On Wed, Mar 13, 2019 at 9:39 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 13.03.19 17:37, Alexander Duyck wrote: > > 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. > > Just as MADV_DONTNEED. So nothing new. Future work. I'm pretty sure you can use MADV_DONTNEED to free up file backed memory, I don't believe this is the case for MADV_FREE, but maybe I am mistaken. On a side note I was just reviewing some stuff related to the reserved bit and on-lining hotplug memory, and it just occurred to me that most the PG_offline bit would be a good means to indicate that we hinted away a page out of the buddy allocator, especially since it is already used by the balloon drivers anyway. We would just have to add a call to make sure we clear it when we call __ClearPageBuddy. It looks like that would currently be in del_page_from_free_area, at least for linux-next. I just wanted to get your thoughts on that as it seems like it might be a good fit. Thanks. - Alex
