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From 03333931c17d9c62ba4063d4e4fec1578c0729a7 Mon Sep 17 00:00:00 2001
From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Sat, 12 May 2012 21:23:23 +0000
Subject: [PATCH] codel: use u16 field instead of 31bits for rec_inv_sqrt
commit 6ff272c9ad65eda219cd975b9da2dbc31cc812ee upstream.
David pointed out gcc might generate poor code with 31bit fields.
Using u16 is more than enough and permits a better code output.
Also make the code intent more readable using constants, fixed point arithmetic
not being trivial for everybody.
Suggested-by: David Miller <davem@davemloft.net>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
include/net/codel.h | 25 +++++++++++++++----------
1 file changed, 15 insertions(+), 10 deletions(-)
--- a/include/net/codel.h
+++ b/include/net/codel.h
@@ -133,13 +133,17 @@ struct codel_params {
struct codel_vars {
u32 count;
u32 lastcount;
- bool dropping:1;
- u32 rec_inv_sqrt:31;
+ bool dropping;
+ u16 rec_inv_sqrt;
codel_time_t first_above_time;
codel_time_t drop_next;
codel_time_t ldelay;
};
+#define REC_INV_SQRT_BITS (8 * sizeof(u16)) /* or sizeof_in_bits(rec_inv_sqrt) */
+/* needed shift to get a Q0.32 number from rec_inv_sqrt */
+#define REC_INV_SQRT_SHIFT (32 - REC_INV_SQRT_BITS)
+
/**
* struct codel_stats - contains codel shared variables and stats
* @maxpacket: largest packet we've seen so far
@@ -173,17 +177,18 @@ static void codel_stats_init(struct code
* http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
* new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
*
- * Here, invsqrt is a fixed point number (< 1.0), 31bit mantissa)
+ * Here, invsqrt is a fixed point number (< 1.0), 32bit mantissa, aka Q0.32
*/
static void codel_Newton_step(struct codel_vars *vars)
{
- u32 invsqrt = vars->rec_inv_sqrt;
- u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 31;
- u64 val = (3LL << 31) - ((u64)vars->count * invsqrt2);
+ u32 invsqrt = ((u32)vars->rec_inv_sqrt) << REC_INV_SQRT_SHIFT;
+ u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 32;
+ u64 val = (3LL << 32) - ((u64)vars->count * invsqrt2);
- val = (val * invsqrt) >> 32;
+ val >>= 2; /* avoid overflow in following multiply */
+ val = (val * invsqrt) >> (32 - 2 + 1);
- vars->rec_inv_sqrt = val;
+ vars->rec_inv_sqrt = val >> REC_INV_SQRT_SHIFT;
}
/*
@@ -195,7 +200,7 @@ static codel_time_t codel_control_law(co
codel_time_t interval,
u32 rec_inv_sqrt)
{
- return t + reciprocal_divide(interval, rec_inv_sqrt << 1);
+ return t + reciprocal_divide(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT);
}
@@ -326,7 +331,7 @@ static struct sk_buff *codel_dequeue(str
codel_Newton_step(vars);
} else {
vars->count = 1;
- vars->rec_inv_sqrt = 0x7fffffff;
+ vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;
}
vars->lastcount = vars->count;
vars->drop_next = codel_control_law(now, params->interval,
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