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Commit 355b590c authored by Eric Dumazet's avatar Eric Dumazet Committed by David S. Miller
Browse files

ipv4: speedup ip_idents_reserve() 



Under stress, ip_idents_reserve() is accessing a contended
cache line twice, with non optimal MESI transactions.

If we place timestamps in separate location, we reduce this
pressure by ~50% and allow atomic_add_return() to issue
a Request for Ownership.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 9dd3c797

net/ipv4/route.c

+11 −9
Original line number Diff line number Diff line
@@ -457,12 +457,9 @@ static struct neighbour *ipv4_neigh_lookup(const struct dst_entry *dst, 
}



#define IP_IDENTS_SZ 2048u

struct ip_ident_bucket {

	atomic_t	id;

	u32		stamp32;

};



static struct ip_ident_bucket *ip_idents __read_mostly;

static atomic_t *ip_idents __read_mostly;

static u32 *ip_tstamps __read_mostly;



/* In order to protect privacy, we add a perturbation to identifiers

 * if one generator is seldom used. This makes hard for an attacker
@@ -470,15 +467,16 @@ static struct ip_ident_bucket *ip_idents __read_mostly; 
 */

u32 ip_idents_reserve(u32 hash, int segs)

{

	struct ip_ident_bucket *bucket = ip_idents + hash % IP_IDENTS_SZ;

	u32 old = ACCESS_ONCE(bucket->stamp32);

	u32 *p_tstamp = ip_tstamps + hash % IP_IDENTS_SZ;

	atomic_t *p_id = ip_idents + hash % IP_IDENTS_SZ;

	u32 old = ACCESS_ONCE(*p_tstamp);

	u32 now = (u32)jiffies;

	u32 delta = 0;



	if (old != now && cmpxchg(&bucket->stamp32, old, now) == old)

	if (old != now && cmpxchg(p_tstamp, old, now) == old)

		delta = prandom_u32_max(now - old);



	return atomic_add_return(segs + delta, &bucket->id) - segs;

	return atomic_add_return(segs + delta, p_id) - segs;

}

EXPORT_SYMBOL(ip_idents_reserve);
@@ -2741,6 +2739,10 @@ int __init ip_rt_init(void) 


	prandom_bytes(ip_idents, IP_IDENTS_SZ * sizeof(*ip_idents));



	ip_tstamps = kcalloc(IP_IDENTS_SZ, sizeof(*ip_tstamps), GFP_KERNEL);

	if (!ip_tstamps)

		panic("IP: failed to allocate ip_tstamps\n");



	for_each_possible_cpu(cpu) {

		struct uncached_list *ul = &per_cpu(rt_uncached_list, cpu);