Re: [PATCH 1/5] Swap Migration V5: LRU operations

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

On Tue, Nov 15, 2005 at 10:46:05AM -0800, Andrew Morton wrote:
> Christoph Lameter <clameter@engr.sgi.com> wrote:
> >
> > On Tue, 15 Nov 2005, Andrew Morton wrote:
> > 
> > > But lru_add_drain_per_cpu() will be called from interrupt context: the IPI
> > > handler.
> > 
> > Ahh.. thought you meant the lru_add_drain run on the local processor.
> >  
> > > I'm asking whether it is safe for the IPI handler to reenable interupts on
> > > all architectures.  It might be so, but I don't recall ever having seen it
> > > discussed, nor have I seen code which does it.
> > 
> > smp_call_function is also used by the slab allocator to drain the 
> > pages. All the spinlocks in there and those of the page allocator (called 
> > for freeing pages) use spin_lock_irqsave. Why is this not used for 
> > lru_add_drain() and friends?
> 
> It's a microoptimisation - lru_add_drain() is always called with local irqs
> enabled, so no need for irqsave.
> 
> I don't think spin_lock_irqsave() is notably more expensive than
> spin_lock_irq() - the cost is in the irq disabling and in the atomic
> operation.

Pardon me, but spin_lock_irqsave() needs to write data to the stack,
which is likely to be cache-cold, so you have to fault the cacheline in 
from slow memory.

And thats much slower than the atomic operation, isnt it?