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[2003:d8:2f18:f900:e0ae:65d5:8bf8:8cfd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3ea4b52b7fcsm4933190f8f.33.2025.09.15.03.39.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Sep 2025 03:39:15 -0700 (PDT) Message-ID: <444e5e05-2dc6-4201-b872-7eb8238c612c@redhat.com> Date: Mon, 15 Sep 2025 12:39:11 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v11 00/15] khugepaged: mTHP support To: Lorenzo Stoakes Cc: Kiryl Shutsemau , Nico Pache , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, dev.jain@arm.com, corbet@lwn.net, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, akpm@linux-foundation.org, baohua@kernel.org, willy@infradead.org, peterx@redhat.com, wangkefeng.wang@huawei.com, usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com, richard.weiyang@gmail.com, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, jannh@google.com, pfalcato@suse.de References: <20250912032810.197475-1-npache@redhat.com> <2c5328ee-fb6e-45cf-a151-868cc8e7ff97@lucifer.local> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 15.09.25 12:35, Lorenzo Stoakes wrote: > On Mon, Sep 15, 2025 at 12:22:07PM +0200, David Hildenbrand wrote: >> On 15.09.25 11:22, Kiryl Shutsemau wrote: >>> On Fri, Sep 12, 2025 at 05:31:51PM -0600, Nico Pache wrote: >>>> On Fri, Sep 12, 2025 at 6:25 AM David Hildenbrand wrote: >>>>> >>>>> On 12.09.25 14:19, Kiryl Shutsemau wrote: >>>>>> On Thu, Sep 11, 2025 at 09:27:55PM -0600, Nico Pache wrote: >>>>>>> The following series provides khugepaged with the capability to collapse >>>>>>> anonymous memory regions to mTHPs. >>>>>>> >>>>>>> To achieve this we generalize the khugepaged functions to no longer depend >>>>>>> on PMD_ORDER. Then during the PMD scan, we use a bitmap to track individual >>>>>>> pages that are occupied (!none/zero). After the PMD scan is done, we do >>>>>>> binary recursion on the bitmap to find the optimal mTHP sizes for the PMD >>>>>>> range. The restriction on max_ptes_none is removed during the scan, to make >>>>>>> sure we account for the whole PMD range. When no mTHP size is enabled, the >>>>>>> legacy behavior of khugepaged is maintained. max_ptes_none will be scaled >>>>>>> by the attempted collapse order to determine how full a mTHP must be to be >>>>>>> eligible for the collapse to occur. If a mTHP collapse is attempted, but >>>>>>> contains swapped out, or shared pages, we don't perform the collapse. It is >>>>>>> now also possible to collapse to mTHPs without requiring the PMD THP size >>>>>>> to be enabled. >>>>>>> >>>>>>> When enabling (m)THP sizes, if max_ptes_none >= HPAGE_PMD_NR/2 (255 on >>>>>>> 4K page size), it will be automatically capped to HPAGE_PMD_NR/2 - 1 for >>>>>>> mTHP collapses to prevent collapse "creep" behavior. This prevents >>>>>>> constantly promoting mTHPs to the next available size, which would occur >>>>>>> because a collapse introduces more non-zero pages that would satisfy the >>>>>>> promotion condition on subsequent scans. >>>>>> >>>>>> Hm. Maybe instead of capping at HPAGE_PMD_NR/2 - 1 we can count >>>>>> all-zeros 4k as none_or_zero? It mirrors the logic of shrinker. >>>>>> >>>>> >>>>> I am all for not adding any more ugliness on top of all the ugliness we >>>>> added in the past. >>>>> >>>>> I will soon propose deprecating that parameter in favor of something >>>>> that makes a bit more sense. >>>>> >>>>> In essence, we'll likely have an "eagerness" parameter that ranges from >>>>> 0 to 10. 10 is essentially "always collapse" and 0 "never collapse if >>>>> not all is populated". >>>> Hi David, >>>> >>>> Do you have any reason for 0-10, I'm guessing these will map to >>>> different max_ptes_none values. >>>> I suggest 0-5, mapping to 0,32,64,128,255,511 >>> >>> That's too x86-64 specific. >>> >>> And the whole idea is not to map to directly, but give kernel wiggle >>> room to play. >> >> Initially we will start out simple and map it directly. But yeah, the idea >> is to give us some more room later. > > I think it's less 'wiggle room' and more us being able to _abstract_ what this > measurement means while reserving the right to adjust this. > > But maybe we are saying the same thing in different ways. > >> >> I had something logarithmic in mind which would roughly be (ignoring the the >> weird -1 for simplicity and expressing it as "used" instead of none-or-zero) >> >> 0 -> ~100% used (~0% none) > > So equivalent to 511 today? > >> 1 -> ~50% used (~50% none) >> 2 -> ~25% used (~75% none) >> 3 -> ~12.5% used (~87.5% none) >> 4 -> ~11.25% used (~88,75% none) >> ... >> 10 -> ~0% used (~100% none) > > So equivalent to 0 today? Yes. > > And with a logarithmic weighting towards values closer to "0% used"? > > This seems sensible given the only reports we've had of non-0/511 uses here are > in that range... > > But ofc this interpretation should be something we determine + treated as an > implementation detail that we can modify later. > >> >> Mapping that to actual THP sizes (#pages in a thp) on an arch will be easy. > > And at different mTHP levels too right? Yes exactly. -- Cheers David / dhildenb