Re: [PATCH 2/2] locking/pvqspinlock: Optionally store lock holder cpu into lock

[Peter Zijlstra]

On Sat, Jul 11, 2020 at 02:21:28PM -0400, Waiman Long wrote:
> The previous patch enables native qspinlock to store lock holder cpu
> number into the lock word when the lock is acquired via the slowpath.
> Since PV qspinlock uses atomic unlock, allowing the fastpath and
> slowpath to put different values into the lock word will further slow
> down the performance. This is certainly undesirable.
> 
> The only way we can do that without too much performance impact is to
> make fastpath and slowpath put in the same value. Still there is a slight
> performance overhead in the additional access to a percpu variable in the
> fastpath as well as the less optimized x86-64 PV qspinlock unlock path.
> 
> A new config option QUEUED_SPINLOCKS_CPUINFO is now added to enable
> distros to decide if they want to enable lock holder cpu information in
> the lock itself for both native and PV qspinlocks across both fastpath
> and slowpath. If this option is not configureed, only native qspinlocks
> in the slowpath will put the lock holder cpu information in the lock
> word.

And this kills it,.. if it doesn't make unconditional sense, we're not
going to do this. It's just too ugly.