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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: Eric Dumazet <edumazet@google.com> Cc: Ben Greear <greearb@candelatech.com> Signed-off-by: David S. Miller <davem@davemloft.net>
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@ -680,7 +680,7 @@ static inline struct sk_buff *skb_act_clone(struct sk_buff *skb, gfp_t gfp_mask,
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#endif
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struct psched_ratecfg {
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u64 rate_bps;
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u64 rate_bytes_ps; /* bytes per second */
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u32 mult;
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u16 overhead;
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u8 shift;
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@ -698,7 +698,7 @@ static inline void psched_ratecfg_getrate(struct tc_ratespec *res,
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const struct psched_ratecfg *r)
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{
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memset(res, 0, sizeof(*res));
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res->rate = r->rate_bps >> 3;
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res->rate = r->rate_bytes_ps;
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res->overhead = r->overhead;
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}
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@ -901,37 +901,33 @@ void dev_shutdown(struct net_device *dev)
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void psched_ratecfg_precompute(struct psched_ratecfg *r,
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const struct tc_ratespec *conf)
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{
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u64 factor;
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u64 mult;
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int shift;
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memset(r, 0, sizeof(*r));
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r->overhead = conf->overhead;
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r->rate_bps = (u64)conf->rate << 3;
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r->rate_bytes_ps = conf->rate;
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r->mult = 1;
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/*
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* Calibrate mult, shift so that token counting is accurate
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* for smallest packet size (64 bytes). Token (time in ns) is
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* computed as (bytes * 8) * NSEC_PER_SEC / rate_bps. It will
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* work as long as the smallest packet transfer time can be
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* accurately represented in nanosec.
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* The deal here is to replace a divide by a reciprocal one
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* in fast path (a reciprocal divide is a multiply and a shift)
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*
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* Normal formula would be :
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* time_in_ns = (NSEC_PER_SEC * len) / rate_bps
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*
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* We compute mult/shift to use instead :
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* time_in_ns = (len * mult) >> shift;
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*
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* We try to get the highest possible mult value for accuracy,
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* but have to make sure no overflows will ever happen.
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*/
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if (r->rate_bps > 0) {
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/*
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* Higher shift gives better accuracy. Find the largest
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* shift such that mult fits in 32 bits.
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*/
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for (shift = 0; shift < 16; shift++) {
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r->shift = shift;
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factor = 8LLU * NSEC_PER_SEC * (1 << r->shift);
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mult = div64_u64(factor, r->rate_bps);
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if (mult > UINT_MAX)
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break;
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}
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if (r->rate_bytes_ps > 0) {
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u64 factor = NSEC_PER_SEC;
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r->shift = shift - 1;
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factor = 8LLU * NSEC_PER_SEC * (1 << r->shift);
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r->mult = div64_u64(factor, r->rate_bps);
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for (;;) {
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r->mult = div64_u64(factor, r->rate_bytes_ps);
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if (r->mult & (1U << 31) || factor & (1ULL << 63))
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break;
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factor <<= 1;
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r->shift++;
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}
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}
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}
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EXPORT_SYMBOL(psched_ratecfg_precompute);
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