Re: [PATCH v2 0/6] kernfs: proposed locking and concurrency improvement

[Ian Kent]

On Thu, 2020-12-17 at 18:09 +0800, Ian Kent wrote:
> On Thu, 2020-12-17 at 16:54 +0800, Fox Chen wrote:
> > On Thu, Dec 17, 2020 at 12:46 PM Ian Kent <raven@xxxxxxxxxx> wrote:
> > > On Tue, 2020-12-15 at 20:59 +0800, Ian Kent wrote:
> > > > On Tue, 2020-12-15 at 16:33 +0800, Fox Chen wrote:
> > > > > On Mon, Dec 14, 2020 at 9:30 PM Ian Kent <raven@xxxxxxxxxx>
> > > > > wrote:
> > > > > > On Mon, 2020-12-14 at 14:14 +0800, Fox Chen wrote:
> > > > > > > On Sun, Dec 13, 2020 at 11:46 AM Ian Kent <
> > > > > > > raven@xxxxxxxxxx
> > > > > > > wrote:
> > > > > > > > On Fri, 2020-12-11 at 10:17 +0800, Ian Kent wrote:
> > > > > > > > > On Fri, 2020-12-11 at 10:01 +0800, Ian Kent wrote:
> > > > > > > > > > > For the patches, there is a mutex_lock in kn-
> > > > > > > > > > > > attr_mutex,
> > > > > > > > > > > as
> > > > > > > > > > > Tejun
> > > > > > > > > > > mentioned here
> > > > > > > > > > > (
> > > > > > > > > > > https://lore.kernel.org/lkml/X8fe0cmu+aq1gi7O@xxxxxxxxxxxxxxx/
> > > > > > > > > > > ),
> > > > > > > > > > > maybe a global
> > > > > > > > > > > rwsem for kn->iattr will be better??
> > > > > > > > > > 
> > > > > > > > > > I wasn't sure about that, IIRC a spin lock could be
> > > > > > > > > > used
> > > > > > > > > > around
> > > > > > > > > > the
> > > > > > > > > > initial check and checked again at the end which
> > > > > > > > > > would
> > > > > > > > > > probably
> > > > > > > > > > have
> > > > > > > > > > been much faster but much less conservative and a
> > > > > > > > > > bit
> > > > > > > > > > more
> > > > > > > > > > ugly
> > > > > > > > > > so
> > > > > > > > > > I just went the conservative path since there was
> > > > > > > > > > so
> > > > > > > > > > much
> > > > > > > > > > change
> > > > > > > > > > already.
> > > > > > > > > 
> > > > > > > > > Sorry, I hadn't looked at Tejun's reply yet and TBH
> > > > > > > > > didn't
> > > > > > > > > remember
> > > > > > > > > it.
> > > > > > > > > 
> > > > > > > > > Based on what Tejun said it sounds like that needs
> > > > > > > > > work.
> > > > > > > > 
> > > > > > > > Those attribute handling patches were meant to allow
> > > > > > > > taking
> > > > > > > > the
> > > > > > > > rw
> > > > > > > > sem read lock instead of the write lock for
> > > > > > > > kernfs_refresh_inode()
> > > > > > > > updates, with the added locking to protect the inode
> > > > > > > > attributes
> > > > > > > > update since it's called from the VFS both with and
> > > > > > > > without
> > > > > > > > the
> > > > > > > > inode lock.
> > > > > > > 
> > > > > > > Oh, understood. I was asking also because lock on kn-
> > > > > > > > attr_mutex
> > > > > > > drags
> > > > > > > concurrent performance.
> > > > > > > 
> > > > > > > > Looking around it looks like kernfs_iattrs() is called
> > > > > > > > from
> > > > > > > > multiple
> > > > > > > > places without a node database lock at all.
> > > > > > > > 
> > > > > > > > I'm thinking that, to keep my proposed change straight
> > > > > > > > forward
> > > > > > > > and on topic, I should just leave
> > > > > > > > kernfs_refresh_inode()
> > > > > > > > taking
> > > > > > > > the node db write lock for now and consider the
> > > > > > > > attributes
> > > > > > > > handling
> > > > > > > > as a separate change. Once that's done we could
> > > > > > > > reconsider
> > > > > > > > what's
> > > > > > > > needed to use the node db read lock in
> > > > > > > > kernfs_refresh_inode().
> > > > > > > 
> > > > > > > You meant taking write lock of kernfs_rwsem for
> > > > > > > kernfs_refresh_inode()??
> > > > > > > It may be a lot slower in my benchmark, let me test it.
> > > > > > 
> > > > > > Yes, but make sure the write lock of kernfs_rwsem is being
> > > > > > taken
> > > > > > not the read lock.
> > > > > > 
> > > > > > That's a mistake I had initially?
