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Commit 6ff272c9 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

codel: use u16 field instead of 31bits for rec_inv_sqrt 



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: default avatarDavid Miller <davem@davemloft.net>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 74833608

include/net/codel.h

+15 −10
Original line number Diff line number Diff line
@@ -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 codel_stats *stats) 
 * 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(codel_time_t t, 
				      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(struct Qdisc *sch, 
			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,