From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zoltan Menyhart Date: Fri, 16 Jun 2006 12:36:52 +0000 Subject: Re: FOR REVIEW: New x86-64 vsyscall vgetcpu() Message-Id: <4492A5E4.9050702@bull.net> List-Id: References: <200606140942.31150.ak@suse.de> <200606161209.25266.ak@suse.de> <44928FB1.5070107@sgi.com> <200606161317.19296.ak@suse.de> <44929CE6.4@sgi.com> In-Reply-To: <44929CE6.4@sgi.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Jes Sorensen Cc: Andi Kleen , Tony Luck , discuss@x86-64.org, linux-kernel@vger.kernel.org, libc-alpha@sourceware.org, vojtech@suse.cz, linux-ia64@vger.kernel.org Just to make sure I understand it correctly... Assuming I have allocated per CPU data (numa control, etc.) pointed at by: void *per_cpu[MAXCPUS]; Assuming a per CPU variable has got an "offset" in each per CPU data area. Accessing this variable can be done as follows: err = vgetcpu(&my_cpu, ...); if (err) goto .... pointer = (typeof pointer) (per_cpu[my_cpu] + offset); // use "pointer"... It is hundred times more long than "__get_per_cpu(var)++". As we do not know when we can be moved to another CPU, "vgetcpu()" has to be called again after a "reasonable short" time. My idea is to map the current task structure at an arch. dependent virtual address into the user space (obviously in RO). #define current ((struct task_struct *) 0x...) No more need to for "vgetcpu()" at all. The example above becomes: pointer = (typeof pointer) (per_cpu[current->thread_info.cpu] + offset); // use "pointer"... As obtaining "pointer" does not cost much, it can be re-calculated at each usage => no problem to know when to recheck it, there is less chance for using the data of a neighbor. Regards, Zoltan Menyhart