Comment 0 for bug 2039575

Bug description by Nils H.:

------------
 Overview:
------------
The following line in the SMC stats code (net/smc/smc_stats.h) caught my attention when using a payload of exactly 4096 bytes:

#define SMC_STAT_PAYLOAD_SUB(_smc_stats, _tech, key, _len, _rc) \
do { \
 typeof(_smc_stats) stats = (_smc_stats); \
 typeof(_tech) t = (_tech); \
 typeof(_len) l = (_len); \
 int _pos = fls64((l) >> 13); \
 typeof(_rc) r = (_rc); \
 int m = SMC_BUF_MAX - 1; \
 this_cpu_inc((*stats).smc[t].key ## _cnt); \
 if (r <= 0) \
  break; \
 _pos = (_pos < m) ? ((l == 1 << (_pos + 12)) ? _pos - 1 : _pos) : m; \ <---
 this_cpu_inc((*stats).smc[t].key ## _pd.buf[_pos]); \
 this_cpu_add((*stats).smc[t].key ## _bytes, r); \
} \
while (0)

With l = 4096, _pos evaluates to -1.

Checking with the following uperf profile:
# cat rr1c-4kx4k---1.xml
<?xml version="1.0"?>
<profile name="TCP_RR">
 <group nprocs="1">
 <!--group nthreads="1"-->
 <!-- if we want to run processes -->
 <!--group nprocs="1"-->
  <transaction iterations="1">
   <flowop type="connect" options="remotehost=<remote IP> protocol=tcp tcp_nodelay" />
  </transaction>
  <transaction iterations="1">
   <flowop type="write" options="size=4096"/>
   <flowop type="read" options="size=4096"/>
  </transaction>
  <transaction iterations="1">
   <flowop type="disconnect" />
  </transaction>
 </group>
</profile>

smcd stats output:
# smcd -d stats reset
SMC-D Connections Summary
  Total connections handled 2
  SMC connections 2 (client 2, server 0)
    v1 0
    v2 2
  Handshake errors 0 (client 0, server 0)
  Avg requests per SMC conn 14.0
  TCP fallback 0 (client 0, server 0)

RX Stats
  Data transmitted (Bytes) 5796 (5.796K)
  Total requests 9
  Buffer full 0 (0.00%)
  Buffer downgrades 0
  Buffer reuses 0
            8KB 16KB 32KB 64KB 128KB 256KB 512KB >512KB
  Bufs 0 0 0 2 0 0 0 1
  Reqs 8 0 0 0 0 0 0 0

TX Stats
  Data transmitted (Bytes) 9960 (9.960K)
  Total requests 19
  Buffer full 0 (0.00%)
  Buffer full (remote) 0 (0.00%)
  Buffer too small 0 (0.00%)
  Buffer too small (remote) 0 (0.00%)
  Buffer downgrades 0
  Buffer reuses 0
            8KB 16KB 32KB 64KB 128KB 256KB 512KB >512KB
  Bufs 0 2 0 0 0 0 0 0
  Reqs 18 0 0 0 0 0 0 1

Extras
  Special socket calls 0
    cork 0
    nodelay 0
    sendpage 0
    splice 0
    urgent data 0

Instead of including the payload in the wrong >512KB buckets, output should be to have 19 reqs in the 8KB buckets for TX stats and 9 reqs in the 8KB bucket for RX stats.

--------
 Repro:
--------
0. Install uperf.
1. Reset SMC-D stats on client and server.
2. Start uperf at server side: "uperf -vs".
3. Update profile with remote IP (server IP) and start uperf at client: "uperf -vai 5 -m rr1c-4kx4k---1.xml" (uperf profile, see above)