On Tue, Mar 12, 2024 at 1:19 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote: > > > > On 3/11/24 09:46, Pasha Tatashin wrote: > > This is follow-up to the LSF/MM proposal [1]. Please provide your > > thoughts and comments about dynamic kernel stacks feature. This is a WIP > > has not been tested beside booting on some machines, and running LKDTM > > thread exhaust tests. The series also lacks selftests, and > > documentations. > > > > This feature allows to grow kernel stack dynamically, from 4KiB and up > > to the THREAD_SIZE. The intend is to save memory on fleet machines. From > > the initial experiments it shows to save on average 70-75% of the kernel > > stack memory. > > > > The average depth of a kernel thread depends on the workload, profiling, > > virtualization, compiler optimizations, and driver implementations. > > However, the table below shows the amount of kernel stack memory before > > vs. after on idling freshly booted machines: > > > > CPU #Cores #Stacks BASE(kb) Dynamic(kb) Saving > > AMD Genoa 384 5786 92576 23388 74.74% > > Intel Skylake 112 3182 50912 12860 74.74% > > AMD Rome 128 3401 54416 14784 72.83% > > AMD Rome 256 4908 78528 20876 73.42% > > Intel Haswell 72 2644 42304 10624 74.89% > > > > Some workloads with that have millions of threads would can benefit > > significantly from this feature. > > > > Ok, first of all, talking about "kernel memory" here is misleading. Hi Peter, I re-read my cover letter, and I do not see where "kernel memory" is mentioned. We are talking about kernel stacks overhead that is proportional to the user workload, as every active thread has an associated kernel stack. The idea is to save memory by not pre-allocating all pages of kernel-stacks, but instead use it as a safeguard when a stack actually becomes deep. Come-up with a solution that can handle rare deeper stacks only when needed. This could be done through faulting on the supported hardware (as proposed in this series), or via pre-map on every schedule event, and checking the access when thread goes off cpu (as proposed by Andy Lutomirski to avoid double faults on x86) . In other words, this feature is only about one very specific type of kernel memory that is not even directly mapped (the feature required vmapped stacks). > Unless your threads are spending nearly all their time sleeping, the > threads will occupy stack and TLS memory in user space as well. Can you please elaborate, what data is contained in the kernel stack when thread is in user space? My series requires thread_info not to be in the stack by depending on THREAD_INFO_IN_TASK. > Second, non-dynamic kernel memory is one of the core design decisions in > Linux from early on. This means there are lot of deeply embedded > assumptions which would have to be untangled. > > Linus would, of course, be the real authority on this, but if someone > would ask me what the fundamental design philosophies of the Linux > kernel are -- the design decisions which make Linux Linux, if you will > -- I would say: > > 1. Non-dynamic kernel memory > 2. Permanent mapping of physical memory The one and two are correlated. Given that all the memory is directly mapped, the kernel core cannot be relocatable, swappable, faultable etc. > 3. Kernel API modeled closely after the POSIX API > (no complicated user space layers) > 4. Fast system call entry/exit (a necessity for a > kernel API based on simple system calls) > 5. Monolithic (but modular) kernel environment > (not cross-privilege, coroutine or message passing) > > Third, *IF* this is something that should be done (and I personally > strongly suspect it should not), at least on x86-64 it probably should > be for FRED hardware only. With FRED, it is possible to set the #PF > event stack level to 1, which will cause an automatic stack switch for > #PF in kernel space (only). However, even in kernel space, #PF can sleep > if it references a user space page, in which case it would have to be > demoted back onto the ring 0 stack (there are multiple ways of doing > that, but it does entail an overhead.) My understanding is that with the proposed approach only double faults are prohibited to be used. Pre-map/check-access could still work, even though it would add some cost to the context switching. Thank you, Pasha
