Messages in this thread | | | Date | Tue, 22 Apr 2008 08:36:04 -0500 | From | Robin Holt <> | Subject | Re: [PATCH 0 of 9] mmu notifier #v12 |
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On Tue, Apr 22, 2008 at 03:21:43PM +0200, Andrea Arcangeli wrote: > On Tue, Apr 22, 2008 at 08:01:20AM -0500, Robin Holt wrote: > > On Tue, Apr 22, 2008 at 02:00:56PM +0200, Andrea Arcangeli wrote: > > > On Tue, Apr 22, 2008 at 09:20:26AM +0200, Andrea Arcangeli wrote: > > > > invalidate_range_start { > > > > spin_lock(&kvm->mmu_lock); > > > > > > > > kvm->invalidate_range_count++; > > > > rmap-invalidate of sptes in range > > > > > > > > > > write_seqlock; write_sequnlock; > > > > I don't think you need it here since invalidate_range_count is already > > elevated which will accomplish the same effect. > > Agreed, seqlock only in range_end should be enough. BTW, the fact
I am a little confused about the value of the seq_lock versus a simple atomic, but I assumed there is a reason and left it at that.
> seqlock is needed regardless of invalidate_page existing or not, > really makes invalidate_page a no brainer not just from the core VM > point of view, but from the driver point of view too. The > kvm_page_fault logic would be the same even if I remove > invalidate_page from the mmu notifier patch but it'd run slower both > when armed and disarmed.
I don't know what you mean by "it'd" run slower and what you mean by "armed and disarmed".
For the sake of this discussion, I will assume "it'd" means the kernel in general and not KVM. With the two call sites for range_begin/range_end, I would agree we have more call sites, but the second is extremely likely to be cache hot.
By disarmed, I will assume you mean no notifiers registered for a particular mm. In that case, the cache will make the second call effectively free. So, for the disarmed case, I see no measurable difference.
For the case where there is a notifier registered, I certainly can see a difference. I am not certain how to quantify the difference as it depends on the callee. In the case of xpmem, our callout is always very expensive for the _start case. Our _end case is very light, but it is essentially the exact same steps we would perform for the _page callout.
When I was discussing this difference with Jack, he reminded me that the GRU, due to its hardware, does not have any race issues with the invalidate_page callout simply doing the tlb shootdown and not modifying any of its internal structures. He then put a caveat on the discussion that _either_ method was acceptable as far as he was concerned. The real issue is getting a patch in that satisfies all needs and not whether there is a seperate invalidate_page callout.
Thanks, Robin
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