Date: Fri, 27 Feb 2015 21:15:13 +0000 (UTC) From: Warner Losh <imp@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r279373 - head/sys/kern Message-ID: <201502272115.t1RLFDfw007282@svn.freebsd.org>
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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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