Re: get_fs_excl/put_fs_excl/has_fs_excl

Re: get_fs_excl/put_fs_excl/has_fs_excl

[Theodore Tso]

On Fri, Apr 24, 2009 at 08:40:47PM +0200, Christoph Hellwig wrote:
> On Thu, Apr 23, 2009 at 09:21:24PM +0200, Jens Axboe wrote:
> > The intent was to add some sort of notification mechanism from the file
> > system to inform the IO scheduler (and others?) that this process is how
> > holding a file system wide resource. So if you have a low priority
> > process getting access to such a resource, you want to boost its
> > priority to avoid higher priority apps getting stuck beind it. Sort of a
> > poor mans priority inheritance.
> > 
> > It would be wonderful if you could kick this process more into gear on
> > the fs side...

I have to agree with Christoph; it would be nice if this were actually
documented somewhere.  Filesystem authors can't do something if they
don't understand what the semantics are and how it is supposed to be
used!

I'm kind of curious why you implemented things in this way, though.
Is there a reason why the bosting is happening deep in the guts of the
cfq code, instead of in blk-core.c when the submission of the block
I/O request is processed?

> So what are the calls in lock_super/unlock_super supposed to be for?
> ->write_super?  While that can sync bits out most of the heavy lifting
> is now done in ->sync_fs for most filesystems.  ->remount_fs?  This is
> going to block all other I/O anyway.  ->put_super?  Surely not :)
> 
> ext3/4 internal bits?  Doesn't seem to be used for any journal related
> activity but mostly as protection against resizing (the whole lock_super
> usage in ext3/4 looks odd to me, interestingly there's none at all in
> ext2.  Maybe someone of the extN crowd should audit and get rid of it in
> favour of a better fs-specific lock)

Yeah, the use of lock_super is definitely very funny in ext3/4.  There
seems to be 3 primary usages; one is blocking write_super(), although
I'm not entirely sure that's needed in all of the places where we do
it.  Another is in protecting the orphan list handling; and the final
one seems to be in the resizing handling.  The last
seems... interesting, especially given this comment:

	/*
	 * We need to protect s_groups_count against other CPUs seeing
	 * inconsistent state in the superblock.
	 *
	 * The precise rules we use are:
	 *
	 * * Writers of s_groups_count *must* hold lock_super
	 * AND
	 * * Writers must perform a smp_wmb() after updating all dependent
	 *   data and before modifying the groups count
	 *
	 * * Readers must hold lock_super() over the access
	 * OR
	 * * Readers must perform an smp_rmb() after reading the groups count
	 *   and before reading any dependent data.
	 *
	 * NB. These rules can be relaxed when checking the group count
	 * while freeing data, as we can only allocate from a block
	 * group after serialising against the group count, and we can
	 * only then free after serialising in turn against that
	 * allocation.
	 */

... but mballoc.c appears not to follow the above protocol at all, as
it relates to using smp_rmb() --- although balloc.c does.  Fortunately
resizes don't happen all that often, but there is definitely some
scary potential problems hiding here, I suspect.

						- Ted

Re: get_fs_excl/put_fs_excl/has_fs_excl

[Jens Axboe]

On Sat, Apr 25 2009, Theodore Tso wrote:
> On Fri, Apr 24, 2009 at 08:40:47PM +0200, Christoph Hellwig wrote:
> > On Thu, Apr 23, 2009 at 09:21:24PM +0200, Jens Axboe wrote:
> > > The intent was to add some sort of notification mechanism from the file
> > > system to inform the IO scheduler (and others?) that this process is how
> > > holding a file system wide resource. So if you have a low priority
> > > process getting access to such a resource, you want to boost its
> > > priority to avoid higher priority apps getting stuck beind it. Sort of a
> > > poor mans priority inheritance.
> > > 
> > > It would be wonderful if you could kick this process more into gear on
> > > the fs side...
> 
> I have to agree with Christoph; it would be nice if this were actually
> documented somewhere.  Filesystem authors can't do something if they
> don't understand what the semantics are and how it is supposed to be
> used!

