The patch titled
Subject: lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable-fix-2
has been added to the -mm tree. Its filename is
lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable-fix-2.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable-fix-2.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable-fix-2.patch
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------------------------------------------------------
From: Nicholas Piggin <npiggin@xxxxxxxxx>
Subject: lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable-fix-2
Explain the requirements for lazy tlb mm refcounting in the comment,
to help with archs that may want to disable this by some means other
than MMU_LAZY_TLB_SHOOTDOWN.
Link: https://lkml.kernel.org/r/1623121605.j47gdpccep.astroid@xxxxxxxxx
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Anton Blanchard <anton@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/Kconfig | 10 ++++++++++
1 file changed, 10 insertions(+)
--- a/arch/Kconfig~lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable-fix-2
+++ a/arch/Kconfig
@@ -419,6 +419,16 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
shootdowns should enable this.
# Use normal mm refcounting for MMU_LAZY_TLB kernel thread references.
+# MMU_LAZY_TLB_REFCOUNT=n can improve the scalability of context switching
+# to/from kernel threads when the same mm is running on a lot of CPUs (a large
+# multi-threaded application), by reducing contention on the mm refcount.
+#
+# This can be disabled if the architecture ensures no CPUs are using an mm as a
+# "lazy tlb" beyond its final refcount (i.e., by the time __mmdrop frees the mm
+# or its kernel page tables). This could be arranged by arch_exit_mmap(), or
+# final exit(2) TLB flush, for example. arch code must also ensure the
+# _lazy_tlb variants of mmgrab/mmdrop are used when dropping the lazy reference
+# to a kthread ->active_mm (non-arch code has been converted already).
config MMU_LAZY_TLB_REFCOUNT
def_bool y
_
Patches currently in -mm which might be from npiggin@xxxxxxxxx are
lazy-tlb-introduce-lazy-mm-refcount-helper-functions.patch
lazy-tlb-introduce-lazy-mm-refcount-helper-functions-fix.patch
lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable.patch
lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable-fix-2.patch
lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option.patch
lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option-fix.patch
powerpc-64s-enable-mmu_lazy_tlb_shootdown.patch
