Re: [PATCH RFC 1/3] mutex: Make more scalable by doing less atomic operations

[Robin Holt <holt@sgi.com>]

On Mon, Apr 08, 2013 at 07:38:39AM -0700, Linus Torvalds wrote:
> On Mon, Apr 8, 2013 at 5:42 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > AFAICS the main performance trade-off is the following: when the owner CPU unlocks
> > the mutex, we'll poll it via a read first, which turns the cacheline into
> > shared-read MESI state. Then we notice that its content signals 'lock is
> > available', and we attempt the trylock again.
> >
> > This increases lock latency in the few-contended-tasks case slightly - and we'd
> > like to know by precisely how much, not just for a generic '10-100 users' case
> > which does not tell much about the contention level.
> 
> We had this problem for *some* lock where we used a "read + cmpxchg"
> in the hotpath and it caused us problems due to two cacheline state
> transitions (first to shared, then to exclusive). It was faster to
> just assume it was unlocked and try to do an immediate cmpxchg.
> 
> But iirc it is a non-issue for this case, because this is only about
> the contended slow path.
> 
> I forget where we saw the case where we should *not* read the initial
> value, though. Anybody remember?

I think you might be remembering ia64.  Fairly early on, I recall there
being a change in the spinlocks where we did not check them before just
trying to acquire.

Thanks,
Robin