On Fri, Jul 17, 2015 at 04:33:42PM -0700, Andy Lutomirski wrote: > On Fri, Jul 17, 2015 at 4:28 PM, Ondřej Bílka <neleai@xxxxxxxxx> wrote: > > On Fri, Jul 17, 2015 at 11:48:14AM -0700, Linus Torvalds wrote: > >> > >> 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. > > Or we implement per-cpu segment registers so you can point gs directly > at percpu data. This is conceptually easy and has no weird ABI > issues. All it needs is an implementation and some good tests. > That only works if you have free register on your arch. As gs there was rfc to teach gcc use it which could give bigger speedup. I didn't see how much this could help yet so I am bit skeptical. > I think the API should be "set gsbase to x + y*(cpu number)". On > x86_64, userspace just allocates a big swath of virtual space and > populates it as needed. > That wouldn't work well if two shared libraries want to use that. You would need to use something like se it to 4096*cpu_number or so. Also we didn't considered yet overhead, as this slows down everything a bit due slower context switches. So will this needs to have widespread performance improvement to be worthwhile. What are use cases to make that pay itself? -- 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
