[PATCH] random: limit overhead of add_interrupt_randomness

[Joern Engel]

fast_mix is noticeably less fast than the name might imply.  Add
rate-limiting to it, so we only run it once per jiffie and cpu for the
painful case of a single interrupt hammering a cpu.  Instead we do the
dumbest possible mixing - we xor the input with the pool without any
shifting whatsoever.  Gathers some randomness at near-zero cost.

Signed-off-by: Joern Engel <joern@xxxxxxxxx>
---
 drivers/char/random.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index b86eae9..7b7f64e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -557,6 +557,7 @@ static void mix_pool_bytes(struct entropy_store *r, const void *in,
 struct fast_pool {
 	__u32		pool[4];
 	unsigned long	last;
+	unsigned long	last_jiffies;
 	unsigned short	count;
 	unsigned char	rotate;
 	unsigned char	last_timer_intr;
@@ -760,6 +761,17 @@ void add_interrupt_randomness(int irq, int irq_flags)
 		input[3] = ip >> 32;
 	}
 
+	/*
+	 * Even fast_mix is slow when dealing with 6-digit interrupt
+	 * rates.  Rate-limit this to once per jiffie.  If we get lots
+	 * of interrupts, this still generates 1.6 bits of entropy per
+	 * second and cpu.  If we get few interrupts, it shouldn't
+	 * substantially change the entropy collection.
+	 */
+	if (fast_pool->last_jiffies == jiffies)
+		return;
+	fast_pool->last_jiffies = jiffies;
+
 	fast_mix(fast_pool, input, sizeof(input));
 
 	if ((fast_pool->count & 1023) &&
-- 
1.7.10.4

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