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[80.187.83.189]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fb482b9easm35213505e9.1.2025.10.10.00.54.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 Oct 2025 00:54:53 -0700 (PDT) Message-ID: <3577f7fd-429a-49c5-973b-38174a67be15@redhat.com> Date: Fri, 10 Oct 2025 09:54:50 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 mm-new 03/11] mm: thp: add support for BPF based THP order selection To: Yafang Shao Cc: Zi Yan , Alexei Starovoitov , Johannes Weiner , Andrew Morton , baolin.wang@linux.alibaba.com, Lorenzo Stoakes , Liam Howlett , npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, usamaarif642@gmail.com, gutierrez.asier@huawei-partners.com, Matthew Wilcox , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Amery Hung , David Rientjes , Jonathan Corbet , 21cnbao@gmail.com, Shakeel Butt , Tejun Heo , lance.yang@linux.dev, Randy Dunlap , bpf , linux-mm , "open list:DOCUMENTATION" , LKML References: <20250930055826.9810-1-laoar.shao@gmail.com> <20250930055826.9810-4-laoar.shao@gmail.com> <27e002e3-b39f-40f9-b095-52da0fbd0fc7@redhat.com> <7723a2c7-3750-44f7-9eb5-4ef64b64fbb8@redhat.com> <96AE1C18-3833-4EB8-9145-202517331DF5@nvidia.com> <129379f6-18c7-4d10-8241-8c6c5596d6d5@redhat.com> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 09.10.25 11:59, Yafang Shao wrote: > On Thu, Oct 9, 2025 at 5:19 PM David Hildenbrand wrote: >> >> On 08.10.25 15:11, Yafang Shao wrote: >>> On Wed, Oct 8, 2025 at 8:07 PM David Hildenbrand wrote: >>>> >>>> On 08.10.25 13:27, Zi Yan wrote: >>>>> On 8 Oct 2025, at 5:04, Yafang Shao wrote: >>>>> >>>>>> On Wed, Oct 8, 2025 at 4:28 PM David Hildenbrand wrote: >>>>>>> >>>>>>> On 08.10.25 10:18, Yafang Shao wrote: >>>>>>>> On Wed, Oct 8, 2025 at 4:08 PM David Hildenbrand wrote: >>>>>>>>> >>>>>>>>> On 03.10.25 04:18, Alexei Starovoitov wrote: >>>>>>>>>> On Mon, Sep 29, 2025 at 10:59 PM Yafang Shao wrote: >>>>>>>>>>> >>>>>>>>>>> +unsigned long bpf_hook_thp_get_orders(struct vm_area_struct *vma, >>>>>>>>>>> + enum tva_type type, >>>>>>>>>>> + unsigned long orders) >>>>>>>>>>> +{ >>>>>>>>>>> + thp_order_fn_t *bpf_hook_thp_get_order; >>>>>>>>>>> + int bpf_order; >>>>>>>>>>> + >>>>>>>>>>> + /* No BPF program is attached */ >>>>>>>>>>> + if (!test_bit(TRANSPARENT_HUGEPAGE_BPF_ATTACHED, >>>>>>>>>>> + &transparent_hugepage_flags)) >>>>>>>>>>> + return orders; >>>>>>>>>>> + >>>>>>>>>>> + rcu_read_lock(); >>>>>>>>>>> + bpf_hook_thp_get_order = rcu_dereference(bpf_thp.thp_get_order); >>>>>>>>>>> + if (WARN_ON_ONCE(!bpf_hook_thp_get_order)) >>>>>>>>>>> + goto out; >>>>>>>>>>> + >>>>>>>>>>> + bpf_order = bpf_hook_thp_get_order(vma, type, orders); >>>>>>>>>>> + orders &= BIT(bpf_order); >>>>>>>>>>> + >>>>>>>>>>> +out: >>>>>>>>>>> + rcu_read_unlock(); >>>>>>>>>>> + return orders; >>>>>>>>>>> +} >>>>>>>>>> >>>>>>>>>> I thought I explained it earlier. >>>>>>>>>> Nack to a single global prog approach. >>>>>>>>> >>>>>>>>> I agree. We should have the option to either specify a policy globally, >>>>>>>>> or more refined for cgroups/processes. >>>>>>>>> >>>>>>>>> It's an interesting question if a program would ever want to ship its >>>>>>>>> own policy: I can see use cases for that. >>>>>>>>> >>>>>>>>> So I agree that we should make it more flexible right from the start. >>>>>>>> >>>>>>>> To achieve per-process granularity, the struct-ops must be embedded >>>>>>>> within the mm_struct as follows: >>>>>>>> >>>>>>>> +#ifdef CONFIG_BPF_MM >>>>>>>> +struct bpf_mm_ops { >>>>>>>> +#ifdef CONFIG_BPF_THP >>>>>>>> + struct bpf_thp_ops