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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>
This commit is contained in:
Eric Dumazet 2014-02-26 14:02:11 -08:00 committed by David S. Miller
parent 20d8435a1c
commit 363ec39235
1 changed files with 57 additions and 2 deletions
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57
include/linux/skbuff.h
View File

@ -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;
union {
ktime_t tstamp;
struct skb_mstamp skb_mstamp;
};
struct sock *sk;
struct net_device *dev;