On 15/05/2025 16:15, Lorenzo Stoakes wrote: > Thanks for coming back to me so quickly, appreciated :) > > I am reacting in a 'WTF' way here, but it's in proportion to the (at least > perceived) magnitude of this change. We really need to be sure this is > right. > Lol I had to rewrite my replies a few times to tone them down. Hopefully I don't come across as aggressive :) > On Thu, May 15, 2025 at 03:50:47PM +0100, Usama Arif wrote: >> >> >> On 15/05/2025 14:55, Lorenzo Stoakes wrote: >>> On Thu, May 15, 2025 at 02:33:29PM +0100, Usama Arif wrote: >>>> This allows to change the THP policy of a process, according to the value >>>> set in arg2, all of which will be inherited during fork+exec: >>> >>> This is pretty confusing. >>> >>> It should be something like 'add a new prctl() option that allows...' etc. >>> >>>> - PR_THP_POLICY_DEFAULT_HUGE: This will set the MMF2_THP_VMA_DEFAULT_HUGE >>>> process flag which changes the default of new VMAs to be VM_HUGEPAGE. The >>>> call also modifies all existing VMAs that are not VM_NOHUGEPAGE >>>> to be VM_HUGEPAGE. >>> >>> This is referring to implementation detail that doesn't matter for an overview, >>> just add a summary here e.g. >>> >>> PR_THP_POLICY_DEFAULT_HUGE - set VM_HUGEPAGE flag in all VMAs by default, >>> including after fork/exec, ignoring global policy. >>> >>> PR_THP_POLICY_DEFAULT_NOHUGE - clear VM_HUGEPAGE flag in all VMAs by default, >>> including after fork/exec, ignoring global policy. >>> >>> PR_THP_POLICY_DEFAULT_SYSTEM - Eliminate any policy set above. >> >> Hi Lorenzo, >> >> Thanks for the review. I will make the cover letter clearer in the next revision. > > The next version should emphatically be an RFC also, please. Your cover letter > should mention you're fundamentally changing mm_struct and VMA logic, and > explain why your use cae is so important that that is justified. > Thanks, will make it RFC and add that I am making changes to mm_struct and VMA logic. >> >>> >>>> This allows systems where the global policy is set to "madvise" >>>> to effectively have THPs always for the process. In an environment >>>> where different types of workloads are stacked on the same machine >>>> whose global policy is set to "madvise", this will allow workloads >>>> that benefit from always having hugepages to do so, without regressing >>>> those that don't. >>> >>> So does this just ignore and override the global policy? I'm not sure I'm >>> comfortable with that. >> >> No. The decision making of when and what order THPs are allowed is not >> changed, i.e. there are no changes in __thp_vma_allowable_orders and >> thp_vma_allowable_orders. David has the same concern as you and this >> current series is implementing what David suggested in >> https://lore.kernel.org/all/3f7ba97d-04d5-4ea4-9f08-6ec3584e0d4c@xxxxxxxxxx/ >> >> It will change the existing VMA (NO)HUGE flags according to >> the prctl. For e.g. doing PR_THP_POLICY_DEFAULT_HUGE will not give >> a THP when global policy is never. > > Umm... > > + case PR_SET_THP_POLICY: > + if (arg3 || arg4 || arg5) > + return -EINVAL; > + if (mmap_write_lock_killable(me->mm)) > + return -EINTR; > + switch (arg2) { > + case PR_THP_POLICY_DEFAULT_HUGE: > + set_bit(MMF2_THP_VMA_DEFAULT_HUGE, &me->mm->flags2); > + process_vmas_thp_default_huge(me->mm); > + break; > + default: > > > Where's the check against never? You're unconditionally setting VM_HUGEPAGE? So this was from the discussion with David. My initial implementation in v1, messed with the policy evaluation in thp_vma_allowable_orders and __thp_vma_allowable_orders. The whole point of doing it this way is that you dont mess with the policy evaluation. hugepage_global_always and hugepage_global_enabled will still evaluate to false when never is set and you will not get a hugepage. But more on it below. > > You're relying on VM_HUGEPAGE being ignored in this instance? But you're still: > > 1. Setting VM_HUGEPAGE everywhere (and breaking VMA merging everywhere). > > 2. Setting MMF2_THP_VMA_DEFAULT_HUGE and making it so PR_GET_THP_POLICY says it > has a policy of default huge even if policy is set to never? > > I'm not ok with that. I'd much rather we do the never check here... > I am ok with that. I can add a check over here that wraps this in: if (hugepage_global_enabled()) ... > Also see hugepage_madvise(). There's arch-specific code that overrides > that, and you're now bypassing that (yes it's for one arch of course but > it's still a thing) > Thanks, I will put if (mm_has_pgste(vma->vm_mm)) return 0; at the start. >> >>> >>> What about if the the policy