Re: [PATCH] bsg : Add support for io vectors in bsg

[Douglas Gilbert <dougg@torque.net>]

James Bottomley wrote:
> On Thu, 2008-01-10 at 16:46 -0500, Pete Wyckoff wrote:
>> James.Bottomley@HansenPartnership.com wrote on Thu, 10 Jan 2008 14:55 -0600:
>>> On Thu, 2008-01-10 at 15:43 -0500, Pete Wyckoff wrote:
>>>> fujita.tomonori@lab.ntt.co.jp wrote on Wed, 09 Jan 2008 09:11 +0900:
>>>>> On Tue, 8 Jan 2008 17:09:18 -0500
>>>>> Pete Wyckoff <pw@osc.edu> wrote:
>>>>>> I took another look at the compat approach, to see if it is feasible
>>>>>> to keep the compat handling somewhere else, without the use of #ifdef
>>>>>> CONFIG_COMPAT and size-comparison code inside bsg.c.  I don't see how.
>>>>>> The use of iovec is within a write operation on a char device.  It's
>>>>>> not amenable to a compat_sys_ or a .compat_ioctl approach.
>>>>>>
>>>>>> I'm partial to #1 because the use of architecture-independent fields
>>>>>> matches the rest of struct sg_io_v4.  But if you don't want to have
>>>>>> another iovec type in the kernel, could we do #2 but just return
>>>>>> -EINVAL if the need for compat is detected?  I.e. change
>>>>>> dout_iovec_count to dout_iovec_length and do the math?
>>>>> If you are ok with removing the write/read interface and just have
>>>>> ioctl, we could can handle comapt stuff like others do. But I think
>>>>> that you (OSD people) really want to keep the write/read
>>>>> interface. Sorry, I think that there is no workaround to support iovec
>>>>> in bsg.
>>>> I don't care about read/write in particular.  But we do need some
>>>> way to launch asynchronous SCSI commands, and currently read/write
>>>> are the only way to do that in bsg.  The reason is to keep multiple
>>>> spindles busy at the same time.
>>> Won't multi-threading the ioctl calls achieve the same effect?  Or do
>>> you trip over BKL there?
>> There's no BKL on (new) ioctls anymore, at least.  A thread per
>> device would be feasible perhaps.  But if you want any sort of
>> pipelining out of the device, esp. in the remote iSCSI case, you
>> need to have a good number of commands outstanding to each device.
>> So a thread per command per device.  Typical iSCSI queue depth of
>> 128 times 16 devices for a small setup is a lot of threads.
> 
> I was actually thinking of a thread per outstanding command.
> 
>> The pthread/pipe latency overhead is not insignificant for fast
>> storage networks too.
>>
>>>> How about these new ioctls instead of read/write:
>>>>
>>>>     SG_IO_SUBMIT - start a new blk_execute_rq_nowait()
>>>>     SG_IO_TEST   - complete and return a previous req
>>>>     SG_IO_WAIT   - wait for a req to finish, interruptibly
>>>>
>>>> Then old write users will instead do ioctl SUBMIT.  Read users will
>>>> do TEST for non-blocking fd, or WAIT for blocking.  And SG_IO could
>>>> be implemented as SUBMIT + WAIT.
>>>>
>>>> Then we can do compat_ioctl and convert up iovecs out-of-line before
>>>> calling the normal functions.
>>>>
>>>> Let me know if you want a patch for this.
>>> Really, the thought of re-inventing yet another async I/O interface
>>> isn't very appealing.
>> I'm fine with read/write, except Tomo is against handling iovecs
>> because of the compat complexity with struct iovec being different
>> on 32- vs 64-bit.  There is a standard way to do "compat" ioctl that
>> hides this handling in a different file (not bsg.c), which is the
>> only reason I'm even considering these ioctls.  I don't care about
>> compat setups per se.
>>
>> Is there another async I/O mechanism?  Userspace builds the CDBs,
>> just needs some way to drop them in SCSI ML.  BSG is almost perfect
>> for this, but doesn't do iovec, leading to lots of memcpy.
> 
> No, it's just that async interfaces in Linux have a long and fairly
> unhappy history.

The sg driver's async interface has been pretty stable for
a long time. The sync SG_IO ioctl is built on top of the
async interface. That makes the async interface extremely
well tested.

The write()/read() async interface in sg does have one
problem: when a command is dispatched via a write()
it would be very useful to get back a tag but that
violates write()'s second argument: 'const void * buf'.
That tag could be useful both for identification of the
response and by task management functions.

I was hoping that the 'flags' field in sgv4 could be used
to implement the variants:
     SG_IO_SUBMIT - start a new blk_execute_rq_nowait()
     SG_IO_TEST   - complete and return a previous req
     SG_IO_WAIT   - wait for a req to finish, interruptibly

that way the existing SG_IO ioctl is sufficient.

And if Tomo doesn't want to do it in the bsg driver,
then it could be done it the sg driver.

Doug Gilbert