linux-ext4 - Re: [PATCH 0/8 v2] Non-blocking AIO

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Date:   Mon, 6 Mar 2017 10:06:43 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Avi Kivity <avi@...lladb.com>, Jan Kara <jack@...e.cz>
Cc:     Goldwyn Rodrigues <rgoldwyn@...e.de>, jack@...e.com,
        hch@...radead.org, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH 0/8 v2] Non-blocking AIO

On 03/06/2017 09:59 AM, Avi Kivity wrote:
> 
> 
> On 03/06/2017 06:08 PM, Jens Axboe wrote:
>> On 03/06/2017 08:59 AM, Avi Kivity wrote:
>>> On 03/06/2017 05:38 PM, Jens Axboe wrote:
>>>> On 03/06/2017 08:29 AM, Avi Kivity wrote:
>>>>> On 03/06/2017 05:19 PM, Jens Axboe wrote:
>>>>>> On 03/06/2017 01:25 AM, Jan Kara wrote:
>>>>>>> On Sun 05-03-17 16:56:21, Avi Kivity wrote:
>>>>>>>>> The goal of the patch series is to return -EAGAIN/-EWOULDBLOCK if
>>>>>>>>> any of these conditions are met. This way userspace can push most
>>>>>>>>> of the write()s to the kernel to the best of its ability to complete
>>>>>>>>> and if it returns -EAGAIN, can defer it to another thread.
>>>>>>>>>
>>>>>>>> Is it not possible to push the iocb to a workqueue?  This will allow
>>>>>>>> existing userspace to work with the new functionality, unchanged. Any
>>>>>>>> userspace implementation would have to do the same thing, so it's not like
>>>>>>>> we're saving anything by pushing it there.
>>>>>>> That is not easy because until IO is fully submitted, you need some parts
>>>>>>> of the context of the process which submits the IO (e.g. memory mappings,
>>>>>>> but possibly also other credentials). So you would need to somehow transfer
>>>>>>> this information to the workqueue.
>>>>>> Outside of technical challenges, the API also needs to return EAGAIN or
>>>>>> start blocking at some point. We can't expose a direct connection to
>>>>>> queue work like that, and let any user potentially create millions of
>>>>>> pending work items (and IOs).
>>>>> You wouldn't expect more concurrent events than the maxevents parameter
>>>>> that was supplied to io_setup syscall; it should have reserved any
>>>>> resources needed.
>>>> Doesn't matter what limit you apply, my point still stands - at some
>>>> point you have to return EAGAIN, or block. Returning EAGAIN without
>>>> the caller having flagged support for that change of behavior would
>>>> be problematic.
>>> Doesn't it already return EAGAIN (or some other error) if you exceed
>>> maxevents?
>> It's a setup thing. We check these limits when someone creates an IO
>> context, and carve out the specified entries form our global pool. Then
>> we free those "resources" when the io context is freed.
>>
>> Right now I can setup an IO context with 1000 entries on it, yet that
>> number has NO bearing on when io_submit() would potentially block or
>> return EAGAIN.
>>
>> We can have a huge gap on the intent signaled by io context setup, and
>> the reality imposed by what actually happens on the IO submission side.
> 
> Isn't that a bug?  Shouldn't that 1001st incomplete io_submit() return 
> EAGAIN?
> 
> Just tested it, and maxevents is not respected for this:
> 
> io_setup(1, [0x7fc64537f000])           = 0
> io_submit(0x7fc64537f000, 10, [{pread, fildes=3, buf=0x1eb4000, 
> nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, 
> offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, 
> {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, 
> fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, 
> buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, 
> nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, 
> offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, 
> {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}]) = 10
> 
> which is unexpected, to me.

ioctx_alloc()
{
        [...]

        /*                                                                      
         * We keep track of the number of available ringbuffer slots, to prevent
         * overflow (reqs_available), and we also use percpu counters for this. 
         *                                                                      
         * So since up to half the slots might be on other cpu's percpu counters
         * and unavailable, double nr_events so userspace sees what they        
         * expected: additionally, we move req_batch slots to/from percpu       
         * counters at a time, so make sure that isn't 0:                       
         */                                                                     
        nr_events = max(nr_events, num_possible_cpus() * 4);                    
        nr_events *= 2;                                    
}


-- 
Jens Axboe