RE: [PATCH] PER_CPU [4/4] - PER_CPU-irq_stat

["Shai Fultheim" <shai@scalex86.org>]

> > diff -Nru a/kernel/softirq.c b/kernel/softirq.c
> > --- a/kernel/softirq.c	2004-07-09 01:34:13 -07:00
> > +++ b/kernel/softirq.c	2004-07-09 01:34:13 -07:00
> > @@ -36,8 +36,8 @@
> >   */
> >
> >  #ifndef __ARCH_IRQ_STAT
> > -irq_cpustat_t irq_stat[NR_CPUS] ____cacheline_aligned;
> > -EXPORT_SYMBOL(irq_stat);
> > +DEFINE_PER_CPU(irq_cpustat_t, irq_stat)
> ____cacheline_maxaligned_in_smp;
> > +EXPORT_PER_CPU_SYMBOL(irq_stat);
> >  #endif
> 
> Is there a need for the cacheline alignment? We want to keep that per
> cpu data area as packed as possible, we only want to explicitly pad if
> we need to (eg other cpus are accessing that variable a lot).
> 
> Also it looks like we will have to push the above change into the other
> architectures.
> 
> Anton
> 

IMHO, we want to keep irq_stat aligned from performance perspectives. You
can never know if the info before and after it in the per-cpu area are going
to be cached (and thefore crossing cache-line boundary will cost us more).  

Anyhow, since that also accesses by other CPUs (not a lot...), I think it's
better to keep it aligned (the utilization of per-cpu areas is so low now
that it doesn't really matter).

--shai