> > > > > > 
> > > > > > Still, that attributes handling is, I think, sufficient to
> > > > > > warrant
> > > > > > a separate change since it looks like it might need work,
> > > > > > the
> > > > > > kernfs
> > > > > > node db probably should be kept stable for those attribute
> > > > > > updates
> > > > > > but equally the existence of an instantiated dentry might
> > > > > > mitigate
> > > > > > the it.
> > > > > > 
> > > > > > Some people might just know whether it's ok or not but I
> > > > > > would
> > > > > > like
> > > > > > to check the callers to work out what's going on.
> > > > > > 
> > > > > > In any case it's academic if GCH isn't willing to consider
> > > > > > the
> > > > > > series
> > > > > > for review and possible merge.
> > > > > > 
> > > > > Hi Ian
> > > > > 
> > > > > I removed kn->attr_mutex and changed read lock to write lock
> > > > > for
> > > > > kernfs_refresh_inode
> > > > > 
> > > > > down_write(&kernfs_rwsem);
> > > > > kernfs_refresh_inode(kn, inode);
> > > > > up_write(&kernfs_rwsem);
> > > > > 
> > > > > 
> > > > > Unfortunate, changes in this way make things worse,  my
> > > > > benchmark
> > > > > runs
> > > > > 100% slower than upstream sysfs.  :(
> > > > > open+read+close a sysfs file concurrently took 1000us.
> > > > > (Currently,
> > > > > sysfs with a big mutex kernfs_mutex only takes ~500us
> > > > > for one open+read+close operation concurrently)
> > > > 
> > > > Right, so it does need attention nowish.
> > > > 
> > > > I'll have a look at it in a while, I really need to get a new
> > > > autofs
> > > > release out, and there are quite a few changes, and testing is
> > > > seeing
> > > > a number of errors, some old, some newly introduced. It's
> > > > proving
> > > > difficult.
> > > 
> > > I've taken a breather for the autofs testing and had a look at
> > > this.
> > 
> > Thanks. :)
> > 
> > > I think my original analysis of this was wrong.
> > > 
> > > Could you try this patch please.
> > > I'm not sure how much difference it will make but, in principle,
> > > it's much the same as the previous approach except it doesn't
> > > increase the kernfs node struct size or mess with the other
> > > attribute handling code.
> > > 
> > > Note, this is not even compile tested.
> > 
> > I failed to apply this patch. So based on the original six patches,
> > I
> > manually removed kn->attr_mutex, and added
> > inode_lock/inode_unlock to those two functions, they were like:
> > 
> > int kernfs_iop_getattr(const struct path *path, struct kstat *stat,
> >                        u32 request_mask, unsigned int query_flags)
> > {
> >         struct inode *inode = d_inode(path->dentry);
> >         struct kernfs_node *kn = inode->i_private;
> > 
> >         inode_lock(inode);
> >         down_read(&kernfs_rwsem);
> >         kernfs_refresh_inode(kn, inode);
> >         up_read(&kernfs_rwsem);
> >         inode_unlock(inode);
> > 
> >         generic_fillattr(inode, stat);
> >         return 0;
> > }
> > 
> > int kernfs_iop_permission(struct inode *inode, int mask)
> > {
> >         struct kernfs_node *kn;
> > 
> >         if (mask & MAY_NOT_BLOCK)
> >                 return -ECHILD;
> > 
> >         kn = inode->i_private;
> > 
> >         inode_lock(inode);
> >         down_read(&kernfs_rwsem);
> >         kernfs_refresh_inode(kn, inode);
> >         up_read(&kernfs_rwsem);
> >         inode_unlock(inode);
> > 
> >         return generic_permission(inode, mask);
> > }
> > 
> > But I couldn't boot the kernel and there was no error on the
> > screen.
> > I guess it was deadlocked on /sys creation?? :D
> 
> Right, I guess the locking documentation is out of date. I'm guessing
> the inode lock is taken somewhere over the .permission() call. If
> that
> usage is consistent it's easy fixed, if the usage is inconsistent
> it's
> hard to deal with and amounts to a bug.