bpf_thp; >>>>>>>> +#endif >>>>>>>> +}; >>>>>>>> +#endif >>>>>>>> + >>>>>>>> /* >>>>>>>> * Opaque type representing current mm_struct flag state. Must be accessed via >>>>>>>> * mm_flags_xxx() helper functions. >>>>>>>> @@ -1268,6 +1281,10 @@ struct mm_struct { >>>>>>>> #ifdef CONFIG_MM_ID >>>>>>>> mm_id_t mm_id; >>>>>>>> #endif /* CONFIG_MM_ID */ >>>>>>>> + >>>>>>>> +#ifdef CONFIG_BPF_MM >>>>>>>> + struct bpf_mm_ops bpf_mm; >>>>>>>> +#endif >>>>>>>> } __randomize_layout; >>>>>>>> >>>>>>>> We should be aware that this will involve extensive changes in mm/. >>>>>>> >>>>>>> That's what we do on linux-mm :) >>>>>>> >>>>>>> It would be great to use Alexei's feedback/experience to come up with >>>>>>> something that is flexible for various use cases. >>>>>> >>>>>> I'm still not entirely convinced that allowing individual processes or >>>>>> cgroups to run independent progs is a valid use case. However, since >>>>>> we have a consensus that this is the right direction, I will proceed >>>>>> with this approach. >>>>>> >>>>>>> >>>>>>> So I think this is likely the right direction. >>>>>>> >>>>>>> It would be great to evaluate which scenarios we could unlock with this >>>>>>> (global vs. per-process vs. per-cgroup) approach, and how >>>>>>> extensive/involved the changes will be. >>>>>> >>>>>> 1. Global Approach >>>>>> - Pros: >>>>>> Simple; >>>>>> Can manage different THP policies for different cgroups or processes. >>>>>> - Cons: >>>>>> Does not allow individual processes to run their own BPF programs. >>>>>> >>>>>> 2. Per-Process Approach >>>>>> - Pros: >>>>>> Enables each process to run its own BPF program. >>>>>> - Cons: >>>>>> Introduces significant complexity, as it requires handling the >>>>>> BPF program's lifecycle (creation, destruction, inheritance) within >>>>>> every mm_struct. >>>>>> >>>>>> 3. Per-Cgroup Approach >>>>>> - Pros: >>>>>> Allows individual cgroups to run their own BPF programs. >>>>>> Less complex than the per-process model, as it can leverage the >>>>>> existing cgroup operations structure. >>>>>> - Cons: >>>>>> Creates a dependency on the cgroup subsystem. >>>>>> might not be easy to control at the per-process level. >>>>> >>>>> Another issue is that how and who to deal with hierarchical cgroup, where one >>>>> cgroup is a parent of another. Should bpf program to do that or mm code >>>>> to do that? I remember hierarchical cgroup is the main reason THP control >>>>> at cgroup level is rejected. If we do per-cgroup bpf control, wouldn't we >>>>> get the same rejection from cgroup folks? >>>> >>>> Valid point. >>>> >>>> I do wonder if that problem was already encountered elsewhere with bpf >>>> and if there is already a solution. >>> >>> Our standard is to run only one instance of a BPF program type >>> system-wide to avoid conflicts. For example, we can't have both >>> systemd and a container runtime running bpf-thp simultaneously. >> >> Right, it's a good question how to combine policies, or "who wins". > > From my perspective, the ideal approach is to have one BPF-THP > instance per mm_struct. This allows for separate managers in different > domains, such as systemd managing BPF-THP for system processes and > containerd for container processes, while ensuring that any single > process is managed by only one BPF-THP. I came to the same conclusion. At least it's a valid start. Maybe we would later want a global fallback BPF-THP prog if none was enabled for a specific MM. But I would expect to start with a per MM way of doing it, it gives you way more flexibility in the long run. -- Cheers David / dhildenb