I don't disagree, the project (unfortunately) never really went
anywhere. THe half-assed implementation was meant to be picked up by fs
people. I guess that's what is happening now, so it's a belated success
:-)

> I'm kind of curious why you implemented things in this way, though.
> Is there a reason why the bosting is happening deep in the guts of the
> cfq code, instead of in blk-core.c when the submission of the block
> I/O request is processed?

You would need to implement a lot more logic in the block layer to
handle it there, as it stands it's basically a scheduler decision. So
the positioning is right imho, the placement of fs hooks is probably
mostly crap and could do with some work.

-- 
Jens Axboe

Re: get_fs_excl/put_fs_excl/has_fs_excl

[Theodore Tso]

On Mon, Apr 27, 2009 at 11:53:39AM +0200, Jens Axboe wrote:
> > I'm kind of curious why you implemented things in this way, though.
> > Is there a reason why the bosting is happening deep in the guts of the
> > cfq code, instead of in blk-core.c when the submission of the block
> > I/O request is processed?
> 
> You would need to implement a lot more logic in the block layer to
> handle it there, as it stands it's basically a scheduler decision. So
> the positioning is right imho, the placement of fs hooks is probably
> mostly crap and could do with some work.

The question is whether you see this in terms of a scheduler decision
or in terms of an I/O priority issue.  At the moment I agree it's a
scheduler decision (which to be honest is implemented in somewhat of a
hacky way --- which I suspect won't bother you since, you yourself
called it "half-assed" :-) which happens to be implemented in the I/O
scheduler.

I tend to think of it more as an I/O priority issue, and specifically,
as you put it, an priority inversion issue, but much of that is no
doubt influenced by how I did the patches to reduce the fsync()
latencies in ext3 and ext4.  And indeed the
get_fs_excl()/put_fs_excl() paradigm doesn't really work well for
ext3/ext4 since all of the work which grabs a filesystem-wide
"exclusive lock" is done in a separate process, kjournald.  Hence with
the exception of freeze and unfreeze --- and while this might be
considered irresponsible for a system administrator to freeze a
filesystem in a ionice'd process, I could imagine a badly written
backup script which created a snapshot while being ionice'd --- ext3/4
can't really very profitably use get_fs_excl()/put_fs_excl().

Maybe ext3/ext4 are a special case, but perhaps we should nevertheless
ask some fundamental design questions about the get/put_fs_excl()
interface.

*) Most filesystems will go to great lengths to avoid having any kind
   of fs-wide "exclusive lock", simply because of the disastrous
   performance impacts.  This is *why* in ext3/ext4, we try to do most
   of the commit work in the context of another process, and normally
   usually we let other filesystem operations run in the "current
   transaction" while we let the "committing transaction" complete.
   If you have too many programs running fsync() this tends to screw
   things up, but that's a separate question.  So in practice, there
   really shouldn't be that many "fs-wide" locks.  

   On the other hand, there can be more subtle forms of I/O priority
   inversion; suppose a low priority process has grabbed a mutex which
   protects a directory, and a high (I/O) priority process needs
   access to the same directory.  Do we care about trying to solve
   that issue?

*) Do we only want to support instances where the fs-wide resource is
   held in kernel-space only, or do we want to support things like the
   FREEZE ioctl, where the filesystem has been frozen --- the very
   definition of an I/O wide resource?  (I would argue no, for
   simplicity's sake but document the fact that the well-written
   program using the FREEZE ioctl should strongly consider bumping up
   its I/O and possibly CPU priority levels to minimize the impact on
   the rest of the system.  Since the FREEZE ioctl requires root
   privileges, it's fair to assume a certain amount of competence by
   the users of this interface.)  If the answer to this question is
   no, then we can add warning/debugging code which warns if the
   filesystem ever tries returning to userspace with an elevated
   get_fs_excl() count.

*) Do we only care about processes whose I/O priority is below the
   default?  (i.e., either in the idle class, or in a low-priority
   best efforts class) What if the concern is a real-time process
   which is being blocked by a default I/O priority process taking its
   time while holding some fs-wide resource?