is 'never'? Does this override that? That seems >>> completely wrong. >> >> No, it won't override it. hugepage_global_always and hugepage_global_enabled >> will still evaluate to false and you wont get a hugepage no matter what prctl >> is set. > > Ack ok I see as above, you're relying on VM_HUGEPAGE enforcing htis. > > You really need to put stuff like this in the cover letter though!! > Sure will do in the next revision, Thanks. >> >>> >>>> - PR_THP_POLICY_DEFAULT_NOHUGE: This will set the MMF2_THP_VMA_DEFAULT_NOHUGE >>>> process flag which changes the default of new VMAs to be VM_NOHUGEPAGE. >>>> The call also modifies all existing VMAs that are not VM_HUGEPAGE >>>> to be VM_NOHUGEPAGE. >>>> This allows systems where the global policy is set to "always" >>>> to effectively have THPs on madvise only for the process. In an >>>> environment where different types of workloads are stacked on the >>>> same machine whose global policy is set to "always", this will allow >>>> workloads that benefit from having hugepages on an madvise basis only >>>> to do so, without regressing those that benefit from having hugepages >>>> always. >>> >>> Wait, so 'no huge' means 'madvise'? What? This is confusing. >> >> >> I probably made the cover letter confusing :) or maybe need to rename the flags. >> >> This flag work as follows: >> >> a) Changes the default flag of new VMAs to be VM_NOHUGEPAGE >> >> b) Modifies all existing VMAs that are not VM_HUGEPAGE to be VM_NOHUGEPAGE >> >> c) Is inherited during fork+exec >> >> I think maybe I should add VMA to the flag names and rename the flags to >> PR_THP_POLICY_DEFAULT_VMA_(NO)HUGE ?? > > Please no :) 'VMA' is implicit re: mappings. If you're touching memory > mappings you're necessarily touching VMAs. > > I know some prctl() (a pathway to many abilities some consider to be > unnatural) uses 'VMA' in some of the endpoints but generally when referring > to specific VMAs no? > > These namesa are already kinda horrible (yes naming is hard, for everyone, > ask me about MADV_POISON/REMEDY) but I think something like: > > PR_DEFAULT_MADV_HUGEPAGE > PR_DEFAULT_MADV_NOHUGEPAGE > > -ish :) > Sure, happy with that, Thanks. >> >>> >>>> - PR_THP_POLICY_DEFAULT_SYSTEM: This will clear the MMF2_THP_VMA_DEFAULT_HUGE >>>> and MMF2_THP_VMA_DEFAULT_NOHUGE process flags. >>>> >>>> These patches are required in rolling out hugepages in hyperscaler >>>> configurations for workloads that benefit from them, where workloads are >>>> stacked anda single THP global policy is likely to be used across the entire >>>> fleet, and prctl will help override it. >>> >>> I don't understand this justification whatsoever. What does 'stacked' mean? And >>> you're not justifying why you'd override the policy? >> >> By stacked I just meant different types of workloads running on the same machine. >> Lets say we have a single server whose global policy is set to madvise. >> You can have a container on that server running some database workload that best >> works with madvise. >> You can have another container on that same server running some AI workload that would >> benefit from having VM_HUGEPAGE set on all new VMAs. We can use prctl >> PR_THP_POLICY_DEFAULT_HUGE to get VM_HUGEPAGE set by default on all new VMAs for that >> container. >> >>> >>> This series has no actual justificaiton here at all? You really need to provide one. >>> >> >> There was a discussion on the usecases in >> https://lore.kernel.org/all/13b68fa0-8755-43d8-8504-d181c2d46134@xxxxxxxxx/ >> >> I tried (and I guess failed :)) to summarize the justification from that thread. > > It's fine, I have most definitely not been as clear as I could be in series > too :>) just need to add a bigger summary. > > Don't afraid to waffle on... (I know I am not... ;) > >> >> I will try and rephrase it here. >> >> In hyperscalers, we have a single THP policy for the entire fleet. >> We have different types of workloads (e.g. AI/compute/databases/etc) >> running on a single server (this is what I meant by 'stacked'). >> Some of these workloads will benefit from always getting THP at fault (or collapsed >> by khugepaged), some of them will benefit by only getting them at madvise. >> >> This series is useful for 2 usecases: >> >> 1) global system policy = madvise, while we want some workloads to get THPs >> at fault and by khugepaged :- some processes (e.g. AI workloads) benefits from getting >> THPs at fault (and collapsed by khugepaged). Other workloads like databases will incur >> regression (either a performance regression or they are completely memory bound and >> even a very slight increase in memory will cause them to OOM). So what these patches >> will do is allow setting prctl(PR_THP_POLICY_DEFAULT_HUGE) on the AI workloads, >> (This is how workloads are deployed in our (Meta's/Facebook) fleet at this moment). >> >> 2) global system policy = always, while we want some workloads to get THPs >> only on madvise basis :- Same reason as 1). What these patches >> will do is allow setting prctl(PR_THP_POLICY_DEFAULT_NOHUGE) on the database >> workloads. >> (We hope this is us (Meta) in the near future, if a majority of workloads show that they >> benefit from always, we flip the default host setting to "always" across the fleet and >> workloads that regress can opt-out and be "madvise". >> New services developed will then be tested with always by default. "always" is also the >> default defconfig option upstream, so I would imagine this is faced by others as well.) > > Right, but I'm not sure you're explaining why prctl(), one of the most cursed, > neglected and frankly evil (maybe exaggerating :P) APIs in the kernel is the way > to do this? > > You do need to summarise why the suggested idea re: BPF, or cgroups, or whatnot > is _totally unworkable_. > > And why not process_madvise() with MADV_HUGEPAGE? > > I'm also not sure fork/exec is a great situation to have, because are you sure > the workloads stay the same across all fork/execs that you're now propagating? > > It feels like this should be a cgroup thing, really. > So I actually dont mind the cgroup implementation (that was actually my first prototype and after that I saw there was someone who had posted it earlier). It was shot down because it wont be hierarchical and doesnt solve it when its not being done in a cgroup. A large proportion of the thread in v1 was discussion with David, Johannes, Zi and Yafang (the bpf THP policy author) on different ways of doing this. >> >> Hope this makes the justification for the patches clearer :) > > Sure, please add this kind of thing to the cover letter to get fewer 'wtf' > reactions :) > > You're doing something really _big_ and _opinonated_ here though, that's > basically fundamentally changing core stuff, so an extended discussion of why > you feel it's so important, why other approaches are not workable, why the > Sauron-spawned Mordor dwelling prctl() API is the way to go, etc. > >> >>>> >>>> v1->v2: >>> >>> Where was the v1? Is it [0]? >>> >>> This seems like a massive change compared to that series? >>> >>> You've renamed it and not referenced the old series, please make sure you link >>> it or somehow let somebody see what this is against, because it makes review >>> difficult. >>> >> >> Yes its the patch you linked below. Sorry should have linked it in this series. >> Its a big change, but it was basically incorporating all feedback from David, >> while trying to achieve a similar goal. Will link it in future series. > > Yeah, again, this should have been an RFC on that basis :) > >> >>> [0]: https://lore.kernel.org/linux-mm/20250507141132.2773275-1-usamaarif642@xxxxxxxxx/ >>> >>>> - change from modifying the THP decision making for the process, to modifying >>>> VMA flags only. This prevents further complicating the logic used to >>>> determine THP order (Thanks David!) >>>> - change from using a prctl per policy change to just using PR_SET_THP_POLICY >>>> and arg2 to set the policy. (Zi Yan) >>>> - Introduce PR_THP_POLICY_DEFAULT_NOHUGE and PR_THP_POLICY_DEFAULT_SYSTEM >>>> - Add selftests and documentation. >>>> >>>> Usama Arif (6): >>>> prctl: introduce PR_THP_POLICY_DEFAULT_HUGE for the process >>>> prctl: introduce PR_THP_POLICY_DEFAULT_NOHUGE for the process >>>> prctl: introduce PR_THP_POLICY_SYSTEM for the process >>>> selftests: prctl: introduce tests for PR_THP_POLICY_DEFAULT_NOHUGE >>>> selftests: prctl: introduce tests for PR_THP_POLICY_DEFAULT_HUGE >>>> docs: transhuge: document process level THP controls >>>> >>>> Documentation/admin-guide/mm/transhuge.rst | 40 +++ >>>> include/linux/huge_mm.h | 4 + >>>> include/linux/mm_types.h | 14 + >>>> include/uapi/linux/prctl.h | 6 + >>>> kernel/fork.c | 1 + >>>> kernel/sys.c | 35 +++ >>>> mm/huge_memory.c | 56 ++++ >>>> mm/vma.c | 2 + >>>> tools/include/uapi/linux/prctl.h | 6 + >>>> .../trace/beauty/include/uapi/linux/prctl.h | 6 + >>>> tools/testing/selftests/prctl/Makefile | 2 +- >>>> tools/testing/selftests/prctl/thp_policy.c | 286 ++++++++++++++++++ >>>> 12 files changed, 457 insertions(+), 1 deletion(-) >>>> create mode 100644 tools/testing/selftests/prctl/thp_policy.c >>>> >>>> -- >>>> 2.47.1 >>>> >>
