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kernel: fix accounttime() for HZ >= 1000

"milli-CPU's" is too low resolution for the decaying load average
calculation when HZ >= 1000.

--- a/sys/src/9/port/proc.c

+++ b/sys/src/9/port/proc.c

@@ -1602,14 +1602,14 @@

 	n = perfticks();

 	per = n - m->perf.last;

 	m->perf.last = n;

-	per = (m->perf.period*(HZ-1) + per)/HZ;

+	per = ((uvlong)m->perf.period*(HZ-1) + per)/HZ;

 	if(per != 0)

 		m->perf.period = per;

-	m->perf.avg_inidle = (m->perf.avg_inidle*(HZ-1)+m->perf.inidle)/HZ;

+	m->perf.avg_inidle = ((uvlong)m->perf.avg_inidle*(HZ-1)+m->perf.inidle)/HZ;

 	m->perf.inidle = 0;

-	m->perf.avg_inintr = (m->perf.avg_inintr*(HZ-1)+m->perf.inintr)/HZ;

+	m->perf.avg_inintr = ((uvlong)m->perf.avg_inintr*(HZ-1)+m->perf.inintr)/HZ;

 	m->perf.inintr = 0;

 	/* only one processor gets to compute system load averages */

@@ -1627,8 +1627,9 @@

 	 */

 	n = nrun;

 	nrun = 0;

-	n = (nrdy+n)*1000;

-	m->load = (m->load*(HZ-1)+n)/HZ;

+	n = (nrdy+n)*1000*100;

+	load = ((uvlong)load*(HZ-1)+n)/HZ;

+	m->load = load/100;

 }

 int