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Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.97.1 #2 (Red Hat Linux)) id 1sZ7FD-00000005Cm6-2pw8; Wed, 31 Jul 2024 11:11:47 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 4D900300820; Wed, 31 Jul 2024 13:11:47 +0200 (CEST) Date: Wed, 31 Jul 2024 13:11:47 +0200 From: Peter Zijlstra To: Thomas Gleixner Cc: 20240621164406.256314-1-kirill.shutemov@linux.intel.com, kirill.shutemov@linux.intel.com, ardb@kernel.org, bp@alien8.de, brijesh.singh@amd.com, corbet@lwn.net, dave.hansen@linux.intel.com, hpa@zytor.com, jan.kiszka@siemens.com, jgross@suse.com, kbingham@kernel.org, linux-doc@vger.kernel.org, linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, luto@kernel.org, michael.roth@amd.com, mingo@redhat.com, rick.p.edgecombe@intel.com, sandipan.das@amd.com, thomas.lendacky@amd.com, x86@kernel.org Subject: Re: [PATCH 0/3] x86: Make 5-level paging support unconditional for x86-64 Message-ID: <20240731111147.GA33588@noisy.programming.kicks-ass.net> References: <80734605-1926-4ac7-9c63-006fe3ea6b6a@amd.com> <87wml16hye.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87wml16hye.ffs@tglx> X-Rspam-User: X-Stat-Signature: hfg8rdty6spiw8u8wzxo36x9rmkpmtcd X-Rspamd-Queue-Id: 551C280007 X-Rspamd-Server: rspam11 X-HE-Tag: 1722424318-341497 X-HE-Meta: U2FsdGVkX18BLxG5k/tARKyKdQ5D3y+wxytWYu44JPh9t9XussOVsso05AQsspGOOWN9KQu99usObKliX8l69EgNWFNrhKFq8o9b1gYftBB4KW2Hq4uV2SBs8RXK2QtOQAVTG5k1vjJEO7wIof9gd+J89HIr0NrQBc7GWSqiPQeBNICQy5WR8l8FOso6EUSTw7WfOjqKCinZ4yeq7Ip/acuEruax9V8JbELZ2LIrmUZr+mINwXqym5I6iCAJ8FZ3eNphTnynRcmaCZ7IAKScksnasqFrG3OEjVUmkHU17ZdQU1tlvIKUg3xt/lHxSN0NoOFwhz6G8iahpo8V6pIutecS6Jqlk1YOegu+FPytw6xmOTMxFwZ2C/pYOWmIxwfhcWEeDqpmDrb6/Xx2ku8DNKQe+mwCoAWa0jR+xpW3Iula8Qku/GhbXmmQunWr0IxrLFpVroWgo6ZVhB6B1rhkAvN7/WX6Uvu8CM9ySRZE+5Q2VimvqYe/RiWDJ0WWtGsKRvMlJGcYWSyIu5jO8jmROyE+6dQHOfjHqMUzkdiBMarrX6jZx4yWCbKgEcW8aUfAnWAqxgl9IlhYfLkz87HgS78TxgqthF65UwiePiShXOrgDXd2EZSuAThR/lTzyyGXIiS0xu0TjUlocTimSvHUEN7GUBvkzFFqNXdcaWZYPRc0ML7zpq8q+DyaMcnn/m1P7AG2s+/62yvpsTcXev4v5SDYtw5jrfgkb72iC6qtFrE9NpjmU53j+iGDlMOFYerESimJvAh+QZ7/zfXDdUNaiSpvbAYxUT1KFBFORWI49ChRa9mPIPZMk9xNPbpiwkomCyqBYJJdX5qbzRh2MUS4ON6sVNhEZaD9PFBLZzQeryvCGI0q8GAstBed6XIYQqcvCpO8eVBnbQ2jBqJ70BhoSWtVu2FOX/4HOd99t+/HKlOie/wXk8EfL/5jccH48vfjUaHarsMQ7eEYDpWj9KQ cNmzNr0W KZwZEubzGwwqXTZL45fPc66IpLiumA9+fBDhv+t5ypWDuqNDtFOfU4uDS+P/iA4phHopu5wpLfhmhDzO21FxqlJKGQNVGcIClp2jvfzbiNNV1QuEYq2cCBFw7rqQaya2gAy0AJsXatGBZssetZ2AMeAfHeXwAN2EAaM1c5HqnJrrAXHQ= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000006, 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 Wed, Jul 31, 2024 at 11:15:05AM +0200, Thomas Gleixner wrote: > On Wed, Jul 31 2024 at 14:27, Shivank Garg wrote: > > lmbench:lat_pagefault: Metric- page-fault time (us) - Lower is better > > 4-Level PT 5-Level PT % Change > > THP-never Mean:0.4068 Mean:0.4294 5.56 > > 95% CI:0.4057-0.4078 95% CI:0.4287-0.4302 > > > > THP-Always Mean: 0.4061 Mean: 0.4288 % Change > > 95% CI: 0.4051-0.4071 95% CI: 0.4281-0.4295 5.59 > > > > Inference: > > 5-level page table shows increase in page-fault latency but it does > > not significantly impact other benchmarks. > > 5% regression on lmbench is a NONO. > > 5-level page tables add a cost in every hardware page table walk. That's > a matter of fact and there is absolutely no reason to inflict this cost > on everyone. > > The solution to this to make the 5-level mechanics smarter by evaluating > whether the machine has enough memory to require 5-level tables and > select the depth at boot time. I gotta mention (again) that its a pain we can't mix and match like s390. They default run their userspace on 4 level, even if the kernel runs 5. Only silly daft userspace that needs more than insane amounts of memory get 5 level.