Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops

Mathieu Desnoyers
2011-09-12T14:47:10Z

* Andi Kleen (andi@firstfloor.org) wrote:
> On Mon, Sep 12, 2011 at 04:05:58PM +0200, Peter Zijlstra wrote:
> > Subject: llist: Remove cpu_relax() usage in cmpxchg loops
> > From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Date: Mon Sep 12 15:50:49 CEST 2011
> >
> > Initial benchmarks show they're a net loss (2 socket wsm):
> >
>
> May still save power.

Looking at kernel/spinlock.c:

void __lockfunc __raw_##op##_lock(locktype##_t *lock) \
{
[...]
while (!raw_##op##_can_lock(lock) && (lock)->break_lock)\
arch_##op##_relax(&lock->raw_lock); \

so basically, in typical locking primitives (spinlock), it looks like
lower power consumption is preferred over getting the raw maximal
performance in fully contented scenarios.

So what is the rationale for making those lock-less lists retry scheme
different from spinlocks here ?

Thanks,

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.