From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andi Kleen Date: Fri, 16 Jun 2006 15:37:06 +0000 Subject: Re: FOR REVIEW: New x86-64 vsyscall vgetcpu() Message-Id: <200606161737.06132.ak@suse.de> List-Id: References: <200606140942.31150.ak@suse.de> <200606161656.40930.ak@suse.de> <4492CEC0.2080102@bull.net> In-Reply-To: <4492CEC0.2080102@bull.net> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Zoltan Menyhart Cc: Jes Sorensen , Tony Luck , discuss@x86-64.org, linux-kernel@vger.kernel.org, libc-alpha@sourceware.org, vojtech@suse.cz, linux-ia64@vger.kernel.org On Friday 16 June 2006 17:31, Zoltan Menyhart wrote: > Andi Kleen wrote: > > > That is not how user space TLS works. It usually has a base a register. > > Can you please give me a real life (simplified) example? On x86-64 it's just %fs:offset. gcc is a bit dumb on this and usually loads the base address from %fs:0 first. > > > This means it cannot be cache colored (because you would need a static > > offset) and you couldn't share task_structs on a page. > > I do not see the problem. Your scheme relies on task_struct fields being on a known offset in the page. But slab cache coloring varies the offset to make the data spread out better in the caches. > Can you explain please? > E.g. the scheduler pulls a task instead of the current one. The CPU > will see "current->thread_info.cpu"-s of all the tasks at the same > offset anyway. It varies relative to the start of page. That was one of the bigger wins relative to the task_struct in stack page of 2.4 had. > > > Also you would make task_struct part of the userland ABI which > > seems like a very very bad idea to me. It means we couldn't change > > it anymore. > > We can make some wrapper, e.g.: > > user_per_cpu_var(name, offset) You would need to wrap everything and likely users would like task_struct so much that they accessed it anyways without your wrappers. > "vgetcpu()" would also be added to the ABI which we couldn't change > easily either. Yes, but it's a defined function. No different from a system call. -Andi