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RFC: another approach for 64-bit network stats

To: Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>
Subject: RFC: another approach for 64-bit network stats
From: Patrick McHardy <kaber@xxxxxxxxx>
Date: Mon, 07 Jul 2003 16:06:05 +0200
Cc: netdev@xxxxxxxxxxx
Sender: netdev-bounce@xxxxxxxxxxx
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.3.1) Gecko/20030618 Debian/1.3.1-3
This patch implements a lockless aproach for 64-bit netstatistics with only a very rare
racecondition. On 64 bit system, nothing is changed. On 32 bit system the (32bit) counter
is checked periodically for overflows. The overflows are saved in counter_high. To detect
overflows, we need to save the counter value when last checked (counter_last), so there is
a 4byte overhead per 64bit counter. The 32-bit values can be accessed as before through
stats->counter, the 64bit values are accessed through a macro NETSTAT64(stats, counter).
Accessing the 64bit values contains the before mentioned race-condition, when the counters
are synced while they are read and an overflow occured the value could be of 4gb. However
the next read will return the correct value and with gigabit speed we only need to sync every ~30s,
so thats much better than racing on every counter update (using 64bit counters directly) and
potentially damaging the counter permanently. The race could be avoided by locking syncs and
reads (not normal counter updates). The patch only breaks binary interfaces, all in-kernel users
can continue to use the 32bit values until they have been changed, userspace software just
needs recompilation, device drivers don't need any changes at all.


Comments ?

Bye,
Patrick
===== include/linux/netdevice.h 1.45 vs edited =====
--- 1.45/include/linux/netdevice.h      Wed Jul  2 09:20:08 2003
+++ edited/include/linux/netdevice.h    Mon Jul  7 15:09:05 2003
@@ -91,41 +91,83 @@
 #endif
 
 /*
+ *   Macros for lockless 64 bit netdevice statistics. On 32-bit arches
+ *   the counter is checked periodically for overflows. The overflows
+ *   are carried in name_high. The updates are not atomic, there is a
+ *   race between updating and reading the counters, however this is a 
+ *   very rare condition.
+ */
+
+#if (BITS_PER_LONG == 64)
+       
+#define DECLARE_NETSTAT64(name)                                                
\
+       unsigned long name
+#define NETSTAT64(stats, name)                                         \
+       ((unsigned long long)(stats)->name)
+       
+#else
+
+#define DECLARE_NETSTAT64(name)                                                
\
+       unsigned long name;                                             \
+       unsigned long name##_high;                                      \
+       unsigned long name##_last
+
+#define NETSTAT64(stats, name)                                           \
+({                                                                       \
+       unsigned long cnt = (stats)->name;                                \
+       int carry = (stats)->name##_last > cnt;                           \
+       ((unsigned long long)((stats)->name##_high + carry) << 32 | cnt); \
+})
+
+#define NETSTAT64_SYNC(stats, name)                                    \
+do {                                                                   \
+       unsigned long cnt = (stats)->name;                              \
+       if ((stats)->name##_last > cnt)                                 \
+               (stats)->name##_high++;                                 \
+       (stats)->name##_last = cnt;                                     \
+} while(0)
+
+/* 32bit overflow about every 34s at full gigabit speed */
+#define NETSTATS64_SYNC_INTERVAL       30
+
+#endif
+
+/*
  *     Network device statistics. Akin to the 2.0 ether stats but
  *     with byte counters.
  */
  
 struct net_device_stats
 {
-       unsigned long   rx_packets;             /* total packets received       
*/
-       unsigned long   tx_packets;             /* total packets transmitted    
*/
-       unsigned long   rx_bytes;               /* total bytes received         
*/
-       unsigned long   tx_bytes;               /* total bytes transmitted      
*/
-       unsigned long   rx_errors;              /* bad packets received         
*/
-       unsigned long   tx_errors;              /* packet transmit problems     
*/
-       unsigned long   rx_dropped;             /* no space in linux buffers    
*/
-       unsigned long   tx_dropped;             /* no space available in linux  
*/
-       unsigned long   multicast;              /* multicast packets received   
*/
-       unsigned long   collisions;
+       DECLARE_NETSTAT64(rx_packets);          /* total packets received       
*/
+       DECLARE_NETSTAT64(tx_packets);          /* total packets transmitted    
*/
+       DECLARE_NETSTAT64(rx_bytes);            /* total bytes received         
*/
+       DECLARE_NETSTAT64(tx_bytes);            /* total bytes transmitted      
*/
+       DECLARE_NETSTAT64(rx_errors);           /* bad packets received         
*/
+       DECLARE_NETSTAT64(tx_errors);           /* packet transmit problems     
*/
+       DECLARE_NETSTAT64(rx_dropped);          /* no space in linux buffers    
*/
+       DECLARE_NETSTAT64(tx_dropped);          /* no space available in linux  
*/
+       DECLARE_NETSTAT64(multicast);           /* multicast packets received   
*/
+       DECLARE_NETSTAT64(collisions);
 
