[PATCH] random: Remove entropy batching

Rather than batching up entropy samples, resulting in longer lock hold times
when we actually process the samples, mix in samples immediately.  The trickle
code should eliminate almost all the additional interrupt-time overhead this
would otherwise incur, with or without locking.
Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/random.c

@@ -238,7 +238,6 @@
 #include <linux/poll.h>
 #include <linux/init.h>
 #include <linux/fs.h>
-#include <linux/workqueue.h>
 #include <linux/genhd.h>
 #include <linux/interrupt.h>
 #include <linux/spinlock.h>
@@ -254,7 +253,6 @@
  */
 #define INPUT_POOL_WORDS 128
 #define OUTPUT_POOL_WORDS 32
-#define BATCH_ENTROPY_SIZE 256
 #define SEC_XFER_SIZE 512
 
 /*
@@ -552,118 +550,6 @@ static void credit_entropy_store(struct entropy_store *r, int nbits)
 	spin_unlock_irqrestore(&r->lock, flags);
 }
 
-/**********************************************************************
- *
- * Entropy batch input management
- *
- * We batch entropy to be added to avoid increasing interrupt latency
- *
- **********************************************************************/
-
-struct sample {
-	__u32 data[2];
-	int credit;
-};
-
-static struct sample *batch_entropy_pool, *batch_entropy_copy;
-static int batch_head, batch_tail;
-static DEFINE_SPINLOCK(batch_lock);
-
-static int batch_max;
-static void batch_entropy_process(void *private_);
-static DECLARE_WORK(batch_work, batch_entropy_process, NULL);
-
-/* note: the size must be a power of 2 */
-static int __init batch_entropy_init(int size, struct entropy_store *r)
-{
-	batch_entropy_pool = kmalloc(size*sizeof(struct sample), GFP_KERNEL);
-	if (!batch_entropy_pool)
-		return -1;
-	batch_entropy_copy = kmalloc(size*sizeof(struct sample), GFP_KERNEL);
-	if (!batch_entropy_copy) {
-		kfree(batch_entropy_pool);
-		return -1;
-	}
-	batch_head = batch_tail = 0;
-	batch_work.data = r;
-	batch_max = size;
-	return 0;
-}
-
-/*
- * Changes to the entropy data is put into a queue rather than being added to
- * the entropy counts directly.  This is presumably to avoid doing heavy
- * hashing calculations during an interrupt in add_timer_randomness().
- * Instead, the entropy is only added to the pool by keventd.
- */
-static void batch_entropy_store(u32 a, u32 b, int num)
-{
-	int new;
-	unsigned long flags;
-
-	if (!batch_max)
-		return;
-
-	spin_lock_irqsave(&batch_lock, flags);
-
-	batch_entropy_pool[batch_head].data[0] = a;
-	batch_entropy_pool[batch_head].data[1] = b;
-	batch_entropy_pool[batch_head].credit = num;
-
-	if (((batch_head - batch_tail) & (batch_max - 1)) >= (batch_max / 2))
-		schedule_delayed_work(&batch_work, 1);
-
-	new = (batch_head + 1) & (batch_max - 1);
-	if (new == batch_tail)
-		DEBUG_ENT("batch entropy buffer full\n");
-	else
-		batch_head = new;
-
-	spin_unlock_irqrestore(&batch_lock, flags);
-}
-
-/*
- * Flush out the accumulated entropy operations, adding entropy to the
- * input pool.  If that pool has enough entropy, alternate
- * between randomizing the data of all pools.
- */
-static void batch_entropy_process(void *private_)
-{
-	struct entropy_store *r	= (struct entropy_store *) private_, *p;
-	int max_entropy = r->poolinfo->POOLBITS;
-	unsigned head, tail;
-
-	/* Mixing into the pool is expensive, so copy over the batch
-	 * data and release the batch lock. The pool is at least half
-	 * full, so don't worry too much about copying only the used
-	 * part.
-	 */
-	spin_lock_irq(&batch_lock);
-
-	memcpy(batch_entropy_copy, batch_entropy_pool,
-	       batch_max * sizeof(struct sample));
-
-	head = batch_head;
-	tail = batch_tail;
-	batch_tail = batch_head;
-
-	spin_unlock_irq(&batch_lock);
-
-	p = r;
-	while (head != tail) {
-		if (r->entropy_count >= max_entropy) {
-			r = (r == &blocking_pool) ? &input_pool :
-				&blocking_pool;
-			max_entropy = r->poolinfo->POOLBITS;
-		}
-		add_entropy_words(r, batch_entropy_copy[tail].data, 2);
-		credit_entropy_store(r, batch_entropy_copy[tail].credit);
-		tail = (tail + 1) & (batch_max - 1);
-	}
-	if (p->entropy_count >= random_read_wakeup_thresh)
-		wake_up_interruptible(&random_read_wait);
-}
-
 /*********************************************************************
  *
  * Entropy input management
@@ -692,9 +578,12 @@ static struct timer_rand_state *irq_timer_state[NR_IRQS];
  */
 static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 {
-	cycles_t data;
-	long delta, delta2, delta3, time;
-	int entropy = 0;
+	struct {
+		cycles_t cycles;
+		long jiffies;
+		unsigned num;
+	} sample;
+	long delta, delta2, delta3;
 
 	preempt_disable();
 	/* if over the trickle threshold, use only 1 in 4096 samples */
@@ -702,16 +591,20 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 	    (__get_cpu_var(trickle_count)++ & 0xfff))
 		goto out;
 
+	sample.jiffies = jiffies;
+	sample.cycles = get_cycles();
+	sample.num = num;
+	add_entropy_words(&input_pool, (u32 *)&sample, sizeof(sample)/4);
+
 	/*
 	 * Calculate number of bits of randomness we probably added.
 	 * We take into account the first, second and third-order deltas
 	 * in order to make our estimate.
 	 */
-	time = jiffies;
 
 	if (!state->dont_count_entropy) {
-		delta = time - state->last_time;
-		state->last_time = time;
+		delta = sample.jiffies - state->last_time;
+		state->last_time = sample.jiffies;
 
 		delta2 = delta - state->last_delta;
 		state->last_delta = delta;
@@ -735,20 +628,13 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 		 * Round down by 1 bit on general principles,
 		 * and limit entropy entimate to 12 bits.
 		 */
-		entropy = min_t(int, fls(delta>>1), 11);
+		credit_entropy_store(&input_pool,
+				     min_t(int, fls(delta>>1), 11));
 	}
 
-	/*
-	 * Use get_cycles() if implemented, otherwise fall back to
-	 * jiffies.
-	 */
-	data = get_cycles();
-	if (data)
-		num ^= (u32)((data >> 31) >> 1);
-	else
-		data = time;
+	if(input_pool.entropy_count >= random_read_wakeup_thresh)
+		wake_up_interruptible(&random_read_wait);
 
-	batch_entropy_store(num, data, entropy);
 out:
 	preempt_enable();
 }
@@ -1273,9 +1159,6 @@ static void init_std_data(struct entropy_store *r)
 
 static int __init rand_initialize(void)
 {
-	if (batch_entropy_init(BATCH_ENTROPY_SIZE, &input_pool))
-		return -1;
-
 	init_std_data(&input_pool);
 	init_std_data(&blocking_pool);
 	init_std_data(&nonblocking_pool);