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

From: Ingo Molnar
Date: Sat Jul 08 2017 - 04:36:25 EST

* Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> 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?