[RFC 00/14] Dynamic Kernel Stacks

[Pasha Tatashin]

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.

[1] https://lore.kernel.org/all/CA+CK2bBYt9RAVqASB2eLyRQxYT5aiL0fGhUu3TumQCyJCNTWvw@xxxxxxxxxxxxxx

Pasha Tatashin (14):
  task_stack.h: remove obsolete __HAVE_ARCH_KSTACK_END check
  fork: Clean-up ifdef logic around stack allocation
  fork: Clean-up naming of vm_strack/vm_struct variables in vmap stacks
    code
  fork: Remove assumption that vm_area->nr_pages equals to THREAD_SIZE
  fork: check charging success before zeroing stack
  fork: zero vmap stack using clear_page() instead of memset()
  fork: use the first page in stack to store vm_stack in cached_stacks
  fork: separate vmap stack alloction and free calls
  mm/vmalloc: Add a get_vm_area_node() and vmap_pages_range_noflush()
    public functions
  fork: Dynamic Kernel Stacks
  x86: add support for Dynamic Kernel Stacks
  task_stack.h: Clean-up stack_not_used() implementation
  task_stack.h: Add stack_not_used() support for dynamic stack
  fork: Dynamic Kernel Stack accounting

 arch/Kconfig                     |  33 +++
 arch/x86/Kconfig                 |   1 +
 arch/x86/kernel/traps.c          |   3 +
 arch/x86/mm/fault.c              |   3 +
 include/linux/mmzone.h           |   3 +
 include/linux/sched.h            |   2 +-
 include/linux/sched/task_stack.h |  94 ++++++--
 include/linux/vmalloc.h          |  15 ++
 kernel/fork.c                    | 388 ++++++++++++++++++++++++++-----
 kernel/sched/core.c              |   1 +
 mm/internal.h                    |   9 -
 mm/vmalloc.c                     |  24 ++
 mm/vmstat.c                      |   3 +
 13 files changed, 487 insertions(+), 92 deletions(-)

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2.44.0.278.ge034bb2e1d-goog