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>

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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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net/sched/sch_fq.c

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