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net_sched: sch_fq: avoid touching f->next from fq_gc()

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A significant amount of cpu cycles is spent in fq_gc()

When fq_gc() does its lookup in the rb-tree, it needs the
following fields from struct fq_flow :

f->sk       (lookup key in the rb-tree)
f->fq_node  (anchor in the rb-tree)
f->next     (used to determine if the flow is detached)
f->age      (used to determine if the flow is candidate for gc)

This unfortunately spans two cache lines (assuming 64 bytes cache lines)

We can avoid using f->next, if we use the low order bit of f->{age|tail}

This low order bit is 0, if f->tail points to an sk_buff.
We set the low order bit to 1, if the union contains a jiffies value.

Combined with the following patch, this makes sure we only need
to bring into cpu caches one cache line per flow.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2020-05-02 19:54:18 -07:00 committed by David S. Miller
parent ee1bd483cc
commit dde0a648fc
1 changed files with 13 additions and 8 deletions
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21
net/sched/sch_fq.c
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@ -70,14 +70,14 @@ struct fq_flow {
struct sk_buff *head; /* list of skbs for this flow : first skb */
union {
struct sk_buff *tail; /* last skb in the list */
unsigned long age; /* jiffies when flow was emptied, for gc */
unsigned long age; /* (jiffies | 1UL) when flow was emptied, for gc */
};
struct rb_node fq_node; /* anchor in fq_root[] trees */
struct sock *sk;
int qlen; /* number of packets in flow queue */
int credit;
u32 socket_hash; /* sk_hash */
struct fq_flow *next; /* next pointer in RR lists, or &detached */
struct fq_flow *next; /* next pointer in RR lists */
struct rb_node rate_node; /* anchor in q->delayed tree */
u64 time_next_packet;
@ -126,20 +126,25 @@ struct fq_sched_data {
struct qdisc_watchdog watchdog;
};
/* special value to mark a detached flow (not on old/new list) */
static struct fq_flow detached, throttled;
/*
* f->tail and f->age share the same location.
* We can use the low order bit to differentiate if this location points
* to a sk_buff or contains a jiffies value, if we force this value to be odd.
* This assumes f->tail low order bit must be 0 since alignof(struct sk_buff) >= 2
*/
static void fq_flow_set_detached(struct fq_flow *f)
{
f->next = &detached;
f->age = jiffies;
f->age = jiffies | 1UL;
}
static bool fq_flow_is_detached(const struct fq_flow *f)
{
return f->next == &detached;
return !!(f->age & 1UL);
}
/* special value to mark a throttled flow (not on old/new list) */
static struct fq_flow throttled;
static bool fq_flow_is_throttled(const struct fq_flow *f)
{
return f->next == &throttled;