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[2003:d8:2f28:7c00:a95e:ac49:f2ad:ab84]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a364d2636bsm14163272f8f.99.2025.05.20.08.54.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 20 May 2025 08:54:43 -0700 (PDT) Message-ID: <9b44fe43-155d-457d-81ce-a2c1fb86521a@redhat.com> Date: Tue, 20 May 2025 17:54:42 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 0/5] mm, bpf: BPF based THP adjustment To: Lorenzo Stoakes , Yafang Shao Cc: akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, hannes@cmpxchg.org, usamaarif642@gmail.com, gutierrez.asier@huawei-partners.com, willy@infradead.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, bpf@vger.kernel.org, linux-mm@kvack.org References: <20250520060504.20251-1-laoar.shao@gmail.com> <746e8123-2332-41c8-851b-787cb8c144a1@redhat.com> <849decad-ab38-4a1a-8532-f518a108d8c6@lucifer.local> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: xbrk7gqgwk8bnff6fiq775xwe41w9f46 X-Rspamd-Queue-Id: AA6CB18000E X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1747756488-496120 X-HE-Meta: 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 EKt12j6j 6iiYpNdQrrDsoUmtQhhOneX+NFmsw3nXtuPBeEuxWR3GCJYKxGfCt0CiAwRKQ0Zjgn+Klbf1y1hX/OmNh8iWBkAoMkN2JRmbBypDPrku2Sh5E2Xs3Jjq6npHTqRUWC/ObJEnDhxnWw5MGPEHehmCK/w6e7yuuvY+8e2PWnnUS4HZsaGcxqlIOSu8gxhb1cpsC6u7+b2RGJ42pgxtTspgRYcdLBw1e+nVKCKvLT+ThMuboDjAIjWToZ+0I2tQiw0CmuBX9CBcOkw9X/vEv+/37qP3dAzYnbtwtJE081dND2hAWNL071O4aksichs6Z98+nMvk21sAW1s5nqAwfh3LJaAH8/JWoQXIQYrxWJ4O1jmcPPjkC5exFr/M6+3e1/QDThHxkESgcLaoQLT/FdfP60He/7lR3xTNvQlx9CxquRVAOc4L3NXmvBLYjEMdRqrOtP4LUpGKaWIHCHYrxWcKdc54Dn9Dv5v4Br4/R 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: >> I totally agree with you that the key point here is how to define the >> API. As I replied to David, I believe we have two fundamental >> principles to adjust the THP policies: >> 1. Selective Benefit: Some tasks benefit from THP, while others do not. >> 2. Conditional Safety: THP allocation is safe under certain conditions >> but not others. >> >> Therefore, I believe we can define these APIs based on the established >> principles - everything else constitutes implementation details, even >> if core MM internals need to change. > > But if we're looking to make the concept of THP go away, we really need to > go further than this. Yeah. I might be wrong, but I also don't think doing control on a per-process level etc would be the right solution long-term. In a world where we do stuff automatically ("auto" mode), we would be much smarter about where to place a (m)THP, and which size we would use. One might use bpf to control the allocation policy. But I don't think this would be per-process or even per-VMA etc. Sure, we might give hints, but placement decisions should happen on another level (e.g., during page faults, during khugepaged etc). > > The second we have 'bpf program that figures out whether THP should be > used' we are permanently tied to the idea of THP on/off being a thing. > > I mean any future stuff that makes THP more automagic will probably involve > having new modes for the legacy THP > /sys/kernel/mm/transparent_hugepage/enabled and > /sys/kernel/mm/transparent_hugepage/hugepages-xxkB/enabled Yeah, the plan is to have "auto" in /sys/kernel/mm/transparent_hugepage/enabled and just have all other sizes "inherit" that option. And have a Kconfig that just enables that as default. Once we're there, just phase out the interface long-term. That's the plan. Now we "only" have to figure out how to make the placement actually better ;) > > But if people are super reliant on this stuff it's potentially really > limiting. > > I think you said in another post here that you were toying with the notion > of exposing somehow the madvise() interface and having that be the 'stable > API' of sorts? > > That definitely sounds more sensible than something that very explicitly > interacts with THP. > > Of course we have Usama's series and my proposed series for extending > process_madvise() along those lines also. Yes. -- Cheers, David / dhildenb