Subject: Re: [PATCH v2 0/9] Remove spin_unlock_wait()

From: "Paul E. McKenney" <[email-masked]>
Date: Sat, 8 Jul 2017 04:39:46 -0700

On Sat, Jul 08, 2017 at 10:35:43AM +0200, Ingo Molnar wrote:
> 
> * Manfred Spraul <[email-masked]> wrote:
> 
> > Hi Ingo,
> > 
> > On 07/07/2017 10:31 AM, Ingo Molnar wrote:
> > > 
> > > There's another, probably just as significant advantage: queued_spin_unlock_wait()
> > > is 'read-only', while spin_lock()+spin_unlock() dirties the lock cache line. On
> > > any bigger system this should make a very measurable difference - if
> > > spin_unlock_wait() is ever used in a performance critical code path.
> > At least for ipc/sem:
> > Dirtying the cacheline (in the slow path) allows to remove a smp_mb() in the
> > hot path.
> > So for sem_lock(), I either need a primitive that dirties the cacheline or
> > sem_lock() must continue to use spin_lock()/spin_unlock().
> 
> Technically you could use spin_trylock()+spin_unlock() and avoid the lock acquire 
> spinning on spin_unlock() and get very close to the slow path performance of a 
> pure cacheline-dirtying behavior.
> 
> But adding something like spin_barrier(), which purely dirties the lock cacheline, 
> would be even faster, right?

Interestingly enough, the arm64 and powerpc implementations of
spin_unlock_wait() were very close to what it sounds like you are
describing.

							Thanx, Paul