Hi George,
On Sun, Aug 09, 2020 at 06:30:17PM +0000, George Spelvin wrote:
> Even something simple like buffering 8 TSC samples, and adding them
> at 32-bit offsets across the state every 8th call, would make a huge
> difference.
Doing testing on real hardware showed that retrieving the TSC on every
call had a non negligible cost, causing a loss of 2.5% on the accept()
rate and 4% on packet rate when using iptables -m statistics. However
I reused your idea of accumulating old TSCs to increase the uncertainty
about their exact value, except that I retrieve it only on 1/8 calls
and use the previous noise in this case. With this I observe the same
performance as plain 5.8. Below are the connection rates accepted on
a single core :
5.8 5.8+patch 5.8+patch+tsc
192900-197900 188800->192200 194500-197500 (conn/s)
This was on a core i7-8700K. I looked at the asm code for the function
and it remains reasonably light, in the same order of complexity as the
original one, so I think we could go with that.
My proposed change is below, in case you have any improvements to suggest.
Regards,
Willy
diff --git a/lib/random32.c b/lib/random32.c
index 2b048e2ea99f..a12d63028106 100644
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -317,6 +317,8 @@ static void __init prandom_state_selftest(void)
struct siprand_state {
unsigned long v[4];
+ unsigned long noise;
+ unsigned long count;
};
static DEFINE_PER_CPU(struct siprand_state, net_rand_state) __latent_entropy;
@@ -334,7 +336,7 @@ static DEFINE_PER_CPU(struct siprand_state, net_rand_state)
__latent_entropy;
#define K0 (0x736f6d6570736575 ^ 0x6c7967656e657261 )
#define K1 (0x646f72616e646f6d ^ 0x7465646279746573 )
-#elif BITS_PER_LONG == 23
+#elif BITS_PER_LONG == 32
/*
* On 32-bit machines, we use HSipHash, a reduced-width version of SipHash.
* This is weaker, but 32-bit machines are not used for high-traffic
@@ -375,6 +377,12 @@ static u32 siprand_u32(struct siprand_state *s)
{
unsigned long v0 = s->v[0], v1 = s->v[1], v2 = s->v[2], v3 = s->v[3];
+ if (++s->count >= 8) {
+ v3 ^= s->noise;
+ s->noise += random_get_entropy();
+ s->count = 0;
+ }
+
SIPROUND(v0, v1, v2, v3);
SIPROUND(v0, v1, v2, v3);
s->v[0] = v0; s->v[1] = v1; s->v[2] = v2; s->v[3] = v3;
