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.
Unless your threads are spending nearly all their time sleeping, the
threads will occupy stack and TLS memory in user space as well.
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
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.)
-hpa
