1 files changed, 20 insertions, 24 deletions

diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 20224086cc28..4626cef4b76e 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -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)
+        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);