On 3/13/19 8:17 AM, David Hildenbrand 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 :) I will do that. If we don't end up finding any appropriate page flag to track the age of free page. I am wondering if I can somehow use bitmap to track the free count for each PFN. > -- Regards Nitesh
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