lazy tlb: allow lazy tlb mm refcounting to be configurable

Add CONFIG_MMU_TLB_REFCOUNT which enables refcounting of the lazy tlb mm
when it is context switched.  This can be disabled by architectures that
don't require this refcounting if they clean up lazy tlb mms when the last
refcount is dropped.  Currently this is always enabled, so the patch
introduces no functional change.

Link: https://lkml.kernel.org/r/20230203071837.1136453-4-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/Documentation/mm/active_mm.rst b/Documentation/mm/active_mm.rst
index 45d89f8fb3a80b48e403922b11c1e8ca03476b11..d096fc091e2330da3e60d199d49a85cbf390f2ba 100644 (file)
--- a/Documentation/mm/active_mm.rst
+++ b/Documentation/mm/active_mm.rst
@@ -2,6 +2,12 @@
 Active MM
 =========
 
+Note, the mm_count refcount may no longer include the "lazy" users
+(running tasks with ->active_mm == mm && ->mm == NULL) on kernels
+with CONFIG_MMU_LAZY_TLB_REFCOUNT=n. Taking and releasing these lazy
+references must be done with mmgrab_lazy_tlb() and mmdrop_lazy_tlb()
+helpers, which abstract this config option.
+
 ::
 
  List:       linux-kernel

diff --git a/arch/Kconfig b/arch/Kconfig
index e3511afbb7f2f493854242c16c2429a83800984a..643f8a04141ed1d8bd84e96f19679635c2a763a4 100644 (file)
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -465,6 +465,23 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
          irqs disabled over activate_mm. Architectures that do IPI based TLB
          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.
+#
+# To implement this, an arch *must*:
+# Ensure the _lazy_tlb variants of mmgrab/mmdrop are used when manipulating
+# the lazy tlb reference of a kthread's ->active_mm (non-arch code has been
+# converted already).
+config MMU_LAZY_TLB_REFCOUNT
+       def_bool y
+
 config ARCH_HAVE_NMI_SAFE_CMPXCHG
        bool
 

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 5376caf6fcf3adf1d32156970affd2d4d9c195c6..689dbe81256335379997f99fb8b74a51090de456 100644 (file)
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -82,17 +82,29 @@ static inline void mmdrop_sched(struct mm_struct *mm)
 /* Helpers for lazy TLB mm refcounting */
 static inline void mmgrab_lazy_tlb(struct mm_struct *mm)
 {
-       mmgrab(mm);
+       if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT))
+               mmgrab(mm);
 }
 
 static inline void mmdrop_lazy_tlb(struct mm_struct *mm)
 {
-       mmdrop(mm);
+       if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT)) {
+               mmdrop(mm);
+       } else {
+               /*
+                * mmdrop_lazy_tlb must provide a full memory barrier, see the
+                * membarrier comment finish_task_switch which relies on this.
+                */
+               smp_mb();
+       }
 }
 
 static inline void mmdrop_lazy_tlb_sched(struct mm_struct *mm)
 {
-       mmdrop_sched(mm);
+       if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT))
+               mmdrop_sched(mm);
+       else
+               smp_mb(); /* see mmdrop_lazy_tlb() above */
 }
 
 /**