[PATCH] kfence: allow use of a deferrable timer

[Marco Elver <elver@google.com>]

Allow the use of a deferrable timer, which does not force CPU wake-ups
when the system is idle. A consequence is that the sample interval
becomes very unpredictable, to the point that it is not guaranteed that
the KFENCE KUnit test still passes.

Nevertheless, on power-constrained systems this may be preferable, so
let's give the user the option should they accept the above trade-off.

Signed-off-by: Marco Elver <elver@google.com>
---
 lib/Kconfig.kfence | 12 ++++++++++++
 mm/kfence/core.c   | 15 +++++++++++++--
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
index 912f252a41fc..1cf2ea2a1ac7 100644
--- a/lib/Kconfig.kfence
+++ b/lib/Kconfig.kfence
@@ -45,6 +45,18 @@ config KFENCE_NUM_OBJECTS
 	  pages are required; with one containing the object and two adjacent
 	  ones used as guard pages.
 
+config KFENCE_DEFERRABLE
+	bool "Use a deferrable timer to trigger allocations" if EXPERT
+	help
+	  Use a deferrable timer to trigger allocations. This avoids forcing
+	  CPU wake-ups if the system is idle, at the risk of a less predictable
+	  sample interval.
+
+	  Warning: The KUnit test suite fails with this option enabled - due to
+	  the unpredictability of the sample interval!
+
+	  Say N if you are unsure.
+
 config KFENCE_STATIC_KEYS
 	bool "Use static keys to set up allocations" if EXPERT
 	depends on JUMP_LABEL
diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index f126b53b9b85..451277b41bfb 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -95,6 +95,10 @@ module_param_cb(sample_interval, &sample_interval_param_ops, &kfence_sample_inte
 static unsigned long kfence_skip_covered_thresh __read_mostly = 75;
 module_param_named(skip_covered_thresh, kfence_skip_covered_thresh, ulong, 0644);
 
+/* If true, use a deferrable timer at the risk of unpredictable sample intervals. */
+static bool kfence_deferrable __read_mostly = IS_ENABLED(CONFIG_KFENCE_DEFERRABLE);
+module_param_named(deferrable, kfence_deferrable, bool, 0444);
+
 /* The pool of pages used for guard pages and objects. */
 char *__kfence_pool __read_mostly;
 EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
@@ -740,6 +744,8 @@ late_initcall(kfence_debugfs_init);
 
 /* === Allocation Gate Timer ================================================ */
 
+static struct delayed_work kfence_timer;
+
 #ifdef CONFIG_KFENCE_STATIC_KEYS
 /* Wait queue to wake up allocation-gate timer task. */
 static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
@@ -762,7 +768,6 @@ static DEFINE_IRQ_WORK(wake_up_kfence_timer_work, wake_up_kfence_timer);
  * avoids IPIs, at the cost of not immediately capturing allocations if the
  * instructions remain cached.
  */
-static struct delayed_work kfence_timer;
 static void toggle_allocation_gate(struct work_struct *work)
 {
 	if (!READ_ONCE(kfence_enabled))
@@ -790,7 +795,6 @@ static void toggle_allocation_gate(struct work_struct *work)
 	queue_delayed_work(system_unbound_wq, &kfence_timer,
 			   msecs_to_jiffies(kfence_sample_interval));
 }
-static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
 
 /* === Public interface ===================================================== */
 
@@ -809,8 +813,15 @@ static void kfence_init_enable(void)
 {
 	if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
 		static_branch_enable(&kfence_allocation_key);
+
+	if (kfence_deferrable)
+		INIT_DEFERRABLE_WORK(&kfence_timer, toggle_allocation_gate);
+	else
+		INIT_DELAYED_WORK(&kfence_timer, toggle_allocation_gate);
+
 	WRITE_ONCE(kfence_enabled, true);
 	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
+
 	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
 		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
 		(void *)(__kfence_pool + KFENCE_POOL_SIZE));
-- 
2.35.1.616.g0bdcbb4464-goog