[ovs-dev] Meter measures incorrect when use multi-pmd in ovs2.10

ychen ychen103103 at 163.com
Mon Sep 30 03:13:11 UTC 2019

   I meet a problem that when send packets using netperf in multi-thread mode.
   The reproducing condition is like this:
   1. ovs 2.10 in dpdk usermode with 2 pmd
   2. Set Meter with rate=100,000pps, burst=20,000packet
   3. when use single-thread mode for netperf, Meter behaves correct, and packets larger than 100,0000pps is dropped
       when use multi-thread mode for netperf, we noticed packets come from both 2 pmds, and in this time, Meter measure does not work.

   But Meter behaves correctly in ovs2.8 whether using single pmd or multi-pmd .
   Also, we have merged the patch 42697ca7757b594cc841d944e43ffc17905e3188
  (long_delta_t = now / 1000 - meter->used / 1000)  but problem still exists.

   We researched the meter code, found that meter use time_usec() to compute the delta time in ovs2.10.
   but when call the function dp_netdev_run_meter(),  the input parameter 'now' variable come from pmd->ctx.now
   and pmd->ctx.now may updated when receive packet in function dp_netdev_process_rxq_port()
   so let's suppose the following condition:
   pmd1 receives packet in T1
   pmd2 receives packet in T2 (T2<T1)
   first handle Meter in pmd1, meter->used changed to pmd1->ctx.now
   than handle Meter in pmd2, now = pmd2->ctx.now
   and long_delta_t = now / 1000 - meter->used / 1000 = T2/1000 - T1/1000  which will be a negative value!!!
   then delta time changed with clause:
  delta_t = (long_delta_t > (long long int)meter->max_delta_t)
        ? meter->max_delta_t : (uint32_t)long_delta_t;
 in this time, delta_t = (uint32_t)(T2/1000 - T1/1000)  will be overflow???

now, we just fix this problem by modifing the input parameter 'now' with time_usec().
we don't know whether this code modify has any side effect(performance issue?)

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