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

[Douglas Gilbert]

James Bottomley wrote:
> On Thu, 2008-01-10 at 16:46 -0500, Pete Wyckoff wrote:
> > James.Bottomley@xxxxxxxxxxxxxxxxxxxxx wrote on Thu, 10 Jan 2008 14:55 -0600:
> > > On Thu, 2008-01-10 at 15:43 -0500, Pete Wyckoff wrote:
> > > > fujita.tomonori@xxxxxxxxxxxxx wrote on Wed, 09 Jan 2008 09:11 +0900:
> > > > > On Tue, 8 Jan 2008 17:09:18 -0500
> > > > > Pete Wyckoff <pw@xxxxxxx> 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
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