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@bobo.none
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Anton Blanchard <anton@ozlabs.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>

diff --git a/arch/Kconfig b/arch/Kconfig
index 9dddd38d2bf41f0329fc024865f40e61600c1be4..16e7a3eddb66322964f32cd7a9df4ea753a3a41d 100644 (file)
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -426,6 +426,16 @@ 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