On Mon, Feb 11, 2019 at 04:09:53PM -0800, Alexander Duyck wrote: > On Mon, 2019-02-11 at 17:52 -0500, Michael S. Tsirkin wrote: > > On Mon, Feb 11, 2019 at 01:00:53PM -0800, Alexander Duyck wrote: > > > On Mon, 2019-02-11 at 14:54 -0500, Michael S. Tsirkin wrote: > > > > On Mon, Feb 11, 2019 at 10:10:06AM -0800, Alexander Duyck wrote: > > > > > On Mon, 2019-02-11 at 12:36 -0500, Michael S. Tsirkin wrote: > > > > > > On Mon, Feb 11, 2019 at 08:31:34AM -0800, Alexander Duyck wrote: > > > > > > > On Sat, 2019-02-09 at 19:49 -0500, Michael S. Tsirkin wrote: > > > > > > > > On Mon, Feb 04, 2019 at 10:15:52AM -0800, Alexander Duyck wrote: > > > > > > > > > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> > > > > > > > > > > > > > > > > > > Add guest support for providing free memory hints to the KVM hypervisor for > > > > > > > > > freed pages huge TLB size or larger. I am restricting the size to > > > > > > > > > huge TLB order and larger because the hypercalls are too expensive to be > > > > > > > > > performing one per 4K page. > > > > > > > > > > > > > > > > Even 2M pages start to get expensive with a TB guest. > > > > > > > > > > > > > > Agreed. > > > > > > > > > > > > > > > Really it seems we want a virtio ring so we can pass a batch of these. > > > > > > > > E.g. 256 entries, 2M each - that's more like it. > > > > > > > > > > > > > > The only issue I see with doing that is that we then have to defer the > > > > > > > freeing. Doing that is going to introduce issues in the guest as we are > > > > > > > going to have pages going unused for some period of time while we wait > > > > > > > for the hint to complete, and we cannot just pull said pages back. I'm > > > > > > > not really a fan of the asynchronous nature of Nitesh's patches for > > > > > > > this reason. > > > > > > > > > > > > Well nothing prevents us from doing an extra exit to the hypervisor if > > > > > > we want. The asynchronous nature is there as an optimization > > > > > > to allow hypervisor to do its thing on a separate CPU. > > > > > > Why not proceed doing other things meanwhile? > > > > > > And if the reason is that we are short on memory, then > > > > > > maybe we should be less aggressive in hinting? > > > > > > > > > > > > E.g. if we just have 2 pages: > > > > > > > > > > > > hint page 1 > > > > > > page 1 hint processed? > > > > > > yes - proceed to page 2 > > > > > > no - wait for interrupt > > > > > > > > > > > > get interrupt that page 1 hint is processed > > > > > > hint page 2 > > > > > > > > > > > > > > > > > > If hypervisor happens to be running on same CPU it > > > > > > can process things synchronously and we never enter > > > > > > the no branch. > > > > > > > > > > > > > > > > Another concern I would have about processing this asynchronously is > > > > > that we have the potential for multiple guest CPUs to become > > > > > bottlenecked by a single host CPU. I am not sure if that is something > > > > > that would be desirable. > > > > > > > > Well with a hypercall per page the fix is to block VCPU > > > > completely which is also not for everyone. > > > > > > > > If you can't push a free page hint to host, then > > > > ideally you just won't. That's a nice property of > > > > hinting we have upstream right now. > > > > Host too busy - hinting is just skipped. > > > > > > Right, but if you do that then there is a potential to end up missing > > > hints for a large portion of memory. It seems like you would end up > > > with even bigger issues since then at that point you have essentially > > > leaked memory. > > > I would think you would need a way to resync the host and the guest > > > after something like that. Otherwise you can have memory that will just > > > go unused for an extended period if a guest just goes idle. > > > > Yes and that is my point. Existing hints code will just take a page off > > the free list in that case so it resyncs using the free list. > > > > Something like this could work then: mark up > > hinted pages with a flag (its easy to find unused > > flags for free pages) then when you get an interrupt > > because outstanding hints have been consumed, > > get unflagged/unhinted pages from buddy and pass > > them to host. > > Ugh. This is beginning to sound like yet another daemon that will have > to be running to handle missed sync events. Why a daemon? Not at all. You get an interrupt, you schedule a wq immediately or just do it from the interrupt handler. > I really think that taking an async approach for this will be nothing > but trouble. You are going to have a difficult time maintaining any > sort of coherency no the freelist without the daemon having to take the > zone lock and then notify the host of what is free and what isn't. We seem to be doing fine without zone lock for now. Just plain alloc_pages. > > > > > > > > > > > > Using the huge TLB order became the obvious > > > > > > > > > choice for the order to use as it allows us to avoid fragmentation of higher > > > > > > > > > order memory on the host. > > > > > > > > > > > > > > > > > > I have limited the functionality so that it doesn't work when page > > > > > > > > > poisoning is enabled. I did this because a write to the page after doing an > > > > > > > > > MADV_DONTNEED would effectively negate the hint, so it would be wasting > > > > > > > > > cycles to do so. > > > > > > > > > > > > > > > > Again that's leaking host implementation detail into guest interface. > > > > > > > > > > > > > > > > We are giving guest page hints to host that makes sense, > > > > > > > > weird interactions with other features due to host > > > > > > > > implementation details should be handled by host. > > > > > > > > > > > > > > I don't view this as a host implementation detail, this is guest > > > > > > > feature making use of all pages for debugging. If we are placing poison > > > > > > > values in the page then I wouldn't consider them an unused page, it is > > > > > > > being actively used to store the poison value. > > > > > > > > > > > > Well I guess it's a valid point of view for a kernel hacker, but they are > > > > > > unused from application's point of view. > > > > > > However poisoning is transparent to users and most distro users > > > > > > are not aware of it going on. They