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[2003:d8:2f18:f900:e0ae:65d5:8bf8:8cfd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e76078708bsm17596191f8f.15.2025.09.15.03.22.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Sep 2025 03:22:10 -0700 (PDT) Message-ID: Date: Mon, 15 Sep 2025 12:22:07 +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: Kiryl Shutsemau , Nico Pache Cc: 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, lorenzo.stoakes@oracle.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> 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 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 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) 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) Mapping that to actual THP sizes (#pages in a thp) on an arch will be easy. -- Cheers David / dhildenb