[RFC PATCH 00/12] x86/mm: Implement lockless pgd_alloc()/pgd_free()

[Ingo Molnar <mingo@kernel.org>]

Waiman Long reported 'pgd_lock' contention on high CPU count systems and proposed 
moving pgd_lock on a separate cacheline to eliminate false sharing and to reduce 
some of the lock bouncing overhead.

I think we can do much better: this series eliminates the pgd_list and makes 
pgd_alloc()/pgd_free() lockless.

Now the lockless initialization of the PGD has a few preconditions, which the 
initial part of the series implements:

 - no PGD clearing is allowed, only additions. This makes sense as a single PGD 
   entry covers 512 GB of RAM so the 4K overhead per 0.5TB of RAM mapped is 
   miniscule.

The patches after that convert existing pgd_list users to walk the task list.

PGD locking is kept intact: coherency guarantees between the CPA, vmalloc, 
hotplug, etc. code are unchanged.

The final patches eliminate the pgd_list and thus make pgd_alloc()/pgd_free() 
lockless.

The patches have been boot tested on 64-bit and 32-bit x86 systems.

Architectures not making use of the new facility are unaffected.

Thanks,

	Ingo

=====
Ingo Molnar (12):
  x86/mm/pat: Don't free PGD entries on memory unmap
  x86/mm/hotplug: Remove pgd_list use from the memory hotplug code
  x86/mm/hotplug: Don't remove PGD entries in remove_pagetable()
  x86/mm/hotplug: Simplify sync_global_pgds()
  mm: Introduce arch_pgd_init_late()
  x86/mm: Enable and use the arch_pgd_init_late() method
  x86/virt/guest/xen: Remove use of pgd_list from the Xen guest code
  x86/mm: Remove pgd_list use from vmalloc_sync_all()
  x86/mm/pat/32: Remove pgd_list use from the PAT code
  x86/mm: Make pgd_alloc()/pgd_free() lockless
  x86/mm: Remove pgd_list leftovers
  x86/mm: Simplify pgd_alloc()

 arch/Kconfig                      |   9 ++++
 arch/x86/Kconfig                  |   1 +
 arch/x86/include/asm/pgtable.h    |   3 --
 arch/x86/include/asm/pgtable_64.h |   3 +-
 arch/x86/mm/fault.c               |  24 +++++++----
 arch/x86/mm/init_64.c             |  73 +++++++++++----------------------
 arch/x86/mm/pageattr.c            |  34 ++++++++--------
 arch/x86/mm/pgtable.c             | 129 +++++++++++++++++++++++++++++-----------------------------
 arch/x86/xen/mmu.c                |  34 +++++++++++++---
 fs/exec.c                         |   3 ++
 include/linux/mm.h                |   6 +++
 kernel/fork.c                     |  16 ++++++++
 12 files changed, 183 insertions(+), 152 deletions(-)

-- 
2.1.4