On 1/18/23 10:00 AM, Nicholas Piggin wrote:
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, which is
what existing code does, so the patch is effectively a no-op.
Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is.
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
---
Documentation/mm/active_mm.rst | 6 ++++++
arch/Kconfig | 17 +++++++++++++++++
include/linux/sched/mm.h | 18 +++++++++++++++---
kernel/sched/core.c | 22 ++++++++++++++++++----
kernel/sched/sched.h | 4 +++-
5 files changed, 59 insertions(+), 8 deletions(-)
diff --git a/Documentation/mm/active_mm.rst b/Documentation/mm/active_mm.rst
index 6f8269c284ed..2b0d08332400 100644
--- a/Documentation/mm/active_mm.rst
+++ b/Documentation/mm/active_mm.rst
@@ -4,6 +4,12 @@
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 abstracts this config option.
+
::
List: linux-kernel
diff --git a/arch/Kconfig b/arch/Kconfig
index 12e3ddabac9d..b07d36f08fea 100644
--- 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 dropping the
+# lazy 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 5376caf6fcf3..68bbe8d90c2e 100644
--- 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();
+ }
}
Considering the fact that mmdrop_lazy_tlb() replaced mmdrop() in various
locations in which smp_mb() was not required, this comment might be
confusing. IOW, for the cases in most cases where mmdrop_lazy_tlb()
replaced mmdrop(), this comment was irrelevant, and therefore it now
becomes confusing.
I am not sure the include the smp_mb() here instead of "open-coding" it
helps.
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 above
}
Wrong style of comment.
