On Wed, Mar 13, 2019 at 5:18 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 13.03.19 12:54, Nitesh Narayan Lal wrote: > > > > On 3/12/19 5:13 PM, Alexander Duyck wrote: > >> On Tue, Mar 12, 2019 at 12:46 PM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: > >>> On 3/8/19 4:39 PM, Alexander Duyck wrote: > >>>> On Fri, Mar 8, 2019 at 11:39 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: > >>>>> On 3/8/19 2:25 PM, Alexander Duyck wrote: > >>>>>> On Fri, Mar 8, 2019 at 11:10 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: > >>>>>>> On 3/8/19 1:06 PM, Alexander Duyck wrote: > >>>>>>>> On Thu, Mar 7, 2019 at 6:32 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > >>>>>>>>> On Thu, Mar 07, 2019 at 02:35:53PM -0800, Alexander Duyck wrote: > >>>>>>>>>> The only other thing I still want to try and see if I can do is to add > >>>>>>>>>> a jiffies value to the page private data in the case of the buddy > >>>>>>>>>> pages. > >>>>>>>>> Actually there's one extra thing I think we should do, and that is make > >>>>>>>>> sure we do not leave less than X% off the free memory at a time. > >>>>>>>>> This way chances of triggering an OOM are lower. > >>>>>>>> If nothing else we could probably look at doing a watermark of some > >>>>>>>> sort so we have to have X amount of memory free but not hinted before > >>>>>>>> we will start providing the hints. It would just be a matter of > >>>>>>>> tracking how much memory we have hinted on versus the amount of memory > >>>>>>>> that has been pulled from that pool. > >>>>>>> This is to avoid false OOM in the guest? > >>>>>> Partially, though it would still be possible. Basically it would just > >>>>>> be a way of determining when we have hinted "enough". Basically it > >>>>>> doesn't do us much good to be hinting on free memory if the guest is > >>>>>> already constrained and just going to reallocate the memory shortly > >>>>>> after we hinted on it. The idea is with a watermark we can avoid > >>>>>> hinting until we start having pages that are actually going to stay > >>>>>> free for a while. > >>>>>> > >>>>>>>> It is another reason why we > >>>>>>>> probably want a bit in the buddy pages somewhere to indicate if a page > >>>>>>>> has been hinted or not as we can then use that to determine if we have > >>>>>>>> to account for it in the statistics. > >>>>>>> The one benefit which I can see of having an explicit bit is that it > >>>>>>> will help us to have a single hook away from the hot path within buddy > >>>>>>> merging code (just like your arch_merge_page) and still avoid duplicate > >>>>>>> hints while releasing pages. > >>>>>>> > >>>>>>> I still have to check PG_idle and PG_young which you mentioned but I > >>>>>>> don't think we can reuse any existing bits. > >>>>>> Those are bits that are already there for 64b. I think those exist in > >>>>>> the page extension for 32b systems. If I am not mistaken they are only > >>>>>> used in VMA mapped memory. What I was getting at is that those are the > >>>>>> bits we could think about reusing. > >>>>>> > >>>>>>> If we really want to have something like a watermark, then can't we use > >>>>>>> zone->free_pages before isolating to see how many free pages are there > >>>>>>> and put a threshold on it? (__isolate_free_page() does a similar thing > >>>>>>> but it does that on per request basis). > >>>>>> Right. That is only part of it though since that tells you how many > >>>>>> free pages are there. But how many of those free pages are hinted? > >>>>>> That is the part we would need to track separately and then then > >>>>>> compare to free_pages to determine if we need to start hinting on more > >>>>>> memory or not. > >>>>> Only pages which are isolated will be hinted, and once a page is > >>>>> isolated it will not be counted in the zone free pages. > >>>>> Feel free to correct me if I am wrong. > >>>> You are correct up to here. When we isolate the page it isn't counted > >>>> against the free pages. However after we complete the hint we end up > >>>> taking it out of isolation and returning it to the "free" state, so it > >>>> will be counted against the free pages. > >>>> > >>>>> If I am understanding it correctly you only want to hint the idle pages, > >>>>> is that right? > >>>> Getting back to the ideas from our earlier discussion, we had 3 stages > >>>> for things. Free but not hinted, isolated due to hinting, and free and > >>>> hinted. So what we would need to do is identify the size of the first > >>>> pool that is free and not hinted by knowing the total number of free > >>>> pages, and then subtract the size of the pages that are hinted and > >>>> still free. > >>> To summarize, for now, I think it makes sense to stick with the current > >>> approach as this way we can avoid any locking in the allocation path and > >>> reduce the number of hypercalls for a bunch of MAX_ORDER - 1 page. > >> I'm not sure what you are talking about by "avoid any locking in the > >> allocation path". Are you talking about the spin on idle bit, if so > >> then yes. > > Yeap! > >> However I have been testing your patches and I was correct > >> in the assumption that you forgot to handle the zone lock when you > >> were freeing __free_one_page. > > Yes, these are the steps other than the comments you provided in the > > code. (One of them is to fix release_buddy_page()) > >> I just did a quick copy/paste from your > >> zone lock handling from the guest_free_page_hinting function into the > >> release_buddy_pages function and then I was able to enable multiple > >> CPUs without any issues. > >> > >>> For the next step other than the comments received in the code and what > >>> I mentioned in the cover email, I would like to do the following: > >>> 1. Explore the watermark idea suggested by Alex and bring down memhog > >>> execution time if possible. > >> So there are a few things that are hurting us on the memhog test: > >> 1. The current QEMU patch is only madvising 4K pages at a time, this > >> is disabling THP and hurts the test. > > Makes sense, thanks for pointing this out. > >> > >> 2. The fact that we madvise the pages away makes it so that we have to > >> fault the page back in in order to use it for the memhog test. In > >> order to avoid that penalty we may want to see if we can introduce > >> some sort of "timeout" on the pages so that we are only hinting away > >> old pages that have not been used for some period of time. > > > > Possibly using MADVISE_FREE should also help in this, I will try this as > > well. > > I was asking myself some time ago how MADVISE_FREE will be handled in > case of THP. Please let me know your findings :) The problem with MADVISE_FREE is that it will add additional complication to the QEMU portion of all this as it only applies to anonymous memory if I am not mistaken. That also reminds me that one thing this patch set still doesn't address is what do we do about a direct assigned device or some other form of shared memory where we want to keep the virtual mapping beneath the guest pinned to a given set of physical memory.
