Subject: Re: [patch 4/4] powerpc 2.6.21-rt1: reduce scheduling latency by changing tlb flush size

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 15 May 2007 17:23:09 +1000

On Mon, 2007-05-14 at 16:40 +0200, Arnd Bergmann wrote:
> On Monday 14 May 2007, Tsutomu OWA wrote:
> > --- linux-2.6.21-rt1/include/asm-powerpc/tlbflush.h     2007-04-26 12:08:32.000000000 +0900
> > +++ rt/include/asm-powerpc/tlbflush.h   2007-05-14 16:12:47.000000000 +0900
> > @@ -25,7 +25,12 @@ struct mm_struct;
> >  #include <linux/percpu.h>
> >  #include <asm/page.h>
> >  
> > +#if defined(CONFIG_PPC_CELLEB) && defined(CONFIG_PREEMPT_RT)
> > +/* Since tlb flush takes long time on Celleb, reduce it to 1 when Celleb && RT */
> > +#define PPC64_TLB_BATCH_NR 1
> > +#else
> >  #define PPC64_TLB_BATCH_NR 192
> > +#endif /* defined(CONFIG_PPC_CELLEB) && defined(CONFIG_PREEMPT_RT) */
> 
> With this code, you get silent side-effects of enabling PPC_CELLEB
> along with another platform.
> 
> Maybe instead you should change the hpte_need_flush() to always flush
> when running on the celleb platform and PREEMPT_RT is enabled.

I think it's wrong either way.... Maybe we can make it a variable and
measure how much we can reasonably do in a given time frame at
boot ? :-) Since it depends wether there's an hypervisor etc...

Ben.