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Subject[PATCH 2/4] random: make /dev/urandom scalable for silly userspace programs
Date
On a system with a 4 socket (NUMA) system where a large number of
application threads were all trying to read from /dev/urandom, this
can result in the system spending 80% of its time contending on the
global urandom spinlock. The application should have used its own
PRNG, but let's try to help it from running, lemming-like, straight
over the locking cliff.

Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
---
drivers/char/random.c | 67 +++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 62 insertions(+), 5 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 91d5c2a..b970db6 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -749,6 +749,17 @@ struct crng_state primary_crng = {
};
static DECLARE_WAIT_QUEUE_HEAD(crng_init_wait);

+#ifdef CONFIG_NUMA
+/*
+ * Hack to deal with crazy userspace progams when they are all trying
+ * to access /dev/urandom in parallel. The programs are almost
+ * certainly doing something terribly wrong, but we'll work around
+ * their brain damage.
+ */
+static struct crng_state **crng_node_pool __read_mostly;
+#endif
+
+
static void _initialize_crng(struct crng_state *crng)
{
int i;
@@ -764,11 +775,13 @@ static void _initialize_crng(struct crng_state *crng)
crng->init_time = jiffies - CRNG_RESEED_INTERVAL;
}

+#ifdef CONFIG_NUMA
static void initialize_crng(struct crng_state *crng)
{
_initialize_crng(crng);
spin_lock_init(&crng->lock);
}
+#endif

static int crng_fast_load(__u32 pool[4])
{
@@ -823,19 +836,23 @@ out:
return ret;
}

+static inline void maybe_reseed_primary_crng(void)
+{
+ if (crng_init > 2 &&
+ time_after(jiffies, primary_crng.init_time + CRNG_RESEED_INTERVAL))
+ crng_reseed(&input_pool);
+}
+
static inline void crng_wait_ready(void)
{
wait_event_interruptible(crng_init_wait, crng_ready());
}

-static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
+static void _extract_crng(struct crng_state *crng,
+ __u8 out[CHACHA20_BLOCK_SIZE])
{
unsigned long v, flags;
- struct crng_state *crng = &primary_crng;

- if (crng_init > 2 &&
- time_after(jiffies, crng->init_time + CRNG_RESEED_INTERVAL))
- crng_reseed(&input_pool);
spin_lock_irqsave(&crng->lock, flags);
if (arch_get_random_long(&v))
crng->state[14] ^= v;
@@ -845,6 +862,30 @@ static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
spin_unlock_irqrestore(&crng->lock, flags);
}

+static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
+{
+#ifndef CONFIG_NUMA
+ maybe_reseed_primary_crng();
+ _extract_crng(&primary_crng, out);
+#else
+ int node_id = numa_node_id();
+ struct crng_state *crng = crng_node_pool[node_id];
+
+ if (time_after(jiffies, crng->init_time + CRNG_RESEED_INTERVAL)) {
+ unsigned long flags;
+
+ maybe_reseed_primary_crng();
+ _extract_crng(&primary_crng, out);
+ spin_lock_irqsave(&crng->lock, flags);
+ memcpy(&crng->state[4], out, CHACHA20_KEY_SIZE);
+ crng->state[15] = node_id;
+ crng->init_time = jiffies;
+ spin_unlock_irqrestore(&crng->lock, flags);
+ }
+ _extract_crng(crng, out);
+#endif
+}
+
static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
{
ssize_t ret = 0, i;
@@ -1549,6 +1590,22 @@ static void init_std_data(struct entropy_store *r)
*/
static int rand_initialize(void)
{
+#ifdef CONFIG_NUMA
+ int i;
+ int num_nodes = num_possible_nodes();
+ struct crng_state *crng;
+
+ crng_node_pool = kmalloc(num_nodes * sizeof(void *),
+ GFP_KERNEL|__GFP_NOFAIL);
+
+ for (i=0; i < num_nodes; i++) {
+ crng = kmalloc(sizeof(struct crng_state),
+ GFP_KERNEL | __GFP_NOFAIL);
+ initialize_crng(crng);
+ crng_node_pool[i] = crng;
+
+ }
+#endif
init_std_data(&input_pool);
init_std_data(&blocking_pool);
_initialize_crng(&primary_crng);
--
2.5.0
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