Re: [patch] preempt-smp.patch, 2.6.9-rc1-bk14

[Ingo Molnar <mingo@elte.hu>]

* Zwane Mwaikambo <zwane@linuxpower.ca> wrote:

> > to solve this problem i've introduced a new spinlock field,
> > lock->break_lock, which signals towards the holding CPU that a
> > spinlock-break is requested by another CPU. This field is only set if a
> > CPU is spinning in a spinlock function [at any locking depth], so the
> > default overhead is zero. I've extended cond_resched_lock() to check for
> > this flag - in this case we can also save a reschedule. I've added the
> > lock_need_resched(lock) and need_lockbreak(lock) methods to check for
> > the need to break out of a critical section.
> 
> Doesn't having break_lock within the same cacheline as lock bounce the
> line around more?

in fact this way it bounces less than if it were on a separate
cacheline. Contention causes bouncing anyway. This way we already have
the cacheline dirty and on the local CPU when we set break_lock, which
the lockholder CPU bounces back when it breaks the lock and/or releases
the lock.

	Ingo