From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail143.messagelabs.com (mail143.messagelabs.com [216.82.254.35]) by kanga.kvack.org (Postfix) with SMTP id 8D8186B0047 for ; Fri, 13 Feb 2009 09:31:06 -0500 (EST) From: Nick Piggin Subject: Re: [PATCH] mm: disable preemption in apply_to_pte_range Date: Sat, 14 Feb 2009 01:30:31 +1100 References: <4994BCF0.30005@goop.org> <200902140030.59027.nickpiggin@yahoo.com.au> <1234534611.6519.109.camel@twins> In-Reply-To: <1234534611.6519.109.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200902140130.31985.nickpiggin@yahoo.com.au> Sender: owner-linux-mm@kvack.org To: Peter Zijlstra Cc: Jeremy Fitzhardinge , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Ingo Molnar List-ID: 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. > 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. > > > 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org