[PATCH 1/3] lib/test_mempress_timer: add module to generate kernel allocation pressure

[Jason Miu <jasonmiu@google.com>]

Update lib/Kconfig.debug and lib/Makefile to introduce
test_mempress_timer, a kernel testing module designed to generate
synthetic memory allocation pressure within SoftIRQ contexts.

Bind independent timers to each online CPU to continuously execute
atomic page allocations (GFP_ATOMIC | __GFP_NOWARN). Provide this
workload generator to repeatedly emulate heavy kernel allocation demands.

Add a configurable termination threshold (test_duration_secs) to prevent
permanent test-node lockups, and provide batching control via the
allocs_per_iteration module parameter.

Signed-off-by: Jason Miu <jasonmiu@google.com>
---
 lib/Kconfig.debug         |  11 +++
 lib/Makefile              |   1 +
 lib/test_mempress_timer.c | 140 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 152 insertions(+)
 create mode 100644 lib/test_mempress_timer.c

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1244dcac2294..00d32012e476 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2635,6 +2635,17 @@ config TEST_BITOPS
 
 	  If unsure, say N.
 
+config TEST_MEMPRESS_TIMER
+	tristate "Test module for memory pressure and allocation stall timing"
+	default n
+	help
+	  This builds the "test_mempress_timer" module that can be used to
+	  provoke and profile page allocation stalls and direct reclaim
+	  slowness. It periodically does a atomic page allocation to generate a
+	  memory pressure.
+
+	  If unsure, say N.
+
 config TEST_VMALLOC
 	tristate "Test module for stress/performance analysis of vmalloc allocator"
 	default n
diff --git a/lib/Makefile b/lib/Makefile
index 7f75cc6edf94..0f35c2b78970 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -78,6 +78,7 @@ CFLAGS_test_ubsan.o += $(call cc-disable-warning, unused-but-set-variable)
 UBSAN_SANITIZE_test_ubsan.o := y
 obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o
 obj-$(CONFIG_TEST_LKM) += test_module.o
+obj-$(CONFIG_TEST_MEMPRESS_TIMER) += test_mempress_timer.o
 obj-$(CONFIG_TEST_VMALLOC) += test_vmalloc.o
 obj-$(CONFIG_TEST_WORKQUEUE) += test_workqueue.o
 obj-$(CONFIG_TEST_RHASHTABLE) += test_rhashtable.o
diff --git a/lib/test_mempress_timer.c b/lib/test_mempress_timer.c
new file mode 100644
index 000000000000..a102ac4f6725
--- /dev/null
+++ b/lib/test_mempress_timer.c
@@ -0,0 +1,140 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * test_mempress_timer.c
+ *
+ * Simulates kernelspace memory allocation pressure via timers.
+ * This module binds individual timers to each online CPU to independently
+ * allocate and free physical pages concurrently. It serves as a workload
+ * generator to reproduce and measure page allocation stalls under system
+ * memory contention.
+ *
+ * Module Parameters:
+ *  - allocs_per_iteration: Number of atomic page allocations executed
+ *                          during each timer callback.
+ *                          Default: 1000
+ *  - test_duration_secs:   Total duration in seconds to run the pressure test
+ *                          before automatically disengaging.
+ *                          Default: 900 (15 minutes)
+ *
+ * Usage:
+ * insmod test_mempress_timer.ko test_duration_secs=900 allocs_per_iteration=1000
+ */
+
+#include <linux/gfp.h>
+#include <linux/mm.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/timer.h>
+#include <linux/proc_fs.h>
+#include <linux/skbuff.h>
+#include <net/tcp.h>
+
+struct mempress_timer {
+	struct timer_list timer;
+	struct list_head list;
+	struct list_head page_list;
+	int cpu;
+};
+
+static LIST_HEAD(timers);
+static bool stop_timers;
+
+static unsigned long interval = 2;
+
+static unsigned long allocs_per_iteration = 1000;
+module_param(allocs_per_iteration, long, 0444);
+
+static unsigned long test_duration_secs = 900; /* 15 mins default */
+module_param(test_duration_secs, ulong, 0444);
+
+static unsigned long end_jiffies;
+
+static void alloc_pages_atomics(struct mempress_timer *test)
+{
+	int i;
+	struct page *page;
+
+	for (i = 0; i < allocs_per_iteration; i++) {
+		page = alloc_page(GFP_ATOMIC | __GFP_NOWARN);
+		if (!page) {
+			pr_warn("page allocation failure from cpu %d at iteration %d\n",
+				test->cpu, i);
+			break;
+		}
+		list_add(&page->lru, &test->page_list);
+	}
+}
+
+static void free_pages_atomics(struct mempress_timer *test)
+{
+	struct list_head *page;
+	struct list_head *iter;
+
+	list_for_each_safe(page, iter, &test->page_list)
+		__free_page(container_of(page, struct page, lru));
+	INIT_LIST_HEAD(&test->page_list);
+}
+
+static void atomic_timer_allocator(struct timer_list *timer)
+{
+	struct mempress_timer *test = timer_container_of(test, timer, timer);
+	bool is_expired = (test_duration_secs > 0 &&
+			   time_after(jiffies, end_jiffies));
+
+	if (list_empty(&test->page_list) && !is_expired)
+		alloc_pages_atomics(test);
+	else
+		free_pages_atomics(test);
+
+	if (!READ_ONCE(stop_timers) && !is_expired) {
+		test->timer.expires = jiffies + interval;
+		add_timer_on(&test->timer, test->cpu);
+	} else if (is_expired) {
+		pr_info_once("Duration (%lu secs) reached, stopping.\n", test_duration_secs);
+	}
+}
+
+static int __init mempress_timers_init(void)
+{
+	int cpu;
+	struct mempress_timer *test;
+
+	if (test_duration_secs > 0)
+		end_jiffies = jiffies + (test_duration_secs * HZ);
+
+	for_each_online_cpu(cpu) {
+		test = kzalloc(sizeof(*test), GFP_KERNEL | __GFP_NOFAIL);
+
+		timer_setup(&test->timer, atomic_timer_allocator, 0);
+		list_add(&test->list, &timers);
+		INIT_LIST_HEAD(&test->page_list);
+		test->cpu = cpu;
+
+		/* For start, use 90 seconds. */
+		test->timer.expires = jiffies + (90 * HZ);
+		add_timer_on(&test->timer, test->cpu);
+	}
+
+	return 0;
+}
+module_init(mempress_timers_init);
+
+static void mempress_timers_exit(void)
+{
+	struct mempress_timer *test, *n;
+
+	pr_crit("exiting\n");
+	stop_timers = true;
+
+	list_for_each_entry_safe(test, n, &timers, list) {
+		timer_delete_sync(&test->timer);
+		list_del(&test->list);
+		cond_resched();
+		free_pages_atomics(test);
+		kfree(test);
+	}
+}
+module_exit(mempress_timers_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Test memory pressure from timers");
-- 
2.55.0.691.gc56d675ccc-goog