A customer is using /proc/stat to track cpu usage in a VM and noted
that the iowait and idle times behave strangely when a cpu goes
offline and comes back online.
This patchset addresses two issues that can cause iowait and idle
to fluctuate up and down. With these changes, cpu iowait and idle
now only monotonically increase.
Tom Hromatka (2):
tick-sched: Do not clear the iowait and idle times
/proc/stat: Simplify iowait and idle calculations when cpu is offline
fs/proc/stat.c | 24 ++++++------------------
kernel/time/tick-sched.c | 9 +++++++++
2 files changed, 15 insertions(+), 18 deletions(-)
--
2.25.3
A customer reported that when a cpu goes offline, the iowait and idle
times reported in /proc/stat will sometimes spike. This is being
caused by a different data source being used for these values when a
cpu is offline.
Prior to this patch:
put the system under heavy load so that there is little idle time
user nice system idle iowait
cpu 109515 17 32111 220686 607
take cpu1 offline
user nice system idle iowait
cpu 113742 17 32721 220724 612
bring cpu1 back online
user nice system idle iowait
cpu 118332 17 33430 220687 607
To prevent this, let's use the same data source whether a cpu is
online or not.
With this patch:
put the system under heavy load so that there is little idle time
user nice system idle iowait
cpu 14096 16 4646 157687 426
take cpu1 offline
user nice system idle iowait
cpu 21614 16 7179 157687 426
bring cpu1 back online
user nice system idle iowait
cpu 27362 16 9555 157688 426
Signed-off-by: Tom Hromatka <[email protected]>
---
fs/proc/stat.c | 24 ++++++------------------
1 file changed, 6 insertions(+), 18 deletions(-)
diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 46b3293015fe..35b92539e711 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -47,32 +47,20 @@ static u64 get_iowait_time(struct kernel_cpustat *kcs, int cpu)
static u64 get_idle_time(struct kernel_cpustat *kcs, int cpu)
{
- u64 idle, idle_usecs = -1ULL;
+ u64 idle, idle_usecs;
- if (cpu_online(cpu))
- idle_usecs = get_cpu_idle_time_us(cpu, NULL);
-
- if (idle_usecs == -1ULL)
- /* !NO_HZ or cpu offline so we can rely on cpustat.idle */
- idle = kcs->cpustat[CPUTIME_IDLE];
- else
- idle = idle_usecs * NSEC_PER_USEC;
+ idle_usecs = get_cpu_idle_time_us(cpu, NULL);
+ idle = idle_usecs * NSEC_PER_USEC;
return idle;
}
static u64 get_iowait_time(struct kernel_cpustat *kcs, int cpu)
{
- u64 iowait, iowait_usecs = -1ULL;
-
- if (cpu_online(cpu))
- iowait_usecs = get_cpu_iowait_time_us(cpu, NULL);
+ u64 iowait, iowait_usecs;
- if (iowait_usecs == -1ULL)
- /* !NO_HZ or cpu offline so we can rely on cpustat.iowait */
- iowait = kcs->cpustat[CPUTIME_IOWAIT];
- else
- iowait = iowait_usecs * NSEC_PER_USEC;
+ iowait_usecs = get_cpu_iowait_time_us(cpu, NULL);
+ iowait = iowait_usecs * NSEC_PER_USEC;
return iowait;
}
--
2.25.3
A customer reported that when a cpu goes offline and then comes back
online, the overall cpu idle and iowait data in /proc/stat decreases.
This is wreaking havoc with their cpu usage calculations.
Prior to this patch:
user nice system idle iowait
cpu 1390748 636 209444 9802206 19598
cpu1 178384 75 24545 1392450 3025
take cpu1 offline and bring it back online
user nice system idle iowait
cpu 1391209 636 209682 8453440 16595
cpu1 178440 75 24572 627 0
To prevent this, do not clear the idle and iowait times for the
cpu that has come back online.
With this patch:
user nice system idle iowait
cpu 129913 17 17590 166512 704
cpu1 15916 3 2395 20989 47
take cpu1 offline and bring it back online
user nice system idle iowait
cpu 130089 17 17686 184625 711
cpu1 15942 3 2401 23088 47
Signed-off-by: Tom Hromatka <[email protected]>
---
kernel/time/tick-sched.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3e2dc9b8858c..8103bad7bbd6 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1375,13 +1375,22 @@ void tick_setup_sched_timer(void)
void tick_cancel_sched_timer(int cpu)
{
struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+ ktime_t idle_sleeptime, iowait_sleeptime;
# ifdef CONFIG_HIGH_RES_TIMERS
if (ts->sched_timer.base)
hrtimer_cancel(&ts->sched_timer);
# endif
+ /* save off and restore the idle_sleeptime and the iowait_sleeptime
+ * to avoid discontinuities and ensure that they are monotonically
+ * increasing
+ */
+ idle_sleeptime = ts->idle_sleeptime;
+ iowait_sleeptime = ts->iowait_sleeptime;
memset(ts, 0, sizeof(*ts));
+ ts->idle_sleeptime = idle_sleeptime;
+ ts->iowait_sleeptime = iowait_sleeptime;
}
#endif
--
2.25.3
Ping.
Thanks.
Tom
On 6/10/20 3:05 PM, Tom Hromatka wrote:
> A customer is using /proc/stat to track cpu usage in a VM and noted
> that the iowait and idle times behave strangely when a cpu goes
> offline and comes back online.
>
> This patchset addresses two issues that can cause iowait and idle
> to fluctuate up and down. With these changes, cpu iowait and idle
> now only monotonically increase.
>
> Tom Hromatka (2):
> tick-sched: Do not clear the iowait and idle times
> /proc/stat: Simplify iowait and idle calculations when cpu is offline
>
> fs/proc/stat.c | 24 ++++++------------------
> kernel/time/tick-sched.c | 9 +++++++++
> 2 files changed, 15 insertions(+), 18 deletions(-)
>