sysstat · orasagar · Jun 3, 2026
diff --git a/rd_stats.c b/rd_stats.c
@@ -492,12 +492,20 @@ void compute_ext_disk_stats(struct stats_disk *sdc, struct stats_disk *sdp,
  * sar, sadf, mpstat
  ***************************************************************************
  */
+static unsigned long long get_cpu_interval_delta(unsigned long long prev,
+						 unsigned long long curr)
+{
+	return (curr > prev) ? curr - prev : 0;
+}
+
 unsigned long long get_per_cpu_interval(struct stats_cpu *scc,
 					struct stats_cpu *scp)
 {
 	unsigned long long ishift = 0LL;
+	unsigned long long interval = 0LL;
 
-	if ((scc->cpu_user - scc->cpu_guest) < (scp->cpu_user - scp->cpu_guest)) {
+	if ((scc->cpu_user >= scp->cpu_user) &&
+	    ((scc->cpu_user - scc->cpu_guest) < (scp->cpu_user - scp->cpu_guest))) {
 		/*
 		 * Sometimes the nr of jiffies spent in guest mode given by the guest
 		 * counter in /proc/stat is slightly higher than that included in
@@ -506,7 +514,8 @@ unsigned long long get_per_cpu_interval(struct stats_cpu *scc,
 		ishift += (scp->cpu_user - scp->cpu_guest) -
 		          (scc->cpu_user - scc->cpu_guest);
 	}
-	if ((scc->cpu_nice - scc->cpu_guest_nice) < (scp->cpu_nice - scp->cpu_guest_nice)) {
+	if ((scc->cpu_nice >= scp->cpu_nice) &&
+	    ((scc->cpu_nice - scc->cpu_guest_nice) < (scp->cpu_nice - scp->cpu_guest_nice))) {
 		/*
 		 * Idem for nr of jiffies spent in guest_nice mode.
 		 */
@@ -549,16 +558,22 @@ unsigned long long get_per_cpu_interval(struct stats_cpu *scc,
 	/*
 	 * Don't take cpu_guest and cpu_guest_nice into account
 	 * because cpu_user and cpu_nice already include them.
+	 *
+	 * Some kernels may transiently report per-CPU counters moving
+	 * backwards. Avoid unsigned underflow here: a single regressing
+	 * field must not turn the interval into a huge value, otherwise
+	 * all percentages for the CPU are rounded down to 0.00.
 	 */
-	return ((scc->cpu_user    + scc->cpu_nice   +
-		 scc->cpu_sys     + scc->cpu_iowait +
-		 scc->cpu_idle    + scc->cpu_steal  +
-		 scc->cpu_hardirq + scc->cpu_softirq) -
-		(scp->cpu_user    + scp->cpu_nice   +
-		 scp->cpu_sys     + scp->cpu_iowait +
-		 scp->cpu_idle    + scp->cpu_steal  +
-		 scp->cpu_hardirq + scp->cpu_softirq) +
-		 ishift);
+	interval += get_cpu_interval_delta(scp->cpu_user,    scc->cpu_user);
+	interval += get_cpu_interval_delta(scp->cpu_nice,    scc->cpu_nice);
+	interval += get_cpu_interval_delta(scp->cpu_sys,     scc->cpu_sys);
+	interval += get_cpu_interval_delta(scp->cpu_iowait,  scc->cpu_iowait);
+	interval += get_cpu_interval_delta(scp->cpu_idle,    scc->cpu_idle);
+	interval += get_cpu_interval_delta(scp->cpu_steal,   scc->cpu_steal);
+	interval += get_cpu_interval_delta(scp->cpu_hardirq, scc->cpu_hardirq);
+	interval += get_cpu_interval_delta(scp->cpu_softirq, scc->cpu_softirq);
+
+	return interval + ishift;
 }
 