        /* detailed rx_errors: */
-       unsigned long   rx_length_errors;
-       unsigned long   rx_over_errors;         /* receiver ring buff overflow  
*/
-       unsigned long   rx_crc_errors;          /* recved pkt with crc error    
*/
-       unsigned long   rx_frame_errors;        /* recv'd frame alignment error 
*/
-       unsigned long   rx_fifo_errors;         /* recv'r fifo overrun          
*/
-       unsigned long   rx_missed_errors;       /* receiver missed packet       
*/
+       DECLARE_NETSTAT64(rx_length_errors);
+       DECLARE_NETSTAT64(rx_over_errors);      /* receiver ring buff overflow  
*/
+       DECLARE_NETSTAT64(rx_crc_errors);       /* recved pkt with crc error    
*/
+       DECLARE_NETSTAT64(rx_frame_errors);     /* recv'd frame alignment error 
*/
+       DECLARE_NETSTAT64(rx_fifo_errors);      /* recv'r fifo overrun          
*/
+       DECLARE_NETSTAT64(rx_missed_errors);    /* receiver missed packet       
*/
 
        /* detailed tx_errors */
-       unsigned long   tx_aborted_errors;
-       unsigned long   tx_carrier_errors;
-       unsigned long   tx_fifo_errors;
-       unsigned long   tx_heartbeat_errors;
-       unsigned long   tx_window_errors;
+       DECLARE_NETSTAT64(tx_aborted_errors);
+       DECLARE_NETSTAT64(tx_carrier_errors);
+       DECLARE_NETSTAT64(tx_fifo_errors);
+       DECLARE_NETSTAT64(tx_heartbeat_errors);
+       DECLARE_NETSTAT64(tx_window_errors);
        
        /* for cslip etc */
-       unsigned long   rx_compressed;
-       unsigned long   tx_compressed;
+       DECLARE_NETSTAT64(rx_compressed);
+       DECLARE_NETSTAT64(tx_compressed);
 };
 
 
===== net/core/dev.c 1.89 vs edited =====
--- 1.89/net/core/dev.c Thu Jul  3 09:32:44 2003
+++ edited/net/core/dev.c       Mon Jul  7 13:53:38 2003
@@ -1869,23 +1870,33 @@
        struct net_device_stats *stats = dev->get_stats ? dev->get_stats(dev) :
                                                          NULL;
        if (stats)
-               seq_printf(seq, "%6s:%8lu %7lu %4lu %4lu %4lu %5lu %10lu %9lu "
-                               "%8lu %7lu %4lu %4lu %4lu %5lu %7lu %10lu\n",
-                          dev->name, stats->rx_bytes, stats->rx_packets,
-                          stats->rx_errors,
-                          stats->rx_dropped + stats->rx_missed_errors,
-                          stats->rx_fifo_errors,
-                          stats->rx_length_errors + stats->rx_over_errors +
-                            stats->rx_crc_errors + stats->rx_frame_errors,
-                          stats->rx_compressed, stats->multicast,
-                          stats->tx_bytes, stats->tx_packets,
-                          stats->tx_errors, stats->tx_dropped,
-                          stats->tx_fifo_errors, stats->collisions,
-                          stats->tx_carrier_errors +
-                            stats->tx_aborted_errors +
-                            stats->tx_window_errors +
-                            stats->tx_heartbeat_errors,
-                          stats->tx_compressed);
+               seq_printf(seq, "%6s:%8llu %7llu %4llu %4llu %4llu %5llu "
+                               "%10llu %9llu %8llu %7llu %4llu %4llu %4llu "
+                               "%5llu %7llu %10llu\n",
+                          dev->name,
+                          NETSTAT64(stats, rx_bytes),
+                          NETSTAT64(stats, rx_packets),
+                          NETSTAT64(stats, rx_errors),
+                          NETSTAT64(stats, rx_dropped) +
+                            NETSTAT64(stats, rx_missed_errors),
+                          NETSTAT64(stats, rx_fifo_errors),
+                          NETSTAT64(stats, rx_length_errors) +
+                            NETSTAT64(stats, rx_over_errors) +
+                            NETSTAT64(stats, rx_crc_errors) +
+                            NETSTAT64(stats, rx_frame_errors),
+                          NETSTAT64(stats, rx_compressed),
+                          NETSTAT64(stats, multicast),
+                          NETSTAT64(stats, tx_bytes),
+                          NETSTAT64(stats, tx_packets),
+                          NETSTAT64(stats, tx_errors),
+                          NETSTAT64(stats, tx_dropped),
+                          NETSTAT64(stats, tx_fifo_errors),
+                          NETSTAT64(stats, collisions),
+                          NETSTAT64(stats, tx_carrier_errors) +
+                          NETSTAT64(stats, tx_aborted_errors) +
+                          NETSTAT64(stats, tx_window_errors) +
+                            NETSTAT64(stats, tx_heartbeat_errors),
+                          NETSTAT64(stats, tx_compressed));
        else
                seq_printf(seq, "%6s: No statistics available.\n", dev->name);
 }
@@ -2943,6 +2954,56 @@
        return 0;
 }
 
