The patch titled
Subject: lazy tlb: shoot lazies, a non-refcounting lazy tlb option
has been added to the -mm tree. Its filename is
lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option.patch
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next and is updated
there every 3-4 working days
------------------------------------------------------
From: Nicholas Piggin <npiggin@xxxxxxxxx>
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.
Link: https://lkml.kernel.org/r/20210605014216.446867-4-npiggin@xxxxxxxxx
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Anton Blanchard <anton@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
arch/Kconfig | 13 ++++++++++++
kernel/fork.c | 51 ++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 64 insertions(+)
--- a/arch/Kconfig~lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option
+++ a/arch/Kconfig
@@ -421,6 +421,19 @@ config ARCH_WANT_IRQS_OFF_ACTIVATE_MM
# Use normal mm refcounting for MMU_LAZY_TLB kernel thread references.
config MMU_LAZY_TLB_REFCOUNT
def_bool y
+ depends on !MMU_LAZY_TLB_SHOOTDOWN
+
+# Instead of refcounting the lazy mm struct for kernel thread references
+# (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. To
+# implement this, architectures must use _lazy_tlb variants of mm refcounting
+# when releasing kernel thread mm references, and mm_cpumask must include at
+# least all possible CPUs in which the mm might be lazy, at the time of the
+# final mmdrop. mmgrab/mmdrop in arch/ code must be switched to _lazy_tlb
+# postfix as necessary.
+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
@@ -674,6 +674,53 @@ 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
@@ -683,6 +730,10 @@ 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);
_
Patches currently in -mm which might be from npiggin@xxxxxxxxx are
lazy-tlb-introduce-lazy-mm-refcount-helper-functions.patch
lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable.patch
lazy-tlb-shoot-lazies-a-non-refcounting-lazy-tlb-option.patch
powerpc-64s-enable-mmu_lazy_tlb_shootdown.patch
