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Make sched_random() return an unsigned number, and use uint32_t

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consistently. This also matches the per-cpu pointer declaration
anyway.

This changes the tweak we give to the load from -32..31 to be 0..31
which seems more inline with the rest of the code (- rnd and the -=
64). It should also provide the randomness we need, and may fix a
signedness bug in the old code (it isn't clear that the effect was
intentional as opposed to sloppy, and the right shift of a signed
value is undefined to boot).

This stores sched_balance() behavior when it used random().

Differential Revision: https://reviews.freebsd.org/D1981
This commit is contained in:
Warner Losh 2015-02-27 21:15:12 +00:00
parent e52a2dc83c
commit b250ad3499
1 changed files with 13 additions and 11 deletions
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24
sys/kern/sched_ule.c
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@ -360,17 +360,19 @@ SDT_PROBE_DEFINE2(sched, , , surrender, "struct thread *",
/*
* We need some randomness. Implement the classic Linear Congruential
* generator X_{n+1}=(aX_n+c) mod m. These values are optimized for
* m = 2^32, a = 69069 and c = 5. This is signed so that we can get
* both positive and negative values from it by shifting the value
* right.
* m = 2^32, a = 69069 and c = 5. We only return the upper 16 bits
* of the random state (in the low bits of our answer) to return
* the maximum randomness.
*/
static int sched_random(void)
static uint32_t
sched_random()
{
int rnd, *rndptr;
rndptr = DPCPU_PTR(randomval);
rnd = *rndptr * 69069 + 5;
*rndptr = rnd;
return(rnd);
uint32_t *rndptr;
rndptr = DPCPU_PTR(randomval);
*rndptr = *rndptr * 69069 + 5;
return (*rndptr >> 16);
}
#endif
@ -718,7 +720,7 @@ cpu_search(const struct cpu_group *cg, struct cpu_search *low,
CPU_CLR(cpu, &cpumask);
tdq = TDQ_CPU(cpu);
load = tdq->tdq_load * 256;
rnd = sched_random() >> 26; /* -32 to +31 */
rnd = sched_random() % 32;
if (match & CPU_SEARCH_LOWEST) {
if (cpu == low->cs_prefer)
load -= 64;
@ -882,7 +884,7 @@ sched_balance(void)
return;
balance_ticks = max(balance_interval / 2, 1) +
((sched_random() >> 16) % balance_interval);
(sched_random() % balance_interval);
tdq = TDQ_SELF();
TDQ_UNLOCK(tdq);
sched_balance_group(cpu_top);