From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andi Kleen Date: Sat, 17 Jun 2006 06:55:48 +0000 Subject: Re: [discuss] Re: FOR REVIEW: New x86-64 vsyscall vgetcpu() Message-Id: <200606170855.49123.ak@suse.de> List-Id: References: <200606140942.31150.ak@suse.de> <200606161740.18611.ak@suse.de> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: discuss@x86-64.org Cc: Chase Venters , Brent Casavant , Zoltan Menyhart , Jes Sorensen , Tony Luck , linux-kernel@vger.kernel.org, libc-alpha@sourceware.org, vojtech@suse.cz, linux-ia64@vger.kernel.org On Friday 16 June 2006 23:19, Chase Venters wrote: > On Fri, 16 Jun 2006, Andi Kleen wrote: > >> To this last point, it might be more reasonable to map in a page that > >> contained a new structure with a stable ABI, which mirrored some of > >> the task_struct information, and likely other useful information as > >> needs are identified in the future. In any case, it would be hard > >> to beat a single memory read for performance. > > > > That would mean making the context switch and possibly other > > things slower. > > Well, if every process had a page of its own, what would the context > switch overhead be? For process zero, for thread quite high on x86 because you would need per CPU page tables. Doing that would be extremly nasty because you would potentially need to allocate a new set of page tables every time the process is scheduled to a new CPU it hasn't run on before. If you limit it to a process then you can't get the current CPU from such a mapping because a process can run threaded on multiple CPUs. My reference was more to high suggestion of keeping a second version of task_struct for export. That would require changing everything in task struct that is changed on switch_to and should be exported in the other function too. -Andi