[PATCH 2/2] rcuperf: Measure memory footprint during kfree_rcu() test (v4)

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

During changes to kfree_rcu() code, we often check the amount of free
memory.  As an alternative to checking this manually, this commit adds a
measurement in the test itself.  It measures four times during the test
for available memory, digitally filters these measurements to produce a
running average with a weight of 0.5, and compares this digitally
filtered value with the amount of available memory at the beginning of
the test.

We apply the digital filter only once we are more than 25% into the
test. At the 25% mark, we just read available memory and don't apply any
filtering. This prevents the first sample from skewing the results
as we would not consider memory readings that were before memory was
allocated.

A sample run shows something like:

Total time taken by all kfree'ers: 6369738407 ns, loops: 10000, batches: 764, memory footprint: 216MB

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

---
v1->v2: Minor corrections
v1->v3: Use long long to prevent 32-bit system's overflow
	Handle case where some threads start later than others.
	Start measuring only once 25% into the test. Slightly more accurate.
v3->v4: Simplified test more. Using simple average.

 kernel/rcu/rcuperf.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/rcuperf.c b/kernel/rcu/rcuperf.c
index 1fd0cc72022e..c41f009acbbb 100644
--- a/kernel/rcu/rcuperf.c
+++ b/kernel/rcu/rcuperf.c
@@ -12,6 +12,7 @@
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <linux/init.h>
+#include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/kthread.h>
 #include <linux/err.h>
@@ -616,14 +617,17 @@ DEFINE_KFREE_OBJ(32); // goes on kmalloc-64 slab
 DEFINE_KFREE_OBJ(64); // goes on kmalloc-96 slab
 DEFINE_KFREE_OBJ(96); // goes on kmalloc-128 slab
 
+long long mem_begin;
+
 static int
 kfree_perf_thread(void *arg)
 {
 	int i, loop = 0;
 	long me = (long)arg;
 	void *alloc_ptr;
-
 	u64 start_time, end_time;
+	long mem_samples = 0;
+	long long mem_during = 0;
 
 	VERBOSE_PERFOUT_STRING("kfree_perf_thread task started");
 	set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
@@ -638,7 +642,17 @@ kfree_perf_thread(void *arg)
 			b_rcu_gp_test_started = cur_ops->get_gp_seq();
 	}
 
+	// Prevent "% 0" error below.
+	if (kfree_loops < 4)
+		kfree_loops = 4;
+
 	do {
+		// Start measuring only from when we are at least 25% into the test.
+		if ((loop != 0) && (loop % (kfree_loops / 4) == 0)) {
+			mem_during = mem_during + si_mem_available();
+			mem_samples++;
+		}
+
 		for (i = 0; i < kfree_alloc_num; i++) {
 			int kfree_type = i % 4;
 
@@ -671,6 +685,8 @@ kfree_perf_thread(void *arg)
 		cond_resched();
 	} while (!torture_must_stop() && ++loop < kfree_loops);
 
+	mem_during = (mem_during / mem_samples);
+
 	if (atomic_inc_return(&n_kfree_perf_thread_ended) >= kfree_nrealthreads) {
 		end_time = ktime_get_mono_fast_ns();
 
@@ -679,9 +695,13 @@ kfree_perf_thread(void *arg)
 		else
 			b_rcu_gp_test_finished = cur_ops->get_gp_seq();
 
-		pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld\n",
+		// The "memory footprint" field represents how much in-flight
+		// memory is allocated during the test and waiting to be freed.
+		pr_alert("Total time taken by all kfree'ers: %llu ns, loops: %d, batches: %ld, memory footprint: %lldMB\n",
 		       (unsigned long long)(end_time - start_time), kfree_loops,
-		       rcuperf_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started));
+		       rcuperf_seq_diff(b_rcu_gp_test_finished, b_rcu_gp_test_started),
+		       (mem_begin - mem_during) >> (20 - PAGE_SHIFT));
+
 		if (shutdown) {
 			smp_mb(); /* Assign before wake. */
 			wake_up(&shutdown_wq);
@@ -753,6 +773,8 @@ kfree_perf_init(void)
 		goto unwind;
 	}
 
+	mem_begin = si_mem_available();
+
 	for (i = 0; i < kfree_nrealthreads; i++) {
 		firsterr = torture_create_kthread(kfree_perf_thread, (void *)i,
 						  kfree_reader_tasks[i]);
-- 
2.25.0.rc1.283.g88dfdc4193-goog