On Tue, Jul 21, 2015 at 05:45:26PM +0000, Mathieu Desnoyers wrote: > ----- On Jul 21, 2015, at 11:16 AM, Ondřej Bílka neleai@xxxxxxxxx wrote: > > > On Tue, Jul 21, 2015 at 12:58:13PM +0000, Mathieu Desnoyers wrote: > >> ----- On Jul 21, 2015, at 3:30 AM, Ondřej Bílka neleai@xxxxxxxxx wrote: > >> > >> > On Tue, Jul 21, 2015 at 12:25:00AM +0000, Mathieu Desnoyers wrote: > >> >> >> Does it solve the Wine problem? If Wine uses gs for something and > >> >> >> calls a function that does this, Wine still goes boom, right? > >> >> > > >> >> > So the advantage of just making a global segment descriptor available > >> >> > is that it's not *that* expensive to just save/restore segments. So > >> >> > either wine could do it, or any library users would do it. > >> >> > > >> >> > But anyway, I'm not sure this is a good idea. The advantage of it is > >> >> > that the kernel support really is _very_ minimal. > >> >> > >> >> Considering that we'd at least also want this feature on ARM and > >> >> PowerPC 32/64, and that the gs segment selector approach clashes with > >> >> existing apps (wine), I'm not sure that implementing a gs segment > >> >> selector based approach to cpu number caching would lead to an overall > >> >> decrease in complexity if it leads to performance similar to those of > >> >> portable approaches. > >> >> > >> >> I'm perfectly fine with architecture-specific tweaks that lead to > >> >> fast-path speedups, but if we have to bite the bullet and implement > >> >> an approach based on TLS and registering a memory area at thread start > >> >> through a system call on other architectures anyway, it might end up > >> >> being less complex to add a new system call on x86 too, especially if > >> >> fast path overhead is similar. > >> >> > >> >> But I'm inclined to think that some aspect of the question eludes me, > >> >> especially given the amount of interest generated by the gs-segment > >> >> selector approach. What am I missing ? > >> >> > >> > As I wrote before you don't have to bite bullet as I said before. It > >> > suffices to create 128k element array with cpu for each tid, make that > >> > mmapable file and userspace could get cpu with nearly same performance > >> > without hacks. > >> > >> I don't see how this would be acceptable on memory-constrained embedded > >> systems. They have multiple cores, and performance requirements, so > >> having a fast getcpu would be useful there (e.g. telecom industry), > >> but they clearly cannot afford a 512kB table per process just for that. > >> > > Which just means that you need more complicated api and implementation > > for that but idea stays same. You would need syscalls > > register/deregister_cpuid_idx that would give you index used instead > > tid. A kernel would need to handle that many ids could be registered for > > each thread and resize mmaped file in syscalls. > > I feel we're talking past each other here. What I propose is to implement > a system call that registers a TLS area. It can be invoked at thread start. > The kernel can then keep the current CPU number within that registered > area up-to-date. This system call does not care how the TLS is implemented > underneath. > > My understanding is that you are suggesting a way to speed up TLS accesses > by creating a table indexed by TID. Although it might lead to interesting > speed ups useful when reading the TLS, I don't see how you proposal is > useful in addressing the problem of caching the current CPU number (other > than possibly speeding up TLS accesses). > > Or am I missing something fundamental to your proposal ? > No, I still talk about getting cpu number. My first proposal is that kernel allocates table of current cpu numbers accessed by tid. That could process mmap and get cpu with cpu_tid_table[tid]. As you said that size is problem I replied that you need to be more careful. Instead tid you will use different id that you get with say register_cpucache, store in tls variable and get cpu with cpu_cid_table[cid]. That decreases space used to only threads that use this. A tls speedup was side remark when you would implement per-cpu page then you could speedup tls. As tls access speed and getting tid these are equivalent as you could easily implement one with other. -- 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