 #ifdef SOURCE_SADC

diff --git a/tests/01655 b/tests/01655
@@ -0,0 +1,127 @@
+set -eu
+
+tmpdir="tests/sar-cpu-zero.$$"
+shim_src="$tmpdir/fake-proc-stat.c"
+shim_so="$tmpdir/fake-proc-stat.so"
+data_file="$tmpdir/sa.data"
+sar_out="$tmpdir/sar.out"
+
+cleanup()
+{
+	rm -rf "$tmpdir"
+}
+trap cleanup EXIT HUP INT TERM
+
+mkdir -p "$tmpdir"
+
+cat > "$shim_src" <<'EOF'
+#define _GNU_SOURCE
+#include <dlfcn.h>
+#include <stdio.h>
+#include <string.h>
+
+static FILE *(*real_fopen_fn)(const char *, const char *);
+static FILE *(*real_fopen64_fn)(const char *, const char *);
+static unsigned int proc_stat_open_count;
+
+static const char proc_stat_prev[] =
+"cpu  2000 0 1000 10000 0 0 0 0 0 0\n"
+"cpu0 1000 0 500 5000 0 0 0 0 0 0\n"
+"cpu1 1000 0 500 5000 0 0 0 0 0 0\n"
+"intr 1000\n"
+"ctxt 1000\n"
+"btime 1000\n"
+"processes 100\n"
+"procs_running 1\n"
+"procs_blocked 0\n";
+
+static const char proc_stat_curr[] =
+"cpu  1910 0 910 10100 0 0 0 0 0 0\n"
+"cpu0 900 0 400 5000 0 0 0 0 0 0\n"
+"cpu1 1010 0 510 5100 0 0 0 0 0 0\n"
+"intr 1010\n"
+"ctxt 1010\n"
+"btime 1000\n"
+"processes 101\n"
+"procs_running 1\n"
+"procs_blocked 0\n";
+
+static int is_read_mode(const char *mode)
+{
+	return mode && mode[0] == 'r';
+}
+
+static FILE *fake_proc_stat(void)
+{
+	const char *sample;
+
+	proc_stat_open_count++;
+	sample = proc_stat_open_count <= 2 ? proc_stat_prev : proc_stat_curr;
+
+	return fmemopen((void *) sample, strlen(sample), "r");
+}
+
+FILE *fopen(const char *path, const char *mode)
+{
+	if (!real_fopen_fn)
+		real_fopen_fn = dlsym(RTLD_NEXT, "fopen");
+
+	if (path && !strcmp(path, "/proc/stat") && is_read_mode(mode))
+		return fake_proc_stat();
+
+	return real_fopen_fn(path, mode);
+}
+
+FILE *fopen64(const char *path, const char *mode)
+{
+	if (!real_fopen64_fn)
+		real_fopen64_fn = dlsym(RTLD_NEXT, "fopen64");
+	if (!real_fopen_fn)
+		real_fopen_fn = dlsym(RTLD_NEXT, "fopen");
+
+	if (path && !strcmp(path, "/proc/stat") && is_read_mode(mode))
+		return fake_proc_stat();
+
+	return real_fopen64_fn ? real_fopen64_fn(path, mode) : real_fopen_fn(path, mode);
+}
+EOF
+
+${CC:-cc} -shared -fPIC -o "$shim_so" "$shim_src" -ldl
+
+LC_ALL=C S_TIME_FORMAT=ISO LD_PRELOAD="$shim_so" ./sadc 1 2 "$data_file" >/dev/null
+LC_ALL=C S_TIME_FORMAT=ISO ./sar -P ALL -u ALL -f "$data_file" > "$sar_out"
+
+awk '
+function is_cpu_name(value) {
+	return value == "all" || value ~ /^[0-9]+$/
+}
+
+function is_numeric(value) {
+	return value ~ /^-?[0-9]+([.][0-9]+)?$/
+}
+
+/^Linux/ || /^$/ { next }
+$1 == "Average:" { next }
+{
+	cpu_col = 2
+	if ($2 == "AM" || $2 == "PM")
+		cpu_col = 3
+	if (!is_cpu_name($cpu_col))
+		next
+
+	zero = 1
+	numeric = 0
+	for (i = cpu_col + 1; i <= NF; i++) {
+		if (!is_numeric($i))
+			continue
+		numeric++
+		if ($i != "0" && $i != "0.00")
+			zero = 0
+	}
+	if (numeric >= 6 && zero) {
+		print "unexpected all-zero CPU utilization row: " $0 > "/dev/stderr"
+		bad = 1
+	}
+}
+END { exit bad ? 1 : 0 }
+' "$sar_out"
diff --git a/tests/TLIST b/tests/TLIST
@@ -357,6 +357,7 @@ NOTES:
 01630	LC_ALL=C TZ=GMT ./sadf -p tests/data-wghfreq.tmp -- -m FREQ -P ALL > tests/out.data-wghfreq-sadf-p.tmp
 01640	LC_ALL=C TZ=GMT ./sadf -r tests/data-wghfreq.tmp -- -m FREQ -P ALL > tests/out.data-wghfreq-sadf-r.tmp
 01650	LC_ALL=C TZ=GMT ./sadf -j tests/data-wghfreq.tmp -- -m FREQ -P ALL > tests/out.data-wghfreq-sadf-j.tmp
+01655	LD_PRELOAD fake /proc/stat; ./sadc 1 2 tests/sa.data; ./sar -P ALL -u ALL -f tests/sa.data
 01657	cat tests/out.data-wghfreq-sadf-j.tmp | $VER_JSON >/dev/null;
 01660	LC_ALL=C TZ=GMT ./sadf -x tests/data-wghfreq.tmp -- -m FREQ -P ALL > tests/out.data-wghfreq-sadf-x.tmp
 01667	cat tests/out.data-wghfreq-sadf-x.tmp | $VER_XML --schema xml/sysstat.xsd - >/dev/null;