svn commit: r279373 - head/sys/kern
Warner Losh
imp at FreeBSD.org
Fri Feb 27 21:15:14 UTC 2015
Author: imp
Date: Fri Feb 27 21:15:12 2015
New Revision: 279373
URL: https://svnweb.freebsd.org/changeset/base/279373
Log:
Make sched_random() return an unsigned number, and use uint32_t
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
Modified:
head/sys/kern/sched_ule.c
Modified: head/sys/kern/sched_ule.c
==============================================================================
--- head/sys/kern/sched_ule.c Fri Feb 27 20:32:50 2015 (r279372)
+++ head/sys/kern/sched_ule.c Fri Feb 27 21:15:12 2015 (r279373)
@@ -360,17 +360,19 @@ SDT_PROBE_DEFINE2(sched, , , surrender,
/*
* 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.
- */
-static int sched_random(void)
-{
- int rnd, *rndptr;
- rndptr = DPCPU_PTR(randomval);
- rnd = *rndptr * 69069 + 5;
- *rndptr = rnd;
- return(rnd);
+ * 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 uint32_t
+sched_random()
+{
+ 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, s
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);
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