From: Wanpeng Li <[email protected]>
This can be reproduced by setting up a CPU for nohz_full and pin two tasks
that hog the CPU 100% of the time to that CPU, top reports 70% system time
and 30% user time utilization sometimes. The trace like this:
hog-11980 [015] 341.494491: function: enter_from_user_mode <--
apic_timer_interrupt
<idle>-0 [000] 341.494492: function: smp_apic_timer_interrupt <--
apic_timer_interrupt
hog-11980 [015] 341.494492: function: __context_tracking_exit <--
enter_from_user_mode
<idle>-0 [000] 341.494492: function: irq_enter <--
smp_apic_timer_interrupt
hog-11980 [015] 341.494492: bprint: vtime_delta: diff=0
(now=4295008339 vtime_snap=4295008339)
hog-11980 [015] 341.494492: function: smp_apic_timer_interrupt <--
apic_timer_interrupt
hog-11980 [015] 341.494492: function: irq_enter <--
smp_apic_timer_interrupt
hog-11980 [015] 341.494493: function: tick_sched_timer <--
__hrtimer_run_queues
<idle>-0 [000] 341.494493: function: tick_sched_timer <--
__hrtimer_run_queues
<idle>-0 [000] 341.494493: function: tick_do_update_jiffies64.part.14
<-- tick_sched_do_timer
<idle>-0 [000] 341.494494: function: do_timer <--
tick_do_update_jiffies64.part.14
hog-11980 [015] 341.494494: function: irq_exit <--
smp_apic_timer_interrupt
<idle>-0 [000] 341.494494: bprint: do_timer: updated
jiffies_64=4295008340 ticks=1
hog-11980 [015] 341.494494: function: __context_tracking_enter <--
prepare_exit_to_usermode
hog-11980 [015] 341.494494: function: vtime_user_enter <--
__context_tracking_enter
hog-11980 [015] 341.494495: bprint: vtime_delta: diff=1000000
(now=4295008340 vtime_snap=4295008339)
hog-11980 [015] 341.494495: function: __vtime_account_system <--
vtime_user_enter
hog-11980 [015] 341.494495: bprint: get_vtime_delta:
vtime_snap=4295008339 now=4295008340
hog-11980 [015] 341.494495: function: account_system_time <--
__vtime_account_system
hog-11980 [015] 341.494495: bprint: account_system_time:
cputime=995488
<idle>-0 [000] 341.494497: function: irq_exit <--
smp_apic_timer_interrupt
In this trace, we see the following:
1. On CPU15, we transition from user-space to kernel-space because
of a timer interrupt (it's the tick)
2. vtimer_delta() returns 0, because jiffies didn't change since the
last accounting
3. While CPU15 is executing in kernel-space, jiffies is updated
by CPU0
4. When going back to user-space, vtime_delta() returns non-zero
and the whole time is accounted for system time (observe how
the cputime parameter in account_system_time() is less than 1ms)
After commit ff9a9b4c433 ("sched, time: Switch VIRT_CPU_ACCOUNTING_GEN
to jiffy granularity"), it reduces timing accuracy on nohz_full CPUs to
jiffy based sampling. So the issue can happen if all CPUs fire their ticks
at the same time:
CPU 0 CPU 1
----- -----
exit_user() // no cputime update
tick X update_jiffies
enter_user() // cputime update
exit_user() // no cputime update
tick X+1 update_jiffies
enter_user() // cputime update
The nohz_full CPU receives an interrupt at the same time the timer
interrupt fires on the housekeeping CPU.
This patch offsets the tick to avert all ticks alignment in order
that the vtime sampling does not end up "in phase" with the jiffies
incrementing.
Reported-by: Luiz Capitulino <[email protected]>
Suggested-by: Rik van Riel <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Luiz Capitulino <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
---
kernel/time/tick-sched.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 7fe53be..35d79f7 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1197,8 +1197,12 @@ void tick_setup_sched_timer(void)
/* Get the next period (per-CPU) */
hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());
- /* Offset the tick to avert jiffies_lock contention. */
- if (sched_skew_tick) {
+ /*
+ * Offset the tick to avert jiffies_lock contention, and all ticks
+ * alignment in order that the vtime sampling does not end up "in
+ * phase" with the jiffies incrementing.
+ */
+ if (sched_skew_tick || tick_nohz_full_enabled()) {
u64 offset = ktime_to_ns(tick_period) >> 1;
do_div(offset, num_possible_cpus());
offset *= smp_processor_id();
--
2.7.4
