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Commit 130d3d68 authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

net_sched: psched_ratecfg_precompute() improvements 



Before allowing 64bits bytes rates, refactor
psched_ratecfg_precompute() to get better comments
and increased accuracy.

rate_bps field is renamed to rate_bytes_ps, as we only
have to worry about bytes per second.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Cc: Ben Greear <greearb@candelatech.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 45203a3b

include/net/sch_generic.h

+2 −2
Original line number Diff line number Diff line
@@ -680,7 +680,7 @@ static inline struct sk_buff *skb_act_clone(struct sk_buff *skb, gfp_t gfp_mask, 
#endif



struct psched_ratecfg {

	u64	rate_bps;

	u64	rate_bytes_ps; /* bytes per second */

	u32	mult;

	u16	overhead;

	u8	shift;
@@ -698,7 +698,7 @@ static inline void psched_ratecfg_getrate(struct tc_ratespec *res, 
					  const struct psched_ratecfg *r)

{

	memset(res, 0, sizeof(*res));

	res->rate = r->rate_bps >> 3;

	res->rate = r->rate_bytes_ps;

	res->overhead = r->overhead;

}

net/sched/sch_generic.c

+20 −24
Original line number Diff line number Diff line
@@ -901,37 +901,33 @@ void dev_shutdown(struct net_device *dev) 
void psched_ratecfg_precompute(struct psched_ratecfg *r,

			       const struct tc_ratespec *conf)

{

	u64 factor;

	u64 mult;

	int shift;



	memset(r, 0, sizeof(*r));

	r->overhead = conf->overhead;

	r->rate_bps = (u64)conf->rate << 3;

	r->rate_bytes_ps = conf->rate;

	r->mult = 1;

	/*

	 * Calibrate mult, shift so that token counting is accurate

	 * for smallest packet size (64 bytes).  Token (time in ns) is

	 * computed as (bytes * 8) * NSEC_PER_SEC / rate_bps.  It will

	 * work as long as the smallest packet transfer time can be

	 * accurately represented in nanosec.

	 */

	if (r->rate_bps > 0) {

		/*

		 * Higher shift gives better accuracy.  Find the largest

		 * shift such that mult fits in 32 bits.

	 * The deal here is to replace a divide by a reciprocal one

	 * in fast path (a reciprocal divide is a multiply and a shift)

	 *

	 * Normal formula would be :

	 *  time_in_ns = (NSEC_PER_SEC * len) / rate_bps

	 *

	 * We compute mult/shift to use instead :

	 *  time_in_ns = (len * mult) >> shift;

	 *

	 * We try to get the highest possible mult value for accuracy,

	 * but have to make sure no overflows will ever happen.

	 */

		for (shift = 0; shift < 16; shift++) {

			r->shift = shift;

			factor = 8LLU * NSEC_PER_SEC * (1 << r->shift);

			mult = div64_u64(factor, r->rate_bps);

			if (mult > UINT_MAX)

	if (r->rate_bytes_ps > 0) {

		u64 factor = NSEC_PER_SEC;



		for (;;) {

			r->mult = div64_u64(factor, r->rate_bytes_ps);

			if (r->mult & (1U << 31) || factor & (1ULL << 63))

				break;

			factor <<= 1;

			r->shift++;

		}



		r->shift = shift - 1;

		factor = 8LLU * NSEC_PER_SEC * (1 << r->shift);

		r->mult = div64_u64(factor, r->rate_bps);

	}

}

EXPORT_SYMBOL(psched_ratecfg_precompute);