   If the answer to the previous question is no, it becomes more
   reasonable to consider bump the submission priority of the process
   in question to the highest priority "best efforts" level.  After
   all, if this truly is a "filesystem-wide" resource, then no one is
   going to make forward progress relating to this block device unless
   and until the filesystem-wide lock is resolved.  Also, if we don't
   allow this situation to return to userspace, presumably the
   kernel-code involved will only be writing to the block-device in
   question.  (This might not be entirely true if in the case of the
   sendfile(2) syscall, but currently we can only read from
   filesystems with sendfile, and so presumably a filesystem would
   never call get_fs_excl why servicing a sendfile request.)

*) Is implementing the bulk of this in the cfq scheduler really the
   best place to do this?  To explore something completely different,
   what if the filesystem simply explicitly set I/O priority levels in
   its block I/O submissions, and provided optional callback functions
   which could be used by the page writeback routines to determine the
   appropriate I/O priority level that should be used given a
   particular filesystem and inode number.  (That actually could be
   used to provide another cool function --- we could expose to
   userspace the concept that particular inode should always have its
   I/O go out with a higher priority, perhaps via chattr flag.)

   Basically, the argument here is that we already have the
   appropriate mechanism for ordering I/O requests, which is I/O
   priority mechanism, and the policy really needs to be set by the
   filesystem --- and it might be far more than just "do we have a
   filesystem-wide exclusive lock" or not.

What do other filesystem developers think?

						- Ted

Re: get_fs_excl/put_fs_excl/has_fs_excl

[Jamie Lokier]

Theodore Tso wrote:
> *) Do we only care about processes whose I/O priority is below the
>    default?  (i.e., either in the idle class, or in a low-priority
>    best efforts class) What if the concern is a real-time process
>    which is being blocked by a default I/O priority process taking its
>    time while holding some fs-wide resource?
> 
>    If the answer to the previous question is no, it becomes more
>    reasonable to consider bump the submission priority of the process
>    in question to the highest priority "best efforts" level.  After
>    all, if this truly is a "filesystem-wide" resource, then no one is
>    going to make forward progress relating to this block device unless
>    and until the filesystem-wide lock is resolved.  Also, if we don't
>    allow this situation to return to userspace, presumably the
>    kernel-code involved will only be writing to the block-device in
>    question.  (This might not be entirely true if in the case of the
>    sendfile(2) syscall, but currently we can only read from
>    filesystems with sendfile, and so presumably a filesystem would
>    never call get_fs_excl why servicing a sendfile request.)
> 
> *) Is implementing the bulk of this in the cfq scheduler really the
>    best place to do this?  To explore something completely different,
>    what if the filesystem simply explicitly set I/O priority levels in
>    its block I/O submissions, and provided optional callback functions
>    which could be used by the page writeback routines to determine the
>    appropriate I/O priority level that should be used given a
>    particular filesystem and inode number.  (That actually could be
>    used to provide another cool function --- we could expose to
>    userspace the concept that particular inode should always have its
>    I/O go out with a higher priority, perhaps via chattr flag.)
> 
>    Basically, the argument here is that we already have the
>    appropriate mechanism for ordering I/O requests, which is I/O
>    priority mechanism, and the policy really needs to be set by the
>    filesystem --- and it might be far more than just "do we have a
>    filesystem-wide exclusive lock" or not.

Personally, I'm interested in the following:

    - A process with RT I/O priority and RT CPU priority is reading
      a series of files from disk.  It should be very reliable at this.

    - Other normal I/O priority and normal CPU priority processes are
      reading and writing the disk.

I would like the first process to have a guaranteed minimum I/O
performance: it should continuously make progress, even when it needs
to read some file metadata which overlaps a page affected by the other
processes.  I don't mind all the interference from disk head seeks and
so on, but I would like the I/O that the first process depends on to
have RT I/O priority - including when it's waiting on I/O initiated by
another process and the normal I/O priority queue is full.

