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[209.85.220.65]) by mx.google.com with SMTPS id c11sor4643409iom.117.2019.03.13.15.54.50 for (Google Transport Security); Wed, 13 Mar 2019 15:54:50 -0700 (PDT) Received-SPF: pass (google.com: domain of alexander.duyck@gmail.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=Zp2J1GBY; spf=pass (google.com: domain of alexander.duyck@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=alexander.duyck@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=hI97cvpiFwbqINqVuOvg/SoIet+VipX1FL8Sh+mhPSo=; b=Zp2J1GBY2PZdvuxBzucq8kmyE2K23CMXhXuNXjNf9fuzVrdXzVG3bdIjS1ZfdLV2S7 ioJqAkwmRJxRnM/6NxN4YNhXarvNRZycnK9PUyPORbUfbn83YO9mEq5WLleJcIa7kmV+ IDpKUAjRSz4X5FHid6/o8rK2E0vdQZUKvLElTmL+z9CzhK+UQ/zYUSexpZZqzB/06L2E BVJXVUKx0cy8/DTWQ9EwnQqUO3OMjodIrRO7thGrgIEvG0k1ba3JRVZf+/4QBC/I8Img U9yEI0gTt1KC+6SxErWgzD4UVhdAhkpWHrytgpWzyslbzkjpK1tvIACkSWxrOa9KxvdH AhNw== X-Google-Smtp-Source: APXvYqw3ZG1UU10TbGzDdlbWYuEyTWhm3KqyAdd3EFS4onxj0uEyYP28T3Sv5yA4eD2RAwENvmxZHjtCNcvD0qbmPYg= X-Received: by 2002:a6b:f70a:: with SMTP id k10mr15952688iog.68.1552517690436; Wed, 13 Mar 2019 15:54:50 -0700 (PDT) MIME-Version: 1.0 References: <20190306155048.12868-1-nitesh@redhat.com> <2d9ae889-a9b9-7969-4455-ff36944f388b@redhat.com> <22e4b1cd-38a5-6642-8cbe-d68e4fcbb0b7@redhat.com> <78b604be-2129-a716-a7a6-f5b382c9fb9c@redhat.com> <20190307212845-mutt-send-email-mst@kernel.org> <17d2afa6-556e-ec73-40dc-beac536b3f20@redhat.com> <8f692047-4750-6827-1ee0-d3d354788f09@redhat.com> <41ae8afe-72c9-58e6-0cbb-9375c91ce37a@redhat.com> <1ae522f1-1e98-9eef-324c-29585fe574d6@redhat.com> <8826829a-973d-8117-3fe3-8e33170acfb8@redhat.com> <71d0bd98-ff97-7ed1-1f95-c0d134d0b2a1@redhat.com> In-Reply-To: <71d0bd98-ff97-7ed1-1f95-c0d134d0b2a1@redhat.com> From: Alexander Duyck Date: Wed, 13 Mar 2019 15:54:39 -0700 Message-ID: Subject: Re: [RFC][Patch v9 2/6] KVM: Enables the kernel to isolate guest free pages To: David Hildenbrand Cc: Nitesh Narayan Lal , "Michael S. Tsirkin" , kvm list , LKML , linux-mm , Paolo Bonzini , lcapitulino@redhat.com, pagupta@redhat.com, wei.w.wang@intel.com, Yang Zhang , Rik van Riel , dodgen@google.com, Konrad Rzeszutek Wilk , dhildenb@redhat.com, Andrea Arcangeli Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Mar 13, 2019 at 9:39 AM David Hildenbrand wrote: > > On 13.03.19 17:37, Alexander Duyck wrote: > > On Wed, Mar 13, 2019 at 5:18 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 wrote: > >>>>> On 3/8/19 4:39 PM, Alexander Duyck wrote: > >>>>>> On Fri, Mar 8, 2019 at 11:39 AM Nitesh Narayan Lal wrote: > >>>>>>> On 3/8/19 2:25 PM, Alexander Duyck wrote: > >>>>>>>> On Fri, Mar 8, 2019 at 11:10 AM Nitesh Narayan Lal wrote: > >>>>>>>>> On 3/8/19 1:06 PM, Alexander Duyck wrote: > >>>>>>>>>> On Thu, Mar 7, 2019 at 6:32 PM Michael S. Tsirkin 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