[RFC PATCH 7/7] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

Nicholas Piggin
2020-07-10T01:57:43Z

On big systems, the mm refcount can become highly contented when doing
a lot of context switching with threaded applications (particularly
switching between the idle thread and an application thread).

Abandoning lazy tlb slows switching down quite a bit in the important
user->idle->user cases, so so instead implement a non-refcounted scheme
that causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down
any remaining lazy ones.

On a 16-socket 192-core POWER8 system, a context switching benchmark
with as many software threads as CPUs (so each switch will go in and
out of idle), upstream can achieve a rate of about 1 million context
switches per second. After this patch it goes up to 118 million.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
arch/Kconfig | 16 ++++++++++++++++
arch/powerpc/Kconfig | 1 +
include/linux/sched/mm.h | 6 +++---
kernel/fork.c | 39 +++++++++++++++++++++++++++++++++++++++
4 files changed, 59 insertions(+), 3 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index 2daf8fe6146a..edf69437a971 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -418,6 +418,22 @@ config MMU_LAZY_TLB
help
Enable "lazy TLB" mmu context switching for kernel threads.

+config MMU_LAZY_TLB_REFCOUNT
+ def_bool y
+ depends on MMU_LAZY_TLB
+ depends on !MMU_LAZY_TLB_SHOOTDOWN
+
+config MMU_LAZY_TLB_SHOOTDOWN
+ bool
+ depends on MMU_LAZY_TLB
+ help
+ Instead of refcounting the "lazy tlb" mm struct, which can cause
+ contention with multi-threaded apps on large multiprocessor systems,
+ this option causes __mmdrop to IPI all CPUs in the mm_cpumask and
+ switch to init_mm if they were using the to-be-freed mm as the lazy
+ tlb. Architectures which do not track all possible lazy tlb CPUs in
+ mm_cpumask can not use this (without modification).
+
config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 920c4e3ca4ef..24ac85c868db 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -225,6 +225,7 @@ config PPC
select HAVE_PERF_USER_STACK_DUMP
select MMU_GATHER_RCU_TABLE_FREE
select MMU_GATHER_PAGE_SIZE
+ select MMU_LAZY_TLB_SHOOTDOWN
select HAVE_REGS_AND_STACK_ACCESS_API
select HAVE_RELIABLE_STACKTRACE if PPC_BOOK3S_64 && CPU_LITTLE_ENDIAN
select HAVE_SYSCALL_TRACEPOINTS
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 2c2b20e2ccc7..1067af8039bd 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -53,19 +53,19 @@ void mmdrop(struct mm_struct *mm);
/* Helpers for lazy TLB mm refcounting */
static inline void mmgrab_lazy_tlb(struct mm_struct *mm)
{
- if (IS_ENABLED(CONFIG_MMU_LAZY_TLB))
+ if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT))
mmgrab(mm);
}

static inline void mmdrop_lazy_tlb(struct mm_struct *mm)
{
- if (IS_ENABLED(CONFIG_MMU_LAZY_TLB))
+ if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT))
mmdrop(mm);
}

static inline void mmdrop_lazy_tlb_smp_mb(struct mm_struct *mm)
{
- if (IS_ENABLED(CONFIG_MMU_LAZY_TLB))
+ if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_REFCOUNT))
mmdrop(mm); /* This depends on mmdrop providing a full smp_mb() */
else
smp_mb();
diff --git a/kernel/fork.c b/kernel/fork.c
index 142b23645d82..da0fba9e6079 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -685,6 +685,40 @@ static void check_mm(struct mm_struct *mm)
#define allocate_mm() (kmem_cache_alloc(mm_cachep, GFP_KERNEL))
#define free_mm(mm) (kmem_cache_free(mm_cachep, (mm)))

+static void do_shoot_lazy_tlb(void *arg)
+{
+ struct mm_struct *mm = arg;
+
+ if (current->active_mm == mm) {
+ BUG_ON(current->mm);
+ switch_mm(mm, &init_mm, current);
+ current->active_mm = &init_mm;
+ }
+}
+
+static void do_check_lazy_tlb(void *arg)
+{
+ struct mm_struct *mm = arg;
+
+ BUG_ON(current->active_mm == mm);
+}
+
+static void shoot_lazy_tlbs(struct mm_struct *mm)
+{
+ if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) {
+ smp_call_function_many(mm_cpumask(mm), do_shoot_lazy_tlb, (void *)mm, 1);
+ do_shoot_lazy_tlb(mm);
+ }
+}
+
+static void check_lazy_tlbs(struct mm_struct *mm)
+{
+ if (IS_ENABLED(CONFIG_DEBUG_VM)) {
+ smp_call_function(do_check_lazy_tlb, (void *)mm, 1);
+ do_check_lazy_tlb(mm);
+ }
+}
+
/*
* Called when the last reference to the mm
* is dropped: either by a lazy thread or by
@@ -695,6 +729,11 @@ void __mmdrop(struct mm_struct *mm)
BUG_ON(mm == &init_mm);
WARN_ON_ONCE(mm == current->mm);
WARN_ON_ONCE(mm == current->active_mm);
+
+ /* Ensure no CPUs are using this as their lazy tlb mm */
+ shoot_lazy_tlbs(mm);
+ check_lazy_tlbs(mm);
+
mm_free_pgd(mm);
destroy_context(mm);
mmu_notifier_subscriptions_destroy(mm);
--
2.23.0