Messages in this thread | | | Date | Fri, 13 Feb 2009 11:36:19 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [ltt-dev] [RFC git tree] Userspace RCU (urcu) for Linux (repost) |
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On Fri, Feb 13, 2009 at 01:54:11PM -0500, Mathieu Desnoyers wrote: > * Linus Torvalds (torvalds@linux-foundation.org) wrote: > > > > > > Btw, for user space, if you want to do this all right for something like > > BF. I think the only _correct_ thing to do (in the sense that the end > > result will actually be debuggable) is to essentially give full SMP > > coherency in user space. > > > > It's doable, but rather complicated, and I'm not 100% sure it really ends > > up making sense. The way to do it is to just simply say: > > > > - never map the same page writably on two different cores, and always > > flush the cache (on the receiving side) when you switch a page from one > > core to another. > > > > Now, the kernel can't really do that reasonably, but user space possibly could. > > > > Now, I realize that blackfin doesn't actually even have a MMU or a TLB, so > > by "mapping the same page" in that case we end up really meaning "having a > > shared mapping or thread". I think that _should_ be doable. The most > > trivial approach might be to simply limit all processes with shared > > mappings or CLONE_VM to core 0, and letting core 1 run everything else > > (but you could do it differently: mapping something with MAP_SHARED would > > force you to core 0, but threads would just force the thread group to > > stay on _one_ core, rather than necessarily a fixed one). > > > > Yeah, because of the lack of real memory protection, the kernel can't > > _know_ that processes don't behave badly and access things that they > > didn't explicitly map, but I'm hoping that that is rare. > > > > And yes, if you really want to use threads as a way to do something > > across cores, you'd be screwed - the kenrel would only schedule the > > threads on one CPU. But considering the undefined nature of threading on > > such a cpu, wouldn't that still be preferable? Wouldn't it be nice to have > > the knowledge that user space _looks_ cache-coherent by virtue of the > > kernel just limiting cores appropriately? > > > > And then user space would simply not need to worry as much. Code written > > for another architecture will "just work" on BF SMP too. With the normal > > uclinux limitations, of course. > > > > Linus > > > > I don't know enough about BF to tell for sure, but the other way around > I see that would still permit running threads with shared memory space > on different CPUs is to call a cache flush each time a userspace lock is > taken/released (at the synchronization points where the "magic > test-and-set instruction" is used) _from_ userspace. > > If some more elaborate userspace MT code uses something else than those > basic locks provided by core libraries to synchronize data exchange, > then it would be on its own and have to ensure cache flushing itself.
How about just doing a sched_setaffinity() in the BF case? Sounds like an easy way to implement Linus's suggestion of restricting the multithreaded processes to a single core. I have a hard time losing sleep over the lack of parallelism in the case where the SMP support is at best rudimentary...
> And yes, that would be incredibly costly/slow. This is why RCU-style > reader-sides are good : they have much more relaxed synchronization > constraints. > > I am just thinking that the single-process to a single core solution you > propose above will be somewhat limiting if we end up with a 64-cores > non-cache-coherent architecture. They tend to be especially used for > stuff like video decoding, which is very easy to parallelize when shared > memory is available. But I guess we are not there yet.
If someone invests the silicon for 64 cores, but doesn't provide some semblance of cache coherence, I have to question their sanity. As a kludgey quick fix to get to a dual-proc solution I can understand it, but there is a limit! ;-)
Thanx, Paul
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