net: add skb_mstamp infrastructure

ktime_get() is too expensive on some cases, and we'd like to get
usec resolution timestamps in TCP stack.

This patch adds a light weight facility using a combination of
local_clock() and jiffies samples.

Instead of :

        u64 t0, t1;

        t0 = ktime_get();
        // stuff
        t1 = ktime_get();
        delta_us = ktime_us_delta(t1, t0);

use :
        struct skb_mstamp t0, t1;

        skb_mstamp_get(&t0);
        // stuff
        skb_mstamp_get(&t1);
        delta_us = skb_mstamp_us_delta(&t1, &t0);

Note : local_clock() might have a (bounded) drift between cpus.

Do not use this infra in place of ktime_get() without understanding the
issues.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Larry Brakmo <brakmo@google.com>
Cc: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 11b6925..6d8ed22 100644 (file)
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -32,6 +32,7 @@
 #include <linux/hrtimer.h>
 #include <linux/dma-mapping.h>
 #include <linux/netdev_features.h>
+#include <linux/sched.h>
 #include <net/flow_keys.h>
 
 /* A. Checksumming of received packets by device.
@@ -356,11 +357,62 @@ typedef unsigned int sk_buff_data_t;
 typedef unsigned char *sk_buff_data_t;
 #endif
 
+/**
+ * struct skb_mstamp - multi resolution time stamps
+ * @stamp_us: timestamp in us resolution
+ * @stamp_jiffies: timestamp in jiffies
+ */
+struct skb_mstamp {
+       union {
+               u64             v64;
+               struct {
+                       u32     stamp_us;
+                       u32     stamp_jiffies;
+               };
+       };
+};
+
+/**
+ * skb_mstamp_get - get current timestamp
+ * @cl: place to store timestamps
+ */
+static inline void skb_mstamp_get(struct skb_mstamp *cl)
+{
+       u64 val = local_clock();
+
+       do_div(val, NSEC_PER_USEC);
+       cl->stamp_us = (u32)val;
+       cl->stamp_jiffies = (u32)jiffies;
+}
+
+/**
+ * skb_mstamp_delta - compute the difference in usec between two skb_mstamp
+ * @t1: pointer to newest sample
+ * @t0: pointer to oldest sample
+ */
+static inline u32 skb_mstamp_us_delta(const struct skb_mstamp *t1,
+                                     const struct skb_mstamp *t0)
+{
+       s32 delta_us = t1->stamp_us - t0->stamp_us;
+       u32 delta_jiffies = t1->stamp_jiffies - t0->stamp_jiffies;
+
+       /* If delta_us is negative, this might be because interval is too big,
+        * or local_clock() drift is too big : fallback using jiffies.
+        */
+       if (delta_us <= 0 ||
+           delta_jiffies >= (INT_MAX / (USEC_PER_SEC / HZ)))
+
+               delta_us = jiffies_to_usecs(delta_jiffies);
+
+       return delta_us;
+}
+
+
 /** 
  *     struct sk_buff - socket buffer
  *     @next: Next buffer in list
  *     @prev: Previous buffer in list
- *     @tstamp: Time we arrived
+ *     @tstamp: Time we arrived/left
  *     @sk: Socket we are owned by
  *     @dev: Device we arrived on/are leaving by
  *     @cb: Control buffer. Free for use by every layer. Put private vars here
@@ -429,7 +481,10 @@ struct sk_buff {
        struct sk_buff          *next;
        struct sk_buff          *prev;
 
-       ktime_t                 tstamp;
+       union {
+               ktime_t         tstamp;
+               struct skb_mstamp skb_mstamp;
+       };
 
        struct sock             *sk;
        struct net_device       *dev;