rcuperf: Add kfree_rcu() performance Tests

This test runs kfree_rcu() in a loop to measure performance of the new
kfree_rcu() batching functionality.

The following table shows results when booting with arguments:
rcuperf.kfree_loops=20000 rcuperf.kfree_alloc_num=8000
rcuperf.kfree_rcu_test=1 rcuperf.kfree_no_batch=X

rcuperf.kfree_no_batch=X    # Grace Periods	Test Duration (s)
  X=1 (old behavior)              9133                 11.5
  X=0 (new behavior)              1732                 12.5

On a 16 CPU system with the above boot parameters, we see that the total
number of grace periods that elapse during the test drops from 9133 when
not batching to 1732 when batching (a 5X improvement). The kfree_rcu()
flood itself slows down a bit when batching, though, as shown.

Note that the active memory consumption during the kfree_rcu() flood
does increase to around 200-250MB due to the batching (from around 50MB
without batching). However, this memory consumption is relatively
constant. In other words, the system is able to keep up with the
kfree_rcu() load. The memory consumption comes down considerably if
KFREE_DRAIN_JIFFIES is increased from HZ/50 to HZ/80. A later patch will
reduce memory consumption further by using multiple lists.

Also, when running the test, please disable CONFIG_DEBUG_PREEMPT and
CONFIG_PROVE_RCU for realistic comparisons with/without batching.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ade4e6ec23e0..3ce270b56f3a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3978,6 +3978,23 @@
 			test until boot completes in order to avoid
 			interference.
 
+	rcuperf.kfree_rcu_test= [KNL]
+			Set to measure performance of kfree_rcu() flooding.
+
+	rcuperf.kfree_nthreads= [KNL]
+			The number of threads running loops of kfree_rcu().
+
+	rcuperf.kfree_alloc_num= [KNL]
+			Number of allocations and frees done in an iteration.
+
+	rcuperf.kfree_loops= [KNL]
+			Number of loops doing rcuperf.kfree_alloc_num number
+			of allocations and frees.
+
+	rcuperf.kfree_no_batch= [KNL]
+			Use the non-batching (less efficient) version of kfree_rcu().
+			This is useful for comparing with the batched version.
+
 	rcuperf.nreaders= [KNL]
 			Set number of RCU readers.  The value -1 selects
 			N, where N is the number of CPUs.  A value