[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Andrew Morton]

On Sat,  9 Jun 2012 00:41:03 +0300
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> There's no reason to call rcu_barrier() on every deactivate_locked_super().
> We only need to make sure that all delayed rcu free inodes are flushed
> before we destroy related cache.
> 
> Removing rcu_barrier() from deactivate_locked_super() affects some
> fas paths. E.g. on my machine exit_group() of a last process in IPC
> namespace takes 0.07538s. rcu_barrier() takes 0.05188s of that time.

What an unpleasant patch.  Is final-process-exiting-ipc-namespace a
sufficiently high-frequency operation to justify the change?

I don't really understand what's going on here.  Are you saying that
there is some filesystem against which we run deactivate_locked_super()
during exit_group(), and that this filesystem doesn't use rcu-freeing
of inodes?  The description needs this level of detail, please.


The implementation would be less unpleasant if we could do the
rcu_barrier() in kmem_cache_destroy().  I can't see a way of doing that
without adding a dedicated slab flag, which would require editing all
the filesystems anyway.


(kmem_cache_destroy() already has an rcu_barrier().  Can we do away
with the private rcu games in the vfs and switch to
SLAB_DESTROY_BY_RCU?)

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Al Viro]

On Sat, Jun 09, 2012 at 01:14:46AM +0300, Kirill A. Shutemov wrote:
> > The implementation would be less unpleasant if we could do the
> > rcu_barrier() in kmem_cache_destroy().  I can't see a way of doing that
> > without adding a dedicated slab flag, which would require editing all
> > the filesystems anyway.
> 
> I think rcu_barrier() for all kmem_cache_destroy() would be too expensive.

You've got to be kidding.  Please, show us the codepath that would be hot
enough to make that too expensive and would contain kmem_cache_destroy().
Note that module unload is *not* a hot path - not on any even remotely sane
use.

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Linus Torvalds]

On Fri, Jun 8, 2012 at 3:23 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> Note that module unload is *not* a hot path - not on any even remotely sane
> use.

Actually, I think we've had distributions that basically did a "load
pretty much everything, and let God sort it out" approach to modules.
I know some people *have* actually worried about module load/unload
performance.

Whether it is "remotely sane" I'm not going to argue for, but ..

                Linus

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Andrew Morton]

On Sat, 9 Jun 2012 01:14:46 +0300
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> On Fri, Jun 08, 2012 at 03:02:53PM -0700, Andrew Morton wrote:
> > On Sat,  9 Jun 2012 00:41:03 +0300
> > "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > 
> > > There's no reason to call rcu_barrier() on every deactivate_locked_super().
> > > We only need to make sure that all delayed rcu free inodes are flushed
> > > before we destroy related cache.
> > > 
> > > Removing rcu_barrier() from deactivate_locked_super() affects some
> > > fas paths. E.g. on my machine exit_group() of a last process in IPC
> > > namespace takes 0.07538s. rcu_barrier() takes 0.05188s of that time.
> > 
> > What an unpleasant patch.  Is final-process-exiting-ipc-namespace a
> > sufficiently high-frequency operation to justify the change?

This, please.

> > I don't really understand what's going on here.  Are you saying that
> > there is some filesystem against which we run deactivate_locked_super()
> > during exit_group(), and that this filesystem doesn't use rcu-freeing
> > of inodes?  The description needs this level of detail, please.

You still haven't explained where this deactivate_locked_super() call
is coming from.  Oh well.

> I think the rcu_barrier() is in wrong place. We need it to safely destroy
> inode cache. deactivate_locked_super() is part of umount() path, but all
> filesystems I've checked have inode cache for whole filesystem, not
> per-mount.

Well from a design perspective, putting the rcu_barrier() in the vfs is
the *correct* place.  Individual filesystems shouldn't be hard-coding
knowledge about vfs internal machinery.

A neater implementation might be to add a kmem_cache* argument to
unregister_filesystem().  If that is non-NULL, unregister_filesystem()
does the rcu_barrier() and destroys the cache.  That way we get to
delete (rather than add) a bunch of code from all filesystems and new
and out-of-tree filesystems cannot forget to perform the rcu_barrier().

> > The implementation would be less unpleasant if we could do the
> > rcu_barrier() in kmem_cache_destroy().  I can't see a way of doing that
> > without adding a dedicated slab flag, which would require editing all
> > the filesystems anyway.
> 
> I think rcu_barrier() for all kmem_cache_destroy() would be too expensive.

That is not what I proposed.

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Al Viro]

On Fri, Jun 08, 2012 at 03:25:50PM -0700, Andrew Morton wrote:

> A neater implementation might be to add a kmem_cache* argument to
> unregister_filesystem().  If that is non-NULL, unregister_filesystem()
> does the rcu_barrier() and destroys the cache.  That way we get to
> delete (rather than add) a bunch of code from all filesystems and new
> and out-of-tree filesystems cannot forget to perform the rcu_barrier().

