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 :) -- Thanks, David / dhildenb
