Messages in this thread | | | Subject | Re: [PATCH] mm: disable preemption in apply_to_pte_range | From | Peter Zijlstra <> | Date | Fri, 13 Feb 2009 15:38:58 +0100 |
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On Sat, 2009-02-14 at 01:30 +1100, Nick Piggin wrote: > On Saturday 14 February 2009 01:16:51 Peter Zijlstra wrote: > > On Sat, 2009-02-14 at 00:30 +1100, Nick Piggin wrote: > > > On Friday 13 February 2009 22:48:30 Peter Zijlstra wrote: > > > > On Thu, 2009-02-12 at 17:39 -0800, Jeremy Fitzhardinge wrote: > > > > > In general the model for lazy updates is that you're batching the > > > > > updates in some queue somewhere, which is almost certainly a piece of > > > > > percpu state being maintained by someone. Its therefore broken > > > > > and/or meaningless to have the code making the updates wandering > > > > > between cpus for the duration of the lazy updates. > > > > > > > > > > > If so, should we do the preempt_disable/enable within those > > > > > > functions? Probably not worth the cost, I guess. > > > > > > > > > > The specific rules are that > > > > > arch_enter_lazy_mmu_mode()/arch_leave_lazy_mmu_mode() require you to > > > > > be holding the appropriate pte locks for the ptes you're updating, so > > > > > preemption is naturally disabled in that case. > > > > > > > > Right, except on -rt where the pte lock is a mutex. > > > > > > > > > This all goes a bit strange with init_mm's non-requirement for taking > > > > > pte locks. The caller has to arrange for some kind of serialization > > > > > on updating the range in question, and that could be a mutex. > > > > > Explicitly disabling preemption in enter_lazy_mmu_mode would make > > > > > sense for this case, but it would be redundant for the common case of > > > > > batched updates to usermode ptes. > > > > > > > > I really utterly hate how you just plonk preempt_disable() in there > > > > unconditionally and without very clear comments on how and why. > > > > > > And even on mainline kernels, builds without the lazy mmu mode stuff > > > don't need preemption disabled here either, so it is technically a > > > regression in those cases too. > > > > Well, normally we'd be holding the pte lock, which on regular kernels > > already disable preemption, as Jeremy noted. So in that respect it > > doesn't change things too much. > > But not (necessarily) in the init_mm case.
Right.
> > Its just that slapping preempt_disable()s around like there's not > > tomorrow is horridly annoying, its like using the BKL -- there's no data > > affinity what so ever, so trying to unravel the dependencies a year > > later when you notice its a latency concern is a massive pain in the > > backside. > > Or like using memory barriers. Any of them are OK if they're properly > commented though, I guess.
Yes, given sufficient comments and a good reason most things can be gotten away with ;-)
> > > > I'd rather we'd fix up the init_mm to also have a pte lock. > > > > > > Well that wouldn't fix -rt; there would need to be a preempt_disable > > > within arch_enter_lazy_mmu_mode(), which I think is the cleanest > > > solution. > > > > Hmm, so you're saying we need to be cpu-affine for the lazy mmu stuff? > > Otherwise a -rt would just convert the init_mm pte lock to a mutex along > > with all other pte locks and there'd be no issue. > > Well I don't see any other reason why it should have to use preempt_disable. > Not necessarily just cpu-affine, but perhaps it is using per-cpu data in > non-trivial way so cannot get switched out either.
If the lazy mmu code relies on per-cpu data, then it should be the lazy mmu's responsibility to ensure stuff is properly serialized. Eg. it should do get_cpu_var() and put_cpu_var().
Those constructs can usually be converted to preemptable variants quite easily, as it clearly shows what data needs to be protected.
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