There's often enough more than one cache, so that one is no-go.

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Andrew Morton]

On Fri, 8 Jun 2012 23:27:34 +0100
Al Viro <viro@ZenIV.linux.org.uk> wrote:

> On Fri, Jun 08, 2012 at 03:25:50PM -0700, Andrew Morton wrote:
> 
> > A neater implementation might be to add a kmem_cache* argument to
> > unregister_filesystem().  If that is non-NULL, unregister_filesystem()
> > does the rcu_barrier() and destroys the cache.  That way we get to
> > delete (rather than add) a bunch of code from all filesystems and new
> > and out-of-tree filesystems cannot forget to perform the rcu_barrier().
> 
> There's often enough more than one cache, so that one is no-go.

kmem_cache** ;)

Which filesystems have multiple inode caches?

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Andrew Morton]

On Sat, 9 Jun 2012 02:31:27 +0300
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> On Fri, Jun 08, 2012 at 03:31:20PM -0700, Andrew Morton wrote:
> > On Fri, 8 Jun 2012 23:27:34 +0100
> > Al Viro <viro@ZenIV.linux.org.uk> wrote:
> > 
> > > On Fri, Jun 08, 2012 at 03:25:50PM -0700, Andrew Morton wrote:
> > > 
> > > > A neater implementation might be to add a kmem_cache* argument to
> > > > unregister_filesystem().  If that is non-NULL, unregister_filesystem()
> > > > does the rcu_barrier() and destroys the cache.  That way we get to
> > > > delete (rather than add) a bunch of code from all filesystems and new
> > > > and out-of-tree filesystems cannot forget to perform the rcu_barrier().
> > > 
> > > There's often enough more than one cache, so that one is no-go.
> > 
> > kmem_cache** ;)
> > 
> > Which filesystems have multiple inode caches?
> 
> Multiple inode caches? No.
> Multiple caches with call_rcu() free? See btrfs or gfs2.

OK.  But for those non-inode caches, the rcu treatment is private to
the filesystem.  Hence it is appropriate that the filesystem call
rcu_barrier() for those caches.

But in the case of the inode caches, the rcu treatment is a vfs thing,
so it is the vfs which should perform the rcu_barrier().

This is a red herring - those non-inode caches have nothing to do with
the issue we're dicussing.


So how about open-coding the rcu_barrier() in btrfs and gfs2 for the
non-inode caches (which is the appropriate place), and hand the inode
cache over to the vfs for treatment (which is the appropriate place).

The downside is that btrfs and gfs2 will do an extra rcu_barrier() at
umount time.  Shrug.

If they really want to super-optimise that, they can skip the private
rcu_barrier() call and assume that the vfs will be doing it.  Not a
good idea, IMO.

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Linus Torvalds]

On Fri, Jun 8, 2012 at 4:37 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
>
> So how about open-coding the rcu_barrier() in btrfs and gfs2 for the
> non-inode caches (which is the appropriate place), and hand the inode
> cache over to the vfs for treatment (which is the appropriate place).

The thing is, none of the inode caches are really up to the VFS. They
are per-filesystem caches, that just *embed* an inode as part of the
bigger ext4_inode or whatever. But apart from the fact that the
embedded inode requires them to then use the proper "call_rcu()" stuff
to do the delayed free, they really are pretty much filesystem data
structures. The VFS layer can neither free them or allocate them,
since the VFS layer doesn't even know how big the structures are, or
where the inodes are embedded, or how to initialize them (or even when
to allocate them).

Of course, if you just mean having a VFS wrapper that does

    static void vfs_inode_kmem_cache_destroy(struct kmem_cache *cachep)
    {
        rcu_barrier();
        kmem_cache_destroy(cachep);
    }

then we could do that. Not much better than what Kirill's patch did,
but at least we could have that comment in just one single place.

                Linus

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Andrew Morton]

On Fri, 8 Jun 2012 16:46:47 -0700 Linus Torvalds <torvalds@linux-foundation.org> wrote:

> Of course, if you just mean having a VFS wrapper that does
> 
>     static void vfs_inode_kmem_cache_destroy(struct kmem_cache *cachep)
>     {
>         rcu_barrier();
>         kmem_cache_destroy(cachep);
>     }
> 
> then we could do that. Not much better than what Kirill's patch did,
> but at least we could have that comment in just one single place.

That's conceptually what I meant.  But it has the problem that new and
out-of-tree filesystems might forget to do it.  Which is why I suggest
adding a kmem_cache* argument to unregister_filesystem() for this.

