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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-447f78aeb8csm259530905e9.28.2025.05.27.01.30.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 May 2025 01:30:12 -0700 (PDT) Message-ID: <5f0aadb1-28a8-4be0-bad9-16b738840e57@redhat.com> Date: Tue, 27 May 2025 10:30:10 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 0/5] mm, bpf: BPF based THP adjustment To: Yafang Shao Cc: akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, lorenzo.stoakes@oracle.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> <7d8a9a5c-e0ef-4e36-9e1d-1ef8e853aed4@redhat.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 1B4CDA0007 X-Stat-Signature: w49pj4wj57hgb844u7wit455yzzmsz7a X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1748334616-158183 X-HE-Meta: U2FsdGVkX1/buC6ctQHc1mIihFyZxwP0/Hz+FPZygx9pLwxMFVxKjip/EszkUqwle+yfJLG42oELoeTUVmGv3dYsv6ddFVO44P1KSSwfHgp3P3t7ljFZlC/ANIF8WgfxtxDVEYLkNczQ1TZXZBThBlPd1UY8IByacbCVFRAjwYImDgGbu38anDGXKsWXtH87GTsN0K75CY7WdFKIR9o00D9Q1KV3spux/8n9t1bOePLQ59Ve0rcwtbMzE+tGudd2rIkvtepskRmFXBfaVYaAzQSgWZhFil5GMMvUYUKzYtNbSkAryuJu8iRl2bWCTymdEePsRvqc3NGOt33+TREDGq89VDOzL/mJOrPUlqE0iu6uztlKMv5hI5wOnbQonoXKNkH5GiLzWfads3qAek4sa1GPWzoPaOBug9LiNB8JdUKK094aPJqW16yGW+VthNCBM1LQ6C0HbKPAKMISW3Uy2QQZPNaGRK/5GyZ+gzsmGKGZvhhokZx5mu2PBjFY52NQj/8sXC7X3xx5796/n70gC+rjC7x4iXWdYaXMjg1QN+p1R0FYGgfm+IUnOPba/8iodTyYMYlDxns4hP9dXQwy+7VYBrFS/4YWfD0iL07/VcBwS3FBycMkJK2Gl727C+UXrM0L8Cz6A29tyf0PxruZMNPKKBFArQd/RK9VoHwxX28u5kxVb/ZDsOL+ed7zas3DK8er3N1Z1bLffzFOGNMGK/VIeBlxfIxuR1UpGUp4NrCZakftpqqg7phBpJOm/k+OrNWbIIn4/qun/sJ05sMB9MPDpvanYmPpQ/LABg3nbNj1LrY2tcS5dNEhqyQPeE+lzv7dQDIQTm0XVvEkM+8hTOR4rx76emc80K9NVyrRM9vhd/7Mzsf+WjwLjcSfvINPPtgr/DPfYwVIRsT3YZ0qLvKxrCEcC0ubhzfw0LQTcT29mb1r7h5Yqr/1uJs7w8bcwldR/d1bjVNipoxHs3b 3vt0d8Yp Gjyi/c+tdhdXrQx8ck59ZRM6PgzBvguHh0H92TVnixmH0glaEHjwkPPW/xkFihzCgOc7GU5oPLRXbz7mpBxUCueBxlkmIDXdgPxGcKIVX+TJMRxBskEsK7N34nY/2pYPRv1691imEpbCFCSP97KtA4LD8Cw== 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 don't think we want to add such a mechanism (new mode) where the >> primary configuration mechanism is through bpf. >> >> Maybe bpf could be used as an alternative, but we should look into a >> reasonable alternative first, like the discussed mctrl()/.../ raised in >> the process_madvise() series. >> >> No "bpf" mode in disguise, please :) > > This goal can be readily achieved using a BPF program. In any case, it > is a feasible solution. No BPF-only solution. > >> >>> We could define >>> the API as follows: >>> >>> struct bpf_thp_ops { >>> /** >>> * @task_thp_mode: Get the THP mode for a specific task >>> * >>> * Return: >>> * - TASK_THP_ALWAYS: "always" mode >>> * - TASK_THP_MADVISE: "madvise" mode >>> * - TASK_THP_NEVER: "never" mode >>> * Future modes can also be added. >>> */ >>> int (*task_thp_mode)(struct task_struct *p); >>> }; >>> >>> For observability, we could add a "THP mode" field to >>> /proc/[pid]/status. For example: >>> >>> $ grep "THP mode" /proc/123/status >>> always >>> $ grep "THP mode" /proc/456/status >>> madvise >>> $ grep "THP mode" /proc/789/status >>> never >>> >>> The THP mode for each task would be determined by the attached BPF >>> program based on the task's attributes. We would place the BPF hook in >>> appropriate kernel functions. Note that this setting wouldn't be >>> inherited during fork/exec - the BPF program would make the decision >>> dynamically for each task. >> >> What would be the mode (default) when the bpf program would not be active? >> >>> This approach also enables runtime adjustments to THP modes based on >>> system-wide conditions, such as memory fragmentation or other >>> performance overheads. The BPF program could adapt policies >>> dynamically, optimizing THP behavior in response to changing >>> workloads. >> >> I am not sure that is the proper way to handle these scenarios: I never >> heard that people would be adjusting the system-wide policy dynamically >> in that way either. >> >> Whatever we do, we have to make sure that what we add won't >> over-complicate things in the future. Having tooling dynamically adjust >> the THP policy of processes that coarsely sounds ... very wrong long-term. > > This is just an example demonstrating how BPF can be used to adjust > its flexibility. Notably, all these policies can be implemented > without modifying the kernel. See below on "policy". > >> >> > > As Liam pointed out in another thread, naming is challenging here - >>> "process" might not be the most accurate term for this context. >> >> No, it's not even a per-process thing. It is per MM, and a MM might be >> used by multiple processes ... > > I consistently use 'thread' for the latter case. You can use CLONE_VM without CLONE_THREAD ... Additionally, this > can be implemented per-MM without kernel code modifications. > With a well-designed API, users can even implement custom THP > policies—all without altering kernel code. You can switch between modes, that' all you can do. I wouldn't really call that "custom policy" as it is extremely limited. And that's exactly my point: it's basic switching between modes ... a reasonable policy in the future will make placement decisions and not just state "always/never/madvise". -- Cheers, David / dhildenb