Commit 3d7efa4e authored by Heiko Carstens's avatar Heiko Carstens Committed by Vasily Gorbik
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s390/idle: fix cpu idle time calculation 



The idle time reported in /proc/stat sometimes incorrectly contains
huge values on s390. This is caused by a bug in arch_cpu_idle_time().

The kernel tries to figure out when a different cpu entered idle by
accessing its per-cpu data structure. There is an ordering problem: if
the remote cpu has an idle_enter value which is not zero, and an
idle_exit value which is zero, it is assumed it is idle since
"now". The "now" timestamp however is taken before the idle_enter
value is read.

Which in turn means that "now" can be smaller than idle_enter of the
remote cpu. Unconditionally subtracting idle_enter from "now" can thus
lead to a negative value (aka large unsigned value).

Fix this by moving the get_tod_clock() invocation out of the
loop. While at it also make the code a bit more readable.

A similar bug also exists for show_idle_time(). Fix this is as well.

Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: default avatarVasily Gorbik <gor@linux.ibm.com>
parent a1d863ac

arch/s390/kernel/idle.c

+22 −7
Original line number Diff line number Diff line
@@ -69,18 +69,26 @@ DEVICE_ATTR(idle_count, 0444, show_idle_count, NULL); 
static ssize_t show_idle_time(struct device *dev,

				struct device_attribute *attr, char *buf)

{

	unsigned long long now, idle_time, idle_enter, idle_exit, in_idle;

	struct s390_idle_data *idle = &per_cpu(s390_idle, dev->id);

	unsigned long long now, idle_time, idle_enter, idle_exit;

	unsigned int seq;



	do {

		now = get_tod_clock();

		seq = read_seqcount_begin(&idle->seqcount);

		idle_time = READ_ONCE(idle->idle_time);

		idle_enter = READ_ONCE(idle->clock_idle_enter);

		idle_exit = READ_ONCE(idle->clock_idle_exit);

	} while (read_seqcount_retry(&idle->seqcount, seq));

	idle_time += idle_enter ? ((idle_exit ? : now) - idle_enter) : 0;

	in_idle = 0;

	now = get_tod_clock();

	if (idle_enter) {

		if (idle_exit) {

			in_idle = idle_exit - idle_enter;

		} else if (now > idle_enter) {

			in_idle = now - idle_enter;

		}

	}

	idle_time += in_idle;

	return sprintf(buf, "%llu\n", idle_time >> 12);

}

DEVICE_ATTR(idle_time_us, 0444, show_idle_time, NULL);
@@ -88,17 +96,24 @@ DEVICE_ATTR(idle_time_us, 0444, show_idle_time, NULL); 
u64 arch_cpu_idle_time(int cpu)

{

	struct s390_idle_data *idle = &per_cpu(s390_idle, cpu);

	unsigned long long now, idle_enter, idle_exit;

	unsigned long long now, idle_enter, idle_exit, in_idle;

	unsigned int seq;



	do {

		now = get_tod_clock();

		seq = read_seqcount_begin(&idle->seqcount);

		idle_enter = READ_ONCE(idle->clock_idle_enter);

		idle_exit = READ_ONCE(idle->clock_idle_exit);

	} while (read_seqcount_retry(&idle->seqcount, seq));



	return cputime_to_nsecs(idle_enter ? ((idle_exit ?: now) - idle_enter) : 0);

	in_idle = 0;

	now = get_tod_clock();

	if (idle_enter) {

		if (idle_exit) {

			in_idle = idle_exit - idle_enter;

		} else if (now > idle_enter) {

			in_idle = now - idle_enter;

		}

	}

	return cputime_to_nsecs(in_idle);

}



void arch_cpu_idle_enter(void)

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