Re: [RFC PATCH] Add split_lock

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 08, 2021 at 06:23:38AM +0100, Matthew Wilcox (Oracle) wrote:
> bit_spinlocks are horrible on RT because there's absolutely nowhere
> to put the mutex to sleep on.  They also do not participate in lockdep
> because there's nowhere to put the map.
> 
> Most (all?) bit spinlocks are actually a split lock; logically they
> could be treated as a single spinlock, but for performance, we want to
> split the lock over many objects.  Introduce the split_lock as somewhere
> to store the lockdep map and as somewhere that the RT kernel can put
> a mutex.  It may also let us store a ticket lock for better performance
> on non-RT kernels in the future, but I have left the current cpu_relax()
> implementation intact for now.

I think I like it, but I'm not sure it'll work for RT as is. It's a bit
like qrwlock in that it only uses the internal (split) lock for
contention, but that doesn't work for PI.

I've not recently looked at RT, but I think they simply used to bloat a
number of the data structures with a real lock. Sebastian and Thomas
will know better.