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