[PATCH, RFC 11/12] random: speed up the fast_mix function by a factor of four

[Theodore Ts'o]

By mixing the entropy in chunks of 32-bit words instead of byte by
byte, we can speed up the fast_mix function significantly.  Since it
is called on every single interrupt, on systems with a very heavy
interrupt load, this can make a noticeable different.

Signed-off-by: "Theodore Ts'o" <ty...@mit.edu>
Reported-by: Jörn Engel <jo...@logfs.org>
---
 drivers/char/random.c | 30 +++++++++++++++++-------------
 1 file changed, 17 insertions(+), 13 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 3439b1c..86879d1 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -583,21 +583,26 @@ struct fast_pool {
  * collector.  It's hardcoded for an 128 bit pool and assumes that any
  * locks that might be needed are taken by the caller.
  */
-static void fast_mix(struct fast_pool *f, const void *in, int nbytes)
+static void fast_mix(struct fast_pool *f, __u32 input[4])
 {
-       const char      *bytes = in;
        __u32           w;
-       unsigned        i = f->count;
        unsigned        input_rotate = f->rotate;
 
-       while (nbytes--) {
-               w = rol32(*bytes++, input_rotate & 31) ^ f->pool[i & 3] ^
-                       f->pool[(i + 1) & 3];
-               f->pool[i & 3] = (w >> 3) ^ twist_table[w & 7];
-               input_rotate += (i++ & 3) ? 7 : 14;
-       }
-       f->count = i;
+       w = rol32(input[0], input_rotate) ^ f->pool[0] ^ f->pool[3];
+       f->pool[0] = (w >> 3) ^ twist_table[w & 7];
+       input_rotate = (input_rotate + 14) & 31;
+       w = rol32(input[1], input_rotate) ^ f->pool[1] ^ f->pool[0];
+       f->pool[1] = (w >> 3) ^ twist_table[w & 7];
+       input_rotate = (input_rotate + 7) & 31;
+       w = rol32(input[2], input_rotate) ^ f->pool[2] ^ f->pool[1];
+       f->pool[2] = (w >> 3) ^ twist_table[w & 7];
+       input_rotate = (input_rotate + 7) & 31;
+       w = rol32(input[3], input_rotate) ^ f->pool[3] ^ f->pool[2];
+       f->pool[3] = (w >> 3) ^ twist_table[w & 7];
+       input_rotate = (input_rotate + 7) & 31;
+
        f->rotate = input_rotate;
+       f->count++;
 }
 
 /*
@@ -836,10 +841,9 @@ void add_interrupt_randomness(int irq, int irq_flags)
                input[3] = ip >> 32;
        }
 
-       fast_mix(fast_pool, input, sizeof(input));
+       fast_mix(fast_pool, input);
 
-       if ((fast_pool->count & 1023) &&
-           !time_after(now, fast_pool->last + HZ))
+       if ((fast_pool->count & 63) && !time_after(now, fast_pool->last + HZ))
                return;
 
        fast_pool->last = now;
-- 
1.7.12.rc0.22.gcdd159b

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