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net_sched: psched_ratecfg_precompute() improvements

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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: Eric Dumazet <edumazet@google.com>
Cc: Ben Greear <greearb@candelatech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2013-06-06 13:56:19 -07:00 • committed by David S. Miller
commit 130d3d68b5
2 changed files with 23 additions and 27 deletions
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4
include/net/sch_generic.h
View file

@ -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;
}

46
net/sched/sch_generic.c
View file

@ -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.
* 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.
*/
if (r->rate_bps > 0) {
/*
* Higher shift gives better accuracy. Find the largest
* shift such that mult fits in 32 bits.
*/
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)
break;
}
if (r->rate_bytes_ps > 0) {
u64 factor = NSEC_PER_SEC;
r->shift = shift - 1;
factor = 8LLU * NSEC_PER_SEC * (1 << r->shift);
r->mult = div64_u64(factor, r->rate_bps);
for (;;) {
r->mult = div64_u64(factor, r->rate_bytes_ps);
if (r->mult & (1U << 31) || factor & (1ULL << 63))
break;
factor <<= 1;
r->shift++;
}
}
}
EXPORT_SYMBOL(psched_ratecfg_precompute);