just know that debug kernels > > > > > > are slower. > > > > > > User loading a debug kernel and immediately breaking overcommit > > > > > > is an unpleasant experience. > > > > > > > > > > How would that be any different then a user loading an older kernel > > > > > that doesn't have this feature and breaking overcommit as a result? > > > > > > > > Well old kernel does not have the feature so nothing to debug. > > > > When we have a new feature that goes away in the debug kernel, > > > > that's a big support problem since this leads to heisenbugs. > > > > > > Trying to debug host features from the guest would be a pain anyway as > > > a guest shouldn't even really know what the underlying setup of the > > > guest is supposed to be. > > > > I'm talking about debugging the guest though. > > Right. But my point is if it is a guest feature related to memory that > you are debugging, then disabling the page hinting would probably be an > advisable step anyway since it would have the potential for memory > corruptions itself due to its nature. Oh absolutely. So that's why I wanted debug kernel to be as close as possible to non-debug one in that respect. If one gets a corruption we want it reproducible on debug too. > > > > > I still think it would be better if we left the poisoning enabled in > > > > > such a case and just displayed a warning message if nothing else that > > > > > hinting is disabled because of page poisoning. > > > > > > > > > > One other thought I had on this is that one side effect of page > > > > > poisoning is probably that KSM would be able to merge all of the poison > > > > > pages together into a single page since they are all set to the same > > > > > values. So even with the poisoned pages it would be possible to reduce > > > > > total memory overhead. > > > > > > > > Right. And BTW one thing that host can do is pass > > > > the hinted area to KSM for merging. > > > > That requires an alloc hook to free it though. > > > > > > > > Or we could add a per-VMA byte with the poison > > > > value and use that on host to populate pages on fault. > > > > > > > > > > > > > > > If we can achieve this > > > > > > > and free the page back to the host then even better, but until the > > > > > > > features can coexist we should not use the page hinting while page > > > > > > > poisoning is enabled. > > > > > > > > > > > > Existing hinting in balloon allows them to coexist so I think we > > > > > > need to set the bar just as high for any new variant. > > > > > > > > > > That is what I heard. I will have to look into this. > > > > > > > > It's not doing anything smart right now, just checks > > > > that poison == 0 and skips freeing if not. > > > > But it can be enhanced transparently to guests. > > > > > > Okay, so it probably should be extended to add something like poison > > > page that could replace the zero page for reads to a page that has been > > > unmapped. > > > > > > > > > > This is one of the reasons why I was opposed to just disabling page > > > > > > > poisoning when this feature was enabled in Nitesh's patches. If the > > > > > > > guest has page poisoning enabled it is doing something with the page. > > > > > > > It shouldn't be prevented from doing that because the host wants to > > > > > > > have the option to free the pages. > > > > > > > > > > > > I agree but I think the decision belongs on the host. I.e. > > > > > > hint the page but tell the host it needs to be careful > > > > > > about the poison value. It might also mean we > > > > > > need to make sure poisoning happens after the hinting, not before. > > > > > > > > > > The only issue with poisoning after instead of before is that the hint > > > > > is ignored and we end up triggering a page fault and zero as a result. > > > > > It might make more sense to have an architecture specific call that can > > > > > be paravirtualized to handle the case of poisoning the page for us if > > > > > we have the unused page hint enabled. Otherwise the write to the page > > > > > is a given to invalidate the hint. > > > > > > > > Sounds interesting. So the arch hook will first poison and > > > > then pass the page to the host? > > > > > > > > Or we can also ask the host to poison for us, problem is this forces > > > > host to either always write into page, or call MADV_DONTNEED, > > > > without it could do MADV_FREE. Maybe that is not a big issue. > > > > > > I would think we would ask the host to poison for us. If I am not > > > mistaken both solutions right now are using MADV_DONTNEED. I would tend > > > to lean that way if we are doing page poisoning since the cost for > > > zeroing/poisoning the page on the host could be canceled out by > > > dropping the page poisoning on the guest. > > > > > > Then again since we are doing higher order pages only, and the > > > poisoning is supposed to happen before we get into __free_one_page we > > > would probably have to do both the poisoning, and the poison on fault. > > > > > > Oh that's a nice trick. So in fact if we just make sure > > we never report PAGE_SIZE pages then poisoning will > > automatically happen before reporting? > > So we just need to teach host to poison on fault. > > Sounds cool and we can always optimize further later. > > That is kind of what I was thinking. In the grand scheme of things I > figure most of the expense is in the fault and page zeroing bits of the > code path. I have done a bit of testing today with the patch that just > drops the messages if a device is assigned, and just the hypercall bits > are only causing about a 2.5% regression in performance on a will-it- > scale/page-fault1 test. However if I commit to the full setup with the > madvise, page fault, and zeroing then I am seeing an 11.5% drop in > performance. > > I think in order to really make this pay off we may need to look into > avoiding zeroing or poisoning the page in both the host and the guest. > I will have to look into some things as it looks like there was > somebody from Intel may have been working on doing some work to address > that based on the presentation I found at the link below: > > https://www.lfasiallc.com/wp-content/uploads/2017/11/Use-Hyper-V-Enlightenments-to-Increase-KVM-VM-Performance_Density_Chao-Peng.pdf >
