Re: [RFC PATCH v2 1/2] kernfs: use kernfs_node specific mutex and spinlock.

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jan 04, 2022 at 09:16:03AM +1100, Imran Khan wrote:
> > There is a tradeoff of memory usage and runtime contention that has to
> > be made here, and this might be pushing it in the wrong direction for
> > a lot of systems.
> > 
> Agree. Could you please suggest if this should be made configurable via
> kconfig ? I understand that this would result in 2 versions of some
> functions but it will allow systems with large memories to avoid kernfs
> contention.

No, that way lies madness and horrible bugs and a codebase no one will
be able to maintain.

> We are seeing the launch time of some DB workloads adversely getting
> affected with this contention.

What workloads?  sysfs should NEVER be in the fast-path of any normal
operation, including booting.  What benchmark or real-work is having
problems here?

> Also based on recent movement of kernfs_rwsem into kernfs_root, do you
> think that the above mentioned mutex and spinlock can be moved to
> kernfs_root as well. Although that change would not help in my current
> case, but it could avoid similar contentions between different users of
> kernfs like cgroup and sysfs

Why would you want to move it if you do not see a measured benefit?

thanks,

greg k-h