Re: [PATCH kernfs 1/3] kernfs: switch global kernfs_idr_lock to per-fs lock

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, Apr 12, 2025 at 07:50:54PM +0800, alexjlzheng@gmail.com wrote:
> On Sat, 12 Apr 2025 08:12:22 +0200, gregkh@linuxfoundation.org wrote:
> > On Sat, Apr 13, 2025 at 02:31:07AM +0800, alexjlzheng@gmail.com wrote:
> > > From: Jinliang Zheng <alexjlzheng@tencent.com>
> > > 
> > > The kernfs implementation has big lock granularity(kernfs_idr_lock) so
> > > every kernfs-based(e.g., sysfs, cgroup) fs are able to compete the lock.
> > > 
> > > This patch switches the global kernfs_idr_lock to per-fs lock, which
> > > put the spinlock into kernfs_root.
> > > 
> > > Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
> > > ---
> > >  fs/kernfs/dir.c             | 14 +++++++-------
> > >  fs/kernfs/kernfs-internal.h |  1 +
> > >  2 files changed, 8 insertions(+), 7 deletions(-)
> > 
> > What kind of testing / benchmark did you do for this series that shows
> > that this works, AND that this actually is measureable?  What workload
> > are you doing that causes these changes to be needed?
> 
> Thank you for your reply. :)
> 
> We are trying to implement a kernfs-based filesystem that will have
> multiple instances running at the same time, i.e., multiple kernfs_roots.

I don't think that kernfs is meant for that very well, what is that
filesystem going to be for?

> While investigating the kernfs implementation, we found some global locks
> that would cause noticeable lock contention when there are many filesystem
> instances.
> 
> Fortunately, we found that some optimizations have been made in [1], which
> moved kernfs_rwsem into kernfs_root. But there are still some global locks
> left.
> 
> We think it is also necessary to switch the remaining global locks to
> per-fs. Moreover, we strongly agree with Tejun Heo's point in [1]:
> 
>   "... this is the right thing to do even if there is no concrete
>    performance argument (not saying there isn't). It's just weird to
>    entangle these completely unrelated users in a single rwsem."
> 
> We think kernfs will be widely used to build other filesystems, so we
> strongly recommend switching global locks to per-fs.

I don't strongly object, but I would like to see some real-world numbers first.

thanks,

greg k-h