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[209.85.220.65]) by mx.google.com with SMTPS id l62sor4242493itl.9.2019.03.13.09.37.24 for (Google Transport Security); Wed, 13 Mar 2019 09:37:24 -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=gI0V78kS; 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=+l/+tlArTYVIAgdmiEc6xCw65Apc5s/BXyiZi9QW9Ds=; b=gI0V78kSUW2pr+1vY3HihXY2OlcwcimsXS1wx8dSg94GqtDTw9pSTdKuSZ+mty7VZx Z1vpP8gMeZ9iJsOXgaLMFU0Gm606IeZj8bwPiT1Ct19YoADYcRQTj48868YtJyCuMyvW eNMzqzgXrnZ9GCqs5+L3kmg8PB9maUzl7SsNvtlcfOfGVHmSZlZyS8rLOhTcNz4pEiVJ 6DojFrXhEAaNHryfjEqEOt4d1xCxvZIWS1bsW6IS6QBCIcsR2iBHqZ9wr+9GOb7zDa3x 6aiu81OByPf06kJFnoCIX3avLj3hnUkVqvUj/3B98vb5ytmEXkAeo4O4a0D3jVgWBXsN VtgQ== X-Google-Smtp-Source: APXvYqzkO9ePaB9LM3a/1JO5eXQs89JkCut/x47OEfJ294d7MYsm5Y2Fg96Zp9CtCUgKOS9kexdVfLbBoqqwzF+4GuE= X-Received: by 2002:a24:45e3:: with SMTP id c96mr2208694itd.89.1552495043927; Wed, 13 Mar 2019 09:37:23 -0700 (PDT) MIME-Version: 1.0 References: <20190306155048.12868-1-nitesh@redhat.com> <20190306155048.12868-3-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> In-Reply-To: <8826829a-973d-8117-3fe3-8e33170acfb8@redhat.com> From: Alexander Duyck Date: Wed, 13 Mar 2019 09:37:12 -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 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. 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.