On Tue, Apr 22, 2008 at 08:36:04AM -0500, Robin Holt wrote: > 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.
There's no value for anything but get_user_pages (get_user_pages takes its own lock internally though). I preferred to explain it as a seqlock because it was simpler for reading, but I totally agree in the final implementation it shouldn't be a seqlock. My code was meant to be pseudo-code only. It doesn't even need to be atomic ;). > I don't know what you mean by "it'd" run slower and what you mean by > "armed and disarmed". 1) when armed the time-window where the kvm-page-fault would be blocked would be a bit larger without invalidate_page for no good reason 2) if you were to remove invalidate_page when disarmed the VM could would need two branches instead of one in various places I don't want to waste cycles if not wasting them improves performance both when 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 actually meant for both. > 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 rmap is sure effective free, for do_wp_page it costs one branch for no good reason. > 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 Agreed. > 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. Sure, we have that patch now, I'll send it out in a minute, I was just trying to explain why it makes sense to have an invalidate_page too (which remains the only difference by now), removing it would be a regression on all sides, even if a minor one. ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
