[patch 081/262] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

Andrew Morton
2021-11-05T20:38:57Z

From: Nicholas Piggin <npiggin@gmail.com>
Subject: lazy tlb: shoot lazies, a non-refcounting lazy tlb option

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 instead implement a non-refcounted scheme that
causes __mmdrop() to IPI all CPUs in the mm_cpumask and shoot down any
remaining lazy ones.

Shootdown IPIs are some concern, but they have not been observed to be a
big problem with this scheme (the powerpc implementation generated 314
additional interrupts on a 144 CPU system during a kernel compile). There
are a number of strategies that could be employed to reduce IPIs if they
turn out to be a problem for some workload.

[npiggin@gmail.com: update comments]
Link: https://lkml.kernel.org/r/1623121901.mszkmmum0n.astroid@bobo.none
Link: https://lkml.kernel.org/r/20210605014216.446867-4-npiggin@gmail.com
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>
---

arch/Kconfig | 14 +++++++++++++
kernel/fork.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 65 insertions(+)

--- a/arch/Kconfig~lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option
+++ a/arch/Kconfig
@@ -441,6 +441,20 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
# to a kthread ->active_mm (non-arch code has been converted already).
config MMU_LAZY_TLB_REFCOUNT
def_bool y
+ depends on !MMU_LAZY_TLB_SHOOTDOWN
+
+# This option allows MMU_LAZY_TLB_REFCOUNT=n. It ensures no CPUs are using an
+# mm as a lazy tlb beyond its last reference count, by shooting down these
+# users before the mm is deallocated. __mmdrop() first IPIs all CPUs that may
+# be using the mm as a lazy tlb, so that they may switch themselves to using
+# init_mm for their active mm. mm_cpumask(mm) is used to determine which CPUs
+# may be using mm as a lazy tlb mm.
+#
+# To implement this, an arch must ensure mm_cpumask(mm) contains at least all
+# possible CPUs in which the mm is lazy, and it must meet the requirements for
+# MMU_LAZY_TLB_REFCOUNT=n (see above).
+config MMU_LAZY_TLB_SHOOTDOWN
+ bool

config ARCH_HAVE_NMI_SAFE_CMPXCHG
bool
--- a/kernel/fork.c~lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option
+++ a/kernel/fork.c
@@ -686,6 +686,53 @@ static void check_mm(struct mm_struct *m
#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) {
+ WARN_ON_ONCE(current->mm);
+ current->active_mm = &init_mm;
+ switch_mm(mm, &init_mm, current);
+ }
+}
+
+static void do_check_lazy_tlb(void *arg)
+{
+ struct mm_struct *mm = arg;
+
+ WARN_ON_ONCE(current->active_mm == mm);
+}
+
+static void shoot_lazy_tlbs(struct mm_struct *mm)
+{
+ if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) {
+ /*
+ * IPI overheads have not found to be expensive, but they could
+ * be reduced in a number of possible ways, for example (in
+ * roughly increasing order of complexity):
+ * - A batch of mms requiring IPIs could be gathered and freed
+ * at once.
+ * - CPUs could store their active mm somewhere that can be
+ * remotely checked without a lock, to filter out
+ * false-positives in the cpumask.
+ * - After mm_users or mm_count reaches zero, switching away
+ * from the mm could clear mm_cpumask to reduce some IPIs
+ * (some batching or delaying would help).
+ * - A delayed freeing and RCU-like quiescing sequence based on
+ * mm switching to avoid IPIs completely.
+ */
+ on_each_cpu_mask(mm_cpumask(mm), do_shoot_lazy_tlb, (void *)mm, 1);
+ if (IS_ENABLED(CONFIG_DEBUG_VM))
+ on_each_cpu(do_check_lazy_tlb, (void *)mm, 1);
+ } else {
+ /*
+ * In this case, lazy tlb mms are refounted and would not reach
+ * __mmdrop until all CPUs have switched away and mmdrop()ed.
+ */
+ }
+}
+
/*
* Called when the last reference to the mm
* is dropped: either by a lazy thread or by
@@ -695,6 +742,10 @@ void __mmdrop(struct mm_struct *mm)
{
BUG_ON(mm == &init_mm);
WARN_ON_ONCE(mm == current->mm);
+
+ /* Ensure no CPUs are using this as their lazy tlb mm */
+ shoot_lazy_tlbs(mm);
+
WARN_ON_ONCE(mm == current->active_mm);
mm_free_pgd(mm);
destroy_context(mm);
_