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[2003:cb:c705:4200:ce79:acf6:d832:60df]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-433b45bdbabsm23835385e9.18.2024.11.20.08.20.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 Nov 2024 08:20:11 -0800 (PST) Message-ID: <9286da7a-9923-4a3b-a769-590e8824fa10@redhat.com> Date: Wed, 20 Nov 2024 17:20:09 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/4] KVM: ioctl for populating guest_memfd To: kalyazin@amazon.com, pbonzini@redhat.com, corbet@lwn.net, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Cc: jthoughton@google.com, brijesh.singh@amd.com, michael.roth@amd.com, graf@amazon.de, jgowans@amazon.com, roypat@amazon.co.uk, derekmn@amazon.com, nsaenz@amazon.es, xmarcalx@amazon.com, Sean Christopherson , linux-mm@kvack.org References: <20241024095429.54052-1-kalyazin@amazon.com> <08aeaf6e-dc89-413a-86a6-b9772c9b2faf@amazon.com> <01b0a528-bec0-41d7-80f6-8afe213bd56b@redhat.com> <55b6b3ec-eaa8-494b-9bc7-741fe0c3bc63@amazon.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: roqnyhugtiinyjfjjqdmdm5x1aouhq8o X-Rspam-User: X-Rspamd-Queue-Id: A0BD240007 X-Rspamd-Server: rspam02 X-HE-Tag: 1732119551-96933 X-HE-Meta: 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 AeSGz+U6 tWNfmbNKtyym9yHU1A07hmMFvw6pddz0KJCIWRR1Wk2IaEyT7+pph5N73QlSASRQnDbXcibRrvufnPxleccnmlvhfEBikPTCUEaUyn/+EuLynPsprdJxOtS1mvx/S3dTb/4VGjFBuQOharQOTwzwZtxsegrIM0gSTM6/D/5dL3ORqK9CmzLIPhp3m5TZKjFou3gSAiTY6rrncrP9Up2IXHbsvUjeH8G4WF3icP2NJ1WuqcIR+Pww3B6klJyJP4vjR/jiyayVXM4Bjsjw61TepyyGlV75G83PODnbm7HHxHlzjKEp/5aI/Nkp3bxK0+9riiZDeUDNZ7vzr8VpKk0kbG3DdSGwG+mjKiReUkJTcy+eIliVGYqRisvF0kBdDwt7qcbl87NSAMO58GY0gR8FUN+AYjUzwCiKXm8CJ4fDMWN8M3TUBrZcT4VhqsuYNCXzTHE/k 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: List-Subscribe: List-Unsubscribe: On 20.11.24 16:58, Nikita Kalyazin wrote: > > > On 20/11/2024 15:13, David Hildenbrand wrote: > > Hi! > > Hi! :) > > >> Results: > >> - MAP_PRIVATE: 968 ms > >> - MAP_SHARED: 1646 ms > > > > At least here it is expected to some degree: as soon as the page cache > > is involved map/unmap gets slower, because we are effectively > > maintaining two datastructures (page tables + page cache) instead of > > only a single one (page cache) > > > > Can you make sure that THP/large folios don't interfere in your > > experiments (e.g., madvise(MADV_NOHUGEPAGE))? > > I was using transparent_hugepage=never command line argument in my testing. > > $ cat /sys/kernel/mm/transparent_hugepage/enabled > always madvise [never] > > Is that sufficient to exclude the THP/large folio factor? Yes! > > >> While this logic is intuitive, its performance effect is more > >> significant that I would expect. > > > > Yes. How much of the performance difference would remain if you hack out > > the atomic op just to play with it? I suspect there will still be some > > difference. > > I have tried that, but could not see any noticeable difference in the > overall results. > > It looks like a big portion of the bottleneck has moved from > shmem_get_folio_gfp/folio_mark_uptodate to > finish_fault/__pte_offset_map_lock somehow. I have no good explanation > for why: That's what I assumed. The profiling results can be rather fuzzy and misleading with micro-benchmarks. :( > > Orig: > - 69.62% do_fault > + 44.61% __do_fault > + 20.26% filemap_map_pages > + 3.48% finish_fault > Hacked: > - 67.39% do_fault > + 32.45% __do_fault > + 21.87% filemap_map_pages > + 11.97% finish_fault > > Orig: > - 3.48% finish_fault > - 1.28% set_pte_range > 0.96% folio_add_file_rmap_ptes > - 0.91% __pte_offset_map_lock > 0.54% _raw_spin_lock > Hacked: > - 11.97% finish_fault > - 8.59% __pte_offset_map_lock > - 6.27% _raw_spin_lock > preempt_count_add > 1.00% __pte_offset_map > - 1.28% set_pte_range > - folio_add_file_rmap_ptes > __mod_node_page_state > > > Note that we might improve allocation times with guest_memfd when > > allocating larger folios. > > I suppose it may not always be an option depending on requirements to > consistency of the allocation latency. Eg if a large folio isn't > available at the time, the performance would degrade to the base case > (please correct me if I'm missing something). Yes, there are cons to that. > >> Heh, now I spot that your comment was as reply to a series. > > Yeah, sorry if it wasn't obvious. > >> If your ioctl is supposed to to more than "allocating memory" like >> MAP_POPULATE/MADV_POPULATE+* ... then POPULATE is a suboptimal choice. >> Because for allocating memory, we would want to use fallocate() instead. >> I assume you want to "allocate+copy"? > > Yes, the ultimate use case is "allocate+copy". > >> I'll note that, as we're moving into the direction of moving >> guest_memfd.c into mm/guestmem.c, we'll likely want to avoid "KVM_*" >> ioctls, and think about something generic. > > Good point, thanks. Are we at the stage where some concrete API has > been proposed yet? I might have missed that. People are working on it, and we're figuring out some remaining details (e.g., page_type to intercept folio_put() ). I assume we'll see a new RFC soonish (famous last words), but it's not been proposed yet. > >> Any clue how your new ioctl will interact with the WIP to have shared >> memory as part of guest_memfd? For example, could it be reasonable to >> "populate" the shared memory first (via VMA) and then convert that >> "allocated+filled" memory to private? > > No, I can't immediately see why it shouldn't work. My main concern > would probably still be about the latency of the population stage as I > can't see why it would improve compared to what we have now, because my > feeling is this is linked with the sharedness property of guest_memfd. If the problem is the "pagecache" overhead, then yes, it will be a harder nut to crack. But maybe there are some low-hanging fruits to optimize? Finding the main cause for the added overhead would be interesting. -- Cheers, David / dhildenb