Yes, it is called, both shared on open, and exclusive on open
create, and without the inode lock at all at the start of path
resolution.

That can't really be called a VFS bug since .permission() is
meant to check permissions not update the inode.

This is probably what lead to the attr patches I had.

If a suitable place to put a local per-object lock can't be
found for this, other than in the kernfs_node, then it's a
real problem from a contention POV.

What could be done is to make the kernfs node attr_mutex
a pointer and dynamically allocate it but even that is too
costly a size addition to the kernfs node structure as
Tejun has said.

Those patches I referred to clearly aren't finished because
the eighth one is empty, which followed a patch I have titled
"kernfs: make attr_mutex a local kernfs node lock".

I obviously gave up on it when the series was rejected.
But I'll give it some more thought.

Ian

> 
> I'll have another look at it.
> 
> Also, it sounds like I'm working from a more recent series.
> 
> I had 8 patches, dropped the last three and added the one I posted.
> If I can work out what's going on I'll post the series for you to
> check.
> 
> Ian
> 
> > > kernfs: use kernfs read lock in .getattr() and .permission()
> > > 
> > > From: Ian Kent <raven@xxxxxxxxxx>
> > > 
> > > From Documenation/filesystems.rst and (slightly outdated)
> > > comments
> > > in fs/attr.c the inode i_rwsem is used for attribute handling.
> > > 
> > > This lock satisfies the requirememnts needed to reduce lock
> > > contention,
> > > namely a per-object lock needs to be used rather than a file
> > > system
> > > global lock with the kernfs node db held stable for read
> > > operations.
> > > 
> > > In particular it should reduce lock contention seen when calling
> > > the
> > > kernfs .permission() method.
> > > 
> > > The inode methods .getattr() and .permission() do not hold the
> > > inode
> > > i_rwsem lock when called as they are usually read operations.
> > > Also
> > > the .permission() method checks for rcu-walk mode and returns
> > > -ECHILD
> > > to the VFS if it is set. So the i_rwsem lock can be used in
> > > kernfs_iop_getattr() and kernfs_iop_permission() to protect the
> > > inode
> > > update done by kernfs_refresh_inode(). Using this lock allows the
> > > kernfs node db write lock in these functions to be changed to a
> > > read
> > > lock.
> > > 
> > > Signed-off-by: Ian Kent <raven@xxxxxxxxxx>
> > > ---
> > >  fs/kernfs/inode.c |   12 ++++++++----
> > >  1 file changed, 8 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
> > > index ddaf18198935..568037e9efe9 100644
> > > --- a/fs/kernfs/inode.c
> > > +++ b/fs/kernfs/inode.c
> > > @@ -189,9 +189,11 @@ int kernfs_iop_getattr(const struct path
> > > *path, struct kstat *stat,
> > >         struct inode *inode = d_inode(path->dentry);
> > >         struct kernfs_node *kn = inode->i_private;
> > > 
> > > -       down_write(&kernfs_rwsem);
> > > +       inode_lock(inode);
> > > +       down_read(&kernfs_rwsem);
> > >         kernfs_refresh_inode(kn, inode);
> > > -       up_write(&kernfs_rwsem);
> > > +       up_read(&kernfs_rwsem);
> > > +       inode_unlock(inode);
> > > 
> > >         generic_fillattr(inode, stat);
> > >         return 0;
> > > @@ -281,9 +283,11 @@ int kernfs_iop_permission(struct inode
> > > *inode,
> > > int mask)
> > > 
> > >         kn = inode->i_private;
> > > 
> > > -       down_write(&kernfs_rwsem);
> > > +       inode_lock(inode);
> > > +       down_read(&kernfs_rwsem);
> > >         kernfs_refresh_inode(kn, inode);
> > > -       up_write(&kernfs_rwsem);
> > > +       up_read(&kernfs_rwsem);
> > > +       inode_unlock(inode);
> > > 
> > >         return generic_permission(inode, mask);
> > >  }
> > > 
> > 
> > thanks,
> > fox