Messages in this thread | | | Date | Sat, 8 Nov 2008 20:10:10 +0100 | From | Ingo Molnar <> | Subject | Re: [git pull] scheduler updates |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Sat, 8 Nov 2008, Arjan van de Ven wrote: > > > > historically it was for early AMD cpus (K7, not sure if early K8 did > > this) where 2 consecutive rdtsc's in the same codestream would get > > reordered compared to eachother, so you could observe the tsc go > > backwards... > > .. but this only happens with two _consecutive_ ones. > > The thing is, nobody sane does that in generic code. The scheduler wants > to have cycles, yes, but two consecutive scheduler invocations will have > spinlocks etc in between. That's true of _all_ sane uses of a TSC. > > I don't see that there is ever any reason to do the barriers for any > normal case. And the cases where it does matter would actually be worth > pointing out (ie making the barriers explicit in those cases, and those > cases only). > > Doing it in get_cycles() and "forgetting about it" may sound like a simple > solution, but it's likely wrong. For example, one of the few cases where > we realy care about time going backwards is gettimeofday() - which uses > tsc, but which also has tons of serializing instructions on its own. > EXCEPT WHEN IT IS a vsyscall! > > But in that case, we don't even have the barrier, because we put it in the > wrong function and 'forgot about it'. Of course, we may not need it > (rdtscp maybe always serializes, I didn't check), but the point is, an > explicit barrier is actually better than one that is hidden. > > So who _really_ needs it? And why not just do it there?
i think, the tree as offered to you, intends to do just that, unless i made some grave (and unintended) mistake somewhere.
The barrier is only present in the vread function: which is the vsyscall-read function, to be used from user-space.
Even in the past, no was actually forgotten or put in the wrong function as far as i can see because previously _everything_ (including the vread method) had the barrier.
The change from me simply removes the barrier from the places that dont need it - exactly for the reason you outlined: the scheduler is both imprecise and has a ton of natural serialization anyway, so it's a non-issue there.
Hm?
Ingo
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