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Thu, 19 Sep 2024 01:15:17 -0700 (PDT) X-Google-Smtp-Source: AGHT+IELZTzATS9eCj4TvVTw5NcipBJLousu8FBRwQPfLaXQqUG1sLeG0u06k/YIT0s37X24+ECm3Q== X-Received: by 2002:a05:600c:83c6:b0:42c:b220:4769 with SMTP id 5b1f17b1804b1-42da2bd7679mr176659325e9.32.1726733716615; Thu, 19 Sep 2024 01:15:16 -0700 (PDT) Received: from [10.131.4.59] ([83.68.141.146]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42e6c718e40sm61104015e9.1.2024.09.19.01.15.14 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 19 Sep 2024 01:15:16 -0700 (PDT) Message-ID: <3ed4ea1f-8646-4a6b-aaee-8177a05f972f@redhat.com> Date: Thu, 19 Sep 2024 10:15:16 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 1/4] KVM: Dynamic sized kvm memslots array To: Peter Xu , qemu-devel@nongnu.org Cc: Vitaly Kuznetsov , Fabiano Rosas , Prasad Pandit , Julia Suvorova , Juraj Marcin , Paolo Bonzini , qemu-stable , Zhiyi Guo References: <20240917163835.194664-1-peterx@redhat.com> <20240917163835.194664-2-peterx@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 17.09.24 18:38, Peter Xu wrote: > Zhiyi reported an infinite loop issue in VFIO use case. The cause of that > was a separate discussion, however during that I found a regression of > dirty sync slowness when profiling. > > Each KVMMemoryListerner maintains an array of kvm memslots. Currently it's > statically allocated to be the max supported by the kernel. However after > Linux commit 4fc096a99e ("KVM: Raise the maximum number of user memslots"), > the max supported memslots reported now grows to some number large enough > so that it may not be wise to always statically allocate with the max > reported. > > What's worse, QEMU kvm code still walks all the allocated memslots entries > to do any form of lookups. It can drastically slow down all memslot > operations because each of such loop can run over 32K times on the new > kernels. > > Fix this issue by making the memslots to be allocated dynamically. > > Here the initial size was set to 16 because it should cover the basic VM > usages, so that the hope is the majority VM use case may not even need to > grow at all (e.g. if one starts a VM with ./qemu-system-x86_64 by default > it'll consume 9 memslots), however not too large to waste memory. > > There can also be even better way to address this, but so far this is the > simplest and should be already better even than before we grow the max > supported memslots. For example, in the case of above issue when VFIO was > attached on a 32GB system, there are only ~10 memslots used. So it could > be good enough as of now. > > In the above VFIO context, measurement shows that the precopy dirty sync > shrinked from ~86ms to ~3ms after this patch applied. It should also apply > to any KVM enabled VM even without VFIO. > > NOTE: we don't have a FIXES tag for this patch because there's no real > commit that regressed this in QEMU. Such behavior existed for a long time, > but only start to be a problem when the kernel reports very large > nr_slots_max value. However that's pretty common now (the kernel change > was merged in 2021) so we attached cc:stable because we'll want this change > to be backported to stable branches. > > Cc: qemu-stable > Reported-by: Zhiyi Guo > Tested-by: Zhiyi Guo > Signed-off-by: Peter Xu > --- Acked-by: David Hildenbrand -- Cheers, David / dhildenb