+#if (BITS_PER_LONG != 64)
+static void netstats64_sync_work(void *);
+static DECLARE_WORK(netstats64_work, netstats64_sync_work, NULL);
+
+static inline void netstats64_schedule_work(void)
+{
+       schedule_delayed_work(&netstats64_work, NETSTATS64_SYNC_INTERVAL * HZ);
+}
+
+static void netstats64_sync_work(void *data)
+{
+       struct net_device *dev;
+       struct net_device_stats *stats;
+
+       read_lock_bh(&dev_base_lock);
+       for (dev = dev_base; dev; dev = dev->next) {
+               stats = dev->get_stats ? dev->get_stats(dev) : NULL;
+               if (!stats)
+                       continue;
+               NETSTAT64_SYNC(stats, rx_packets);
+               NETSTAT64_SYNC(stats, tx_packets);
+               NETSTAT64_SYNC(stats, rx_bytes);
+               NETSTAT64_SYNC(stats, tx_bytes);
+               NETSTAT64_SYNC(stats, rx_errors);
+               NETSTAT64_SYNC(stats, tx_errors);
+               NETSTAT64_SYNC(stats, rx_dropped);
+               NETSTAT64_SYNC(stats, tx_dropped);
+               NETSTAT64_SYNC(stats, multicast);
+               NETSTAT64_SYNC(stats, collisions);
+
+               NETSTAT64_SYNC(stats, rx_length_errors);
+               NETSTAT64_SYNC(stats, rx_over_errors);
+               NETSTAT64_SYNC(stats, rx_crc_errors);
+               NETSTAT64_SYNC(stats, rx_frame_errors);
+               NETSTAT64_SYNC(stats, rx_fifo_errors);
+               NETSTAT64_SYNC(stats, rx_missed_errors);
+               
+               NETSTAT64_SYNC(stats, tx_aborted_errors);
+               NETSTAT64_SYNC(stats, tx_carrier_errors);
+               NETSTAT64_SYNC(stats, tx_fifo_errors);
+               NETSTAT64_SYNC(stats, tx_heartbeat_errors);
+               NETSTAT64_SYNC(stats, tx_window_errors);
+
+               NETSTAT64_SYNC(stats, rx_compressed);
+               NETSTAT64_SYNC(stats, tx_compressed);
+       }
+       read_unlock_bh(&dev_base_lock);
+       netstats64_schedule_work();
+}
+#endif
 
 /*
  *     Initialize the DEV module. At boot time this walks the device list and
@@ -3082,6 +3143,9 @@
 
 #ifdef CONFIG_NET_SCHED
        pktsched_init();
+#endif
+#if (BITS_PER_LONG != 64)
+       netstats64_schedule_work();
 #endif
        rc = 0;
 out:
===== net/core/net-sysfs.c 1.7 vs edited =====
--- 1.7/net/core/net-sysfs.c    Sun Jun 15 10:21:46 2003
+++ edited/net/core/net-sysfs.c Mon Jul  7 13:57:11 2003
@@ -184,9 +184,9 @@
        ssize_t (*store)(struct net_device_stats *, const char *, size_t);
 };
 
-static ssize_t net_device_stat_show(unsigned long var, char *buf)
+static ssize_t net_device_stat_show(unsigned long long var, char *buf)
 {
-       return sprintf(buf, "%ld\n", var);
+       return sprintf(buf, "%lld\n", var);
 }
 
 /* generate a read-only statistics attribute */
@@ -194,7 +194,7 @@
 static ssize_t show_stat_##_NAME(const struct net_device_stats *stats, \
                                 char *buf)                             \
 {                                                                      \
-       return net_device_stat_show(stats->_NAME, buf);                 \
+       return net_device_stat_show(NETSTAT64(stats, _NAME), buf);      \
 }                                                                      \
 static struct netstat_fs_entry net_stat_##_NAME  = {                   \
        .attr = {.name = __stringify(_NAME), .mode = S_IRUGO },         \
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