It's a bit awkward, and the fs can pass in NULL if it knows what it's
doing.  But it's reliable.

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Marco Stornelli]

Il 09/06/2012 02:28, Andrew Morton ha scritto:
> On Fri, 8 Jun 2012 16:46:47 -0700 Linus Torvalds<torvalds@linux-foundation.org>  wrote:
>
>> Of course, if you just mean having a VFS wrapper that does
>>
>>      static void vfs_inode_kmem_cache_destroy(struct kmem_cache *cachep)
>>      {
>>          rcu_barrier();
>>          kmem_cache_destroy(cachep);
>>      }
>>
>> then we could do that. Not much better than what Kirill's patch did,
>> but at least we could have that comment in just one single place.
>
> That's conceptually what I meant.  But it has the problem that new and
> out-of-tree filesystems might forget to do it.  Which is why I suggest
> adding a kmem_cache* argument to unregister_filesystem() for this.
>
> It's a bit awkward, and the fs can pass in NULL if it knows what it's
> doing.  But it's reliable.
> --

The call of rcu_barrier should be mandatory for the "unload fs module" 
problem, right? If the fs is compiled statically maybe we could avoid 
it, but (eventually) this kind of decision is per-fs, so this could be a 
clue that the call of rcu_barrier maybe is inside each fs not in VFS.

Marco

[Ocfs2-devel] [RFC, PATCH, RESEND] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Andrew Morton]

On Sat, 09 Jun 2012 09:06:28 +0200 Marco Stornelli <marco.stornelli@gmail.com> wrote:

> Il 09/06/2012 02:28, Andrew Morton ha scritto:
> > On Fri, 8 Jun 2012 16:46:47 -0700 Linus Torvalds<torvalds@linux-foundation.org>  wrote:
> >
> >> Of course, if you just mean having a VFS wrapper that does
> >>
> >>      static void vfs_inode_kmem_cache_destroy(struct kmem_cache *cachep)
> >>      {
> >>          rcu_barrier();
> >>          kmem_cache_destroy(cachep);
> >>      }
> >>
> >> then we could do that. Not much better than what Kirill's patch did,
> >> but at least we could have that comment in just one single place.
> >
> > That's conceptually what I meant.  But it has the problem that new and
> > out-of-tree filesystems might forget to do it.  Which is why I suggest
> > adding a kmem_cache* argument to unregister_filesystem() for this.
> >
> > It's a bit awkward, and the fs can pass in NULL if it knows what it's
> > doing.  But it's reliable.
> > --
> 
> The call of rcu_barrier should be mandatory for the "unload fs module" 
> problem, right? If the fs is compiled statically maybe we could avoid 
> it, but (eventually) this kind of decision is per-fs, so this could be a 
> clue that the call of rcu_barrier maybe is inside each fs not in VFS.
> 

No, this is unrelated to module unloading and the problem affects
statically linked filesystems also.  The requirement is that all inodes
which are pending rcu freeing be flushed (and freed) before their cache
is destroyed in kmem_cache_destroy().

And...  it seems that I misread what's going on.  The individual
filesystems are doing the rcu freeing of their inodes, so it is
appropriate that they also call rcu_barrier() prior to running
kmem_cache_free().  Which is what Kirill's patch does.  oops.

[Ocfs2-devel] [RFC, PATCH] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Linus Torvalds]

On Fri, Jun 8, 2012 at 3:00 PM, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> IIUC, moving rcu_barrier() up should help, but I can't say that I fully
> understand SLAB_DESTROY_BY_RCU semantics.

.. hmm. I think you may be right. Even if we do move it up, we
probably shouldn't use it.

We don't even want SLAB_DESTROY_BY_RCU, since we do the delayed RCU
free for other reasons anyway, so it would duplicate the RCU delaying
and cause problems. I forgot about that little complication.

We could have a separate "RCU_BARRIER_ON_DESTROY" thing, but that's
just silly too.

Maybe your patch is the right thing.

            Linus

[Ocfs2-devel] [RFC, PATCH] fs: push rcu_barrier() from deactivate_locked_super() to filesystems

[Al Viro]

On Fri, Jun 08, 2012 at 03:06:20PM -0700, Linus Torvalds wrote:
> .. hmm. I think you may be right. Even if we do move it up, we
> probably shouldn't use it.
> 
> We don't even want SLAB_DESTROY_BY_RCU, since we do the delayed RCU
> free for other reasons anyway, so it would duplicate the RCU delaying
> and cause problems. I forgot about that little complication.
> 
> We could have a separate "RCU_BARRIER_ON_DESTROY" thing, but that's
> just silly too.

Why not make that rcu_barrier() in there unconditional?  Where are
we creating/destroying caches often enough for that to become a problem?