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From: Oliver Upton To: Yu Zhao Cc: James Houghton , Sean Christopherson , Andrew Morton , Paolo Bonzini , Albert Ou , Ankit Agrawal , Anup Patel , Atish Patra , Axel Rasmussen , Bibo Mao , Catalin Marinas , David Matlack , David Rientjes , Huacai Chen , James Morse , Jonathan Corbet , Marc Zyngier , Michael Ellerman , Nicholas Piggin , Palmer Dabbelt , Paul Walmsley , Raghavendra Rao Ananta , Ryan Roberts , Shaoqin Huang , Shuah Khan , Suzuki K Poulose , Tianrui Zhao , Will Deacon , Zenghui Yu , kvm-riscv@lists.infradead.org, kvm@vger.kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, loongarch@lists.linux.dev Subject: Re: [PATCH v4 2/7] mm: multi-gen LRU: Have secondary MMUs participate in aging Message-ID: References: <20240529180510.2295118-1-jthoughton@google.com> <20240529180510.2295118-3-jthoughton@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 232FA4000B X-Rspam-User: X-Rspamd-Server: rspam12 X-Stat-Signature: e6jfpfrngq518xgpptfce5dtmhjtpsr7 X-HE-Tag: 1717180888-585073 X-HE-Meta: U2FsdGVkX1+Wmbmc5dfn+Tqe6iX2KDF0+yR6yHEXlznK9qnf3T83XLuv32XwIX2UjdxLlePbxr2CmxVpokZGQrGThTFs6nryVY5thHiKQa8Z/zIRajU22xGIYNWj0P4aSdYVMcFgk10NNgc7NxxycDreLfGuJ0EBUwh5YEPM7bKF+cFJKXYh37eg4zzGu4mEIToBWsD+5xF7UETjAY6DEaWkKkXUa00NHxd8brygWw6HUBRpDM5pT6IwBVBJHdpA44SUH91LdOJdAW7veqWE24N5UPeNsYYEWbo1gGtMcz1/8aU1nZtyPg6g7hFXOqrRodgSAiIoW6QbhZae8M4oAmfOCpl/rF5ZCn9yiSVU2K7+A4fTpUav48k21enpA6B89ViuVbuHCiJ5QxRTchAH88G/my6IkRqW3a+z0YafvhvaOZCtU9Hw2sIiXDsi2eLua6CbYFWKZTHafXCvzqycK+oDQerTckx7ovE+xAHT7Swgu6k+P6MHJZSxaKFpMDiVGiPI44KOrrOJpCVFCs1vQ3CyM9pmGOTq6ucMfMCNcAqcY6hFQpnmf3qkHxWHXU4DX6ETfmGQj60fUBryUi/CzAhjCk2GJJAp3Iy8gqFuQeic3ASMgn6OBa6LmeeMSavkHFbP2A8xEoPOBENGuL1qtR+rTwnF2e7uF4QYNiyJCvO+KC0/bgU68rpQ/pk4LBDCZT8z0q0hW0Z2z+UJ/lHuycqit3qPMp1irqE+Cgm2MLR1r0MlVXSfLQRd44CVNCz0wCbmSIsCybuJDy4l3+8jdzj6XaQYmuzZuuAJndVZLPm/fHOPZmsOoA+6fDjC75Iie3vxP5hWrHovj7H2BZWzhwkw9aau/sfeRvvGzFXB+3o3GuK63Fqs7G8V66DsEmQi0wbwSMxLd0iD5JcGaNJaZ23uKZgj/PtRtC4ZWgl6hdC9LcczINtoHoGsoZ+zYaBBpb0gaJAXe1sWgaAGyjk 1md6As2G E+wiIjPssEsX54+IykB9J62FrhwhWUFwa65s9Wiz49pt4RRq+e9vReSJkFPOt8ld8Je0H4UaAjziOItvPTLxsDS1KRecjU04UyES2PRXFs7Qpkvc= 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 Fri, May 31, 2024 at 10:45:04AM -0600, Yu Zhao wrote: > On Fri, May 31, 2024 at 1:03 AM Oliver Upton wrote: > > > > On Fri, May 31, 2024 at 12:05:48AM -0600, Yu Zhao wrote: > > Let me add back what I said earlier: > > I'm not convinced, but it doesn't mean your point of view is > invalid. If you fully understand the implications of your design > choice and document them, I will not object. Thanks, I appreciate the sentiment. Hopefully we can align here. > > > All optimizations in v2 were measured step by step. Even that bitmap, > > > which might be considered overengineered, brought a readily > > > measuarable 4% improvement in memcached throughput on Altra Max > > > swapping to Optane: > > > > That's great, but taking an iterative approach to the problem allows > > the reviewers and maintainers to come to their own conclusions about > > each optimization independently. Squashing all of that together and > > posting the result doesn't allow for this. > > That's your methodology, which I respect: as I said I won't stand in your way. > > But mine is backed by data, please do respect that as well, Data is useful and expected for changes that aim to improve the performance of a system in one way or another. That is, after all, the sole intention of the work, no? What I'm also looking for is a controlled experiment, where a single independent variable (e.g. locking) can be evaluated against the baseline. All-or-nothing data has limited usefulness. > by doing what I asked: document your justifications. The justification for a series is against the upstream tree, not some out-of-tree stuff. The cover letter explicitly calls out the decision to simplify the patch series along with performance data I can reproduce on my own systems. This is a perfect example of how to contribute changes upstream. > > > What I don't think is acceptable is simplifying those optimizations > > > out without documenting your justifications (I would even call it a > > > design change, rather than simplification, from v3 to v4). > > > > No, sorry, there's nothing wrong with James' approach here. > > Sorry, are you saying "without documenting your justifications" is > nothing wrong? If so, please elaborate. As I mentioned above, the reasoning is adequately documented and the discussion that led to v4 is public. OTOH... > > The discussion that led to the design of v4 happened on list; you were > > on CC. The general consensus on the KVM side was that the bitmap was > > complicated and lacked independent justification. There was ample > > opportunity to voice your concerns before he spent the time on v4. > > Please re-read my previous emails -- I never object to the removal of > the bitmap or James' approach. > > And please stop making assumptions -- I did voice my concerns with > James privately. ^~~~~~~~~ If it happened in private then its no better than having said nothing at all. Please, keep the conversation on-list next time so we can iron out any disagreements there. Otherwise contributors are put in a *very* awkward situation of mediating the on- and off-list dialogue. > > You seriously cannot fault a contributor for respinning their work based > > on the provided feedback. > > Are you saying I faulted James for taking others' feedback? No. Sufficient justification is captured in the public review feedback + series cover letter. Your statement that the approach was changed without justification is unsubstantiated. > Also what do you think about the technical flaws and inaccurate > understandings I pointed out? You seem to have a strong opinion on > your iterate approach, but I hope you didn't choose to overlook the > real meat of this discussion. Serious question: are you not receiving my mail or something? I re-raised my question to you from ages ago about locking on the arm64 MMU. You didn't answer last time, I'd appreciate a reply this time around. Otherwise I couldn't be bothered about the color of the Kconfig bikeshed and don't have anything meaningful to add there. I think the three of you are trending in the right direction. -- Thanks, Oliver