Cap the percent CPU of individual threads at 100% to fix some of the

more obvious imprecision in the previous top changes.

Specifically, top uses a delta of clock_gettime() calls right after
invoking the kern.proc sysctl to fetch the process/thread list to
compute the time delta between the fetches.  However, the kern.proc
sysctl handler does not run in constant time.  It can spin on locks,
be preempted by an interrupt handler, etc.  As a result, the time
between the gathering of stats for individual processes or threads
between subsequent kern.proc handlers can vary.  If a "slow" kern.proc
run is followed by a "fast" kern.proc run, then the threads/processes
at the start of the "slow" run will have a longer time delta than the
threads/processes at the end.  If the clock_gettime() time delta is
not itself skewed by preemption, then the delta may be too short for
a given thread/process resulting in a higher percent CPU than actual.
However, there is no good way to calculate the exact amount of overage,
nor to know which threads to subtract the overage from.  Instead, just
punt and fix the definitely-wrong case of an individual thread having
more than 100% CPU.

Discussed with:	zonk

1 files changed, 2 insertions, 0 deletions

diff --git a/usr.bin/top/machine.c b/usr.bin/top/machine.c
index 92f6c24..de57b94 100644
--- a/usr.bin/top/machine.c
+++ b/usr.bin/top/machine.c
@@ -850,6 +850,8 @@ get_process_info(struct system_info *si, struct process_select *sel,
 			continue;
 
 		PCTCPU(pp) = proc_calc_pctcpu(pp);
+		if (sel->thread && PCTCPU(pp) > 1.0)
+			PCTCPU(pp) = 1.0;
 		if (displaymode == DISP_CPU && !show_idle &&
 		    (!proc_used_cpu(pp) ||
 		     pp->ki_stat == SSTOP || pp->ki_stat == SIDL))