POLLED))
>> +				ret = 0;
>> +			return ret;
>> +		}
>>  		if (ret == -ERESTARTSYS)
>>  			ret = -EINTR;
>>  		req_set_fail(req);
>>  	} else if (!fixed) {
>>  		fd_install(fd, file);
>>  		ret = fd;
>> +		/*
>> +		 * if it's in multishot mode, let's return -EAGAIN to make it go
>> +		 * into fast poll path
>> +		 */
>> +		if ((req->flags & REQ_F_APOLL_MULTISHOT) && force_nonblock &&
>> +		   !(req->flags & REQ_F_POLLED))
>> +			ret2 = -EAGAIN;
>>  	} else {
>>  		ret = io_install_fixed_file(req, file, issue_flags,
>>  					    accept->file_slot - 1);
>>  	}
>> -	__io_req_complete(req, issue_flags, ret, 0);
>> -	return 0;
>> +
>> +	if (req->flags & REQ_F_APOLL_MULTISHOT) {
>> +		spin_lock(&ctx->completion_lock);
>> +		if (io_cqring_fill_event(ctx, req->user_data, ret, 0)) {
>> +			io_commit_cqring(ctx);
>> +			ctx->cq_extra++;
>> +		}
>> +		spin_unlock(&ctx->completion_lock);
>> +		goto retry;
>> +	} else {
>> +		__io_req_complete(req, issue_flags, ret, 0);
>> +	}
>> +	return ret2;
>>  }
>>  
>>  static int io_connect_prep_async(struct io_kiocb *req)
>>
> 

-- 
Pavel Begunkov