From: Aaron Tomlin <atomlin@xxxxxxxxxx>
In the context of the idle task and an adaptive-tick mode/or a nohz_full
CPU, quiet_vmstat() can be called: before stopping the idle tick,
entering an idle state and on exit. In particular, for the latter case,
when the idle task is required to reschedule, the idle tick can remain
stopped and the timer expiration time endless i.e., KTIME_MAX. Now,
indeed before a nohz_full CPU enters an idle state, CPU-specific vmstat
counters should be processed to ensure the respective values have been
reset and folded into the zone specific 'vm_stat[]'. That being said, it
can only occur when: the idle tick was previously stopped, and
reprogramming of the timer is not required.
A customer provided some evidence which indicates that the idle tick was
stopped; albeit, CPU-specific vmstat counters still remained populated.
Thus one can only assume quiet_vmstat() was not invoked on return to the
idle loop.
If I understand correctly, I suspect this divergence might erroneously
prevent a reclaim attempt by kswapd. If the number of zone specific free
pages are below their per-cpu drift value then
zone_page_state_snapshot() is used to compute a more accurate view of
the aforementioned statistic. Thus any task blocked on the NUMA node
specific pfmemalloc_wait queue will be unable to make significant
progress via direct reclaim unless it is killed after being woken up by
kswapd (see throttle_direct_reclaim()).
Consider the following theoretical scenario:
- Note: CPU X is part of 'tick_nohz_full_mask'
1. CPU Y migrated running task A to CPU X that
was in an idle state i.e. waiting for an IRQ;
marked the current task on CPU X to need/or
require a reschedule i.e., set TIF_NEED_RESCHED
and invoked a reschedule IPI to CPU X
(see sched_move_task())
2. CPU X acknowledged the reschedule IPI. Generic
idle loop code noticed the TIF_NEED_RESCHED flag
against the idle task and attempts to exit of the
loop and calls the main scheduler function i.e.
__schedule().
Since the idle tick was previously stopped no
scheduling-clock tick would occur.
So, no deferred timers would be handled
3. Post transition to kernel execution Task A
running on CPU X, indirectly released a few pages
(e.g. see __free_one_page()); CPU X's
'vm_stat_diff[NR_FREE_PAGES]' was updated and zone
specific 'vm_stat[]' update was deferred as per the
CPU-specific stat threshold
4. Task A does invoke exit(2) and the kernel does
remove the task from the run-queue; the idle task
was selected to execute next since there are no
other runnable tasks assigned to the given CPU
(see pick_next_task() and pick_next_task_idle())
5. On return to the idle loop since the idle tick
was already stopped and can remain so (see [1]
below) e.g. no pending soft IRQs, no attempt is
made to zero and fold CPU X's vmstat counters
since reprogramming of the scheduling-clock tick
is not required/or needed (see [2])
...
do_idle
{
__current_set_polling()
tick_nohz_idle_enter()
while (!need_resched()) {
local_irq_disable()
...
/* No polling or broadcast event */
cpuidle_idle_call()
{
if (cpuidle_not_available(drv, dev)) {
tick_nohz_idle_stop_tick()
__tick_nohz_idle_stop_tick(this_cpu_ptr(&tick_cpu_sched))
{
int cpu = smp_processor_id()
if (ts->timer_expires_base)
expires = ts->timer_expires
else if (can_stop_idle_tick(cpu, ts))
(1) -------> expires = tick_nohz_next_event(ts, cpu)
else
return
ts->idle_calls++
if (expires > 0LL) {
tick_nohz_stop_tick(ts, cpu)
{
if (ts->tick_stopped && (expires == ts->next_tick)) {
(2) -------> if (tick == KTIME_MAX || ts->next_tick ==
hrtimer_get_expires(&ts->sched_timer))
return
}
...
}
So, the idea of this patch is to ensure refresh_cpu_vm_stats(false) is
called, when it is appropriate, on return to the idle loop if the idle
tick was previously stopped too.
A trivial test program was used to determine the impact of the proposed
changes and under vanilla. The nanosleep(2) system call was used several
times to suspend execution for a period of time to approximately compute
the number of CPU-cycles in the idle code path. The following is an average
count of CPU-cycles:
Vanilla Modified
Cycles per idle loop 151858 153258 (+1.0%)
Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxx>
---
kernel/time/tick-sched.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
Index: linux-2.6/kernel/time/tick-sched.c
===================================================================
--- linux-2.6.orig/kernel/time/tick-sched.c
+++ linux-2.6/kernel/time/tick-sched.c
@@ -926,13 +926,14 @@ static void tick_nohz_stop_tick(struct t
*/
if (!ts->tick_stopped) {
calc_load_nohz_start();
- quiet_vmstat(false);
ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
ts->tick_stopped = 1;
trace_tick_stop(1, TICK_DEP_MASK_NONE);
}
+ /