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([2a09:80c0:192:0:5dac:bf3d:c41:c3e7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4d9f09622sm5414130f8f.98.2025.05.27.05.19.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 May 2025 05:19:17 -0700 (PDT) Message-ID: Date: Tue, 27 May 2025 14:19:16 +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> <5f0aadb1-28a8-4be0-bad9-16b738840e57@redhat.com> <5d48d0c3-89a3-44da-bc1a-9a4601f146a4@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-Server: rspam06 X-Rspamd-Queue-Id: 1B17A1C000B X-Stat-Signature: rprccdmf5oaafc47ba93rejy7gm35dut X-Rspam-User: X-HE-Tag: 1748348361-915972 X-HE-Meta: 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 PzE7f6GF ivwx2fG9X4chJnEg0jDYqsJye/KgBEfe/TwXrnko3spgFzt8p5uAAJOG/gWQ07OUt/Qv+Z/tYz5RonFjCU4/8E8IsP6y9cy7UkxWDZA2LKEjmsaX8mAaLnRQqpumUlOkcDkvoELghdrfh8kX/TII54G0gIA== 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 27.05.25 11:43, Yafang Shao wrote: > On Tue, May 27, 2025 at 5:27 PM David Hildenbrand wrote: >> >> On 27.05.25 10:40, Yafang Shao wrote: >>> On Tue, May 27, 2025 at 4:30 PM David Hildenbrand wrote: >>>> >>>>>> 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 ... >>> >>> If I understand correctly, this can only occur for shared THP but not >>> anonymous THP. For instance, if either process allocates an anonymous >>> THP, it would trigger the creation of a new MM. Please correct me if >>> I'm mistaken. >> >> What clone(CLONE_VM) will do is essentially create a new process, that >> shares the MM with the original process. Similar to a thread, just that >> the new process will show up in /proc/ as ... a new process, not as a >> thread under /prod/$pid/tasks of the original process. >> >> Both processes will operate on the shared MM struct as if they were >> ordinary threads. No Copy-on-Write involved. >> >> One example use case I've been involved in is async teardown in QEMU [1]. >> >> [1] https://kvm-forum.qemu.org/2022/ibm_async_destroy.pdf > > I understand what you mean, but what I'm really confused about is how > this relates to allocating anonymous THP. If either one allocates > anon THP, it will definitely create a new MM, right ? No. They work on the same address space - same MM. Either can allocate a new anon THP and the other one would be able to modify it. No fork/CoW. I only bring it up because it's two "processes" sharing the same MM. And the THP mode in your proposal would actually be per-MM and not per process. It's confusing ... :) > >> >>> >>>> >>>> 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". >>> >>> Could you please elaborate further on 'make placement decisions'? As >>> previously mentioned, we (including the broader community) really need >>> the user input to determine whether THP allocation is appropriate in a >>> given case. >> >> The glorious future were we make smarter decisions where to actually >> place THPs even in the "always" mode. >> >> E.g., just because we enable "always" for a process does not mean that >> we really want a THP everywhere; quite the opposite. > > So 'always' simply means "the system doesn't guarantee THP allocation > will succeed" ? I mean, with THPs, there are no guarantees, ever :( > If that's the case, we should revisit RFC v1 [0], > where we proposed rejecting THP allocations in certain scenarios for > specific tasks. Hooking into actual page allocation during page faults (e.g., THP size, khugepaged collapse decisions) is IMHO a much better application of ebpf than setting a THP mode per process (or MM ... ) using epbf. So yes, you could drive the system in "always" mode and decide to not allocate THPs during page faults / khugepaged for specific processes. IMHO that also does not contradict the VM_HUGEPAGE / VM_NOHUGEPAGE default setting proposal: VM_HUGEPAGE could feed into the epbf program as yet another parameter to make a decision. -- Cheers, David / dhildenb