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>

1 files changed, 57 insertions, 2 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 11b6925f0e96..6d8ed22accb4 100644
--- 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;