So, I'm not exactly sure, but I think what I need for that is:

    - I/O priority boosting (re-queuing in the elevator) to fix the
      inversion when waiting on I/O which was previously queued with
      normal I/O priority, and

    - Task priority boosting when waiting on a filesystem resource
      which is held by a normal priority task.

(I'm not sure if generic task priority boosting is already addressed to some
extent in the RT-PREEMPT Linux tree.)

-- Jamie

Re: get_fs_excl/put_fs_excl/has_fs_excl

[Theodore Tso]

On Mon, Apr 27, 2009 at 03:47:42PM +0100, Jamie Lokier wrote:
> Personally, I'm interested in the following:
> 
>     - A process with RT I/O priority and RT CPU priority is reading
>       a series of files from disk.  It should be very reliable at this.
> 
>     - Other normal I/O priority and normal CPU priority processes are
>       reading and writing the disk.
> 
> I would like the first process to have a guaranteed minimum I/O
> performance: it should continuously make progress, even when it needs
> to read some file metadata which overlaps a page affected by the other
> processes.

That's pretty easy.  The much harder and much more interesting problem
is if the process with RT I/O and CPU priority is *writing* a series
of files to disk, and not just reading from disk.

> I don't mind all the interference from disk head seeks and
> so on, but I would like the I/O that the first process depends on to
> have RT I/O priority - including when it's waiting on I/O initiated by
> another process and the normal I/O priority queue is full.
> 
> So, I'm not exactly sure, but I think what I need for that is:
> 
>     - I/O priority boosting (re-queuing in the elevator) to fix the
>       inversion when waiting on I/O which was previously queued with
>       normal I/O priority, and
> 
>     - Task priority boosting when waiting on a filesystem resource
>       which is held by a normal priority task.

For the latter, I can't think of a filesystem where we would block a
read operation for long time just because someone was holding some
kind of filesytem-wide lock.  A spinlock, maybe, but the only time it
makes sense to worry about boosting an I/O priority is if we're going
to be blocing a filesystem for milliseconds or more, and not just a
few tens of microseconds.  

All of the latency problems people have been complaining about, such
as the infamous firefox fsync() problem, all involved write
operations, and specifically fsync(), and maybe a heavy read-workload
interfered with a write, but I can't think of a situation where a
real-time read operation would be disrupted by normal priority reads
and writes.

For the former, where a real-time read request gets blocked because
the read request for that block had already been submitted --- at a
lower priority --- that's something that should be solvable purely in
core block layer and in the I/O scheduler layer, I would expect.

						- Ted

Re: get_fs_excl/put_fs_excl/has_fs_excl

[Jamie Lokier]

Theodore Tso wrote:
> On Mon, Apr 27, 2009 at 03:47:42PM +0100, Jamie Lokier wrote:
> > Personally, I'm interested in the following:
> > 
> >     - A process with RT I/O priority and RT CPU priority is reading
> >       a series of files from disk.  It should be very reliable at this.
> > 
> >     - Other normal I/O priority and normal CPU priority processes are
> >       reading and writing the disk.
> > 
> > I would like the first process to have a guaranteed minimum I/O
> > performance: it should continuously make progress, even when it needs
> > to read some file metadata which overlaps a page affected by the other
> > processes.
> 
> That's pretty easy.  The much harder and much more interesting problem
> is if the process with RT I/O and CPU priority is *writing* a series
> of files to disk, and not just reading from disk.

...

> I can't think of a filesystem where we would block a
> read operation for long time just because someone was holding some
> kind of filesytem-wide lock.  A spinlock, maybe, but the only time it
> makes sense to worry about boosting an I/O priority is if we're going
> to be blocing a filesystem for milliseconds or more, and not just a
> few tens of microseconds.

...

> For the former, where a real-time read request gets blocked because
> the read request for that block had already been submitted --- at a
> lower priority --- that's something that should be solvable purely in
> core block layer and in the I/O scheduler layer, I would expect.

That's great to know, thanks.  I will poke at the block layer and I/O
scheduler then, see where it leads.

Thanks,
-- Jamie