From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keith Owens Date: Thu, 08 Feb 2007 06:37:54 +0000 Subject: Re: [RFC Patch]Use ar.kr2 for smp_processor_id Message-Id: <7584.1170916674@kao2.melbourne.sgi.com> List-Id: References: <1170905324.3230.7.camel@linux-znh> In-Reply-To: <1170905324.3230.7.camel@linux-znh> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org Zou Nan hai (on 08 Feb 2007 12:27:31 +0800) wrote: >On Thu, 2007-02-08 at 14:04, Keith Owens wrote: >> Zou Nan hai (on 08 Feb 2007 11:28:44 +0800) wrote: >> >Pin ar.kr2 of each CPU, so that smp_processor_id can use it. >> >> Historically ar.k2 has been reserved for debugging purposes, for >> example in ivt.S. Debuggers often need a location that can be used to >> track progress, it has to be somewhere that does not rely on TLB >> entries and is guaranteed to appear in MCA/INIT records - ar.k2 is >> perfect for this. >> > Ok, seems that current kr3 is only used by ia64_itc_printk_clock? >> Use Tony's suggestion of testing for a change in ar.k3 (guaranteed to >> be unique on every cpu) and caching the corresponding cpu number when >> it changes. >> > But why do we even need to cache it? > > It is already in a register if we put it to kr3. > so smp_processor_id() could be very fast. and later sys_getcpu can >also be very fast. ar.k3 is currently used for the address of the per-cpu data area, which speeds up access to all the per-cpu data. Changing ar.k3 to hold the cpu number means an extra array calculation and lookup for every per-cpu variable, slowing down the rest of the system.