On Fri, Jul 17, 2015 at 11:48:14AM -0700, Linus Torvalds wrote: > On Thu, Jul 16, 2015 at 12:27 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > > > > If we actually bit the bullet and implemented per-cpu mappings > > That's not ever going to happen. > > The Linux VM model of "one page table per VM" is the right one. > Anything else sucks, and makes threading a disaster. > > So you can try to prove me wrong, but seriously, I doubt you'll succeed. > > On x86, if you want per-cpu memory areas, you should basically plan on > using segment registers instead (although other odd state has been > used - there's been the people who use segment limits etc rather than > the *pointer* itself, preferring to use "lsl" to get percpu data. You > could also imaging hiding things in the vector state somewhere if you > control your environment well enough). > Thats correct, problem is that you need some sort of hack like this on archs that otherwise would need syscall to get tid/access tls variable. On x64 and archs that have register for tls this could be implemented relatively easily. Kernel needs to allocate int running_cpu_for_tid[32768]; On context switch it atomically writes to this table running_cpu_for_tid[tid] = cpu; This table is read-only accessible from userspace as mmaped file. Then userspace just needs to access it with three indirections like: __thread tid; char caches[CPU_MAX]; #define getcpu_cache caches[tid > 32768 ? get_cpu() : running_cpu_for_tid[tid]] With more complicated kernel interface you could eliminate one indirection as we would use void * array instead and thread could do syscall to register what values it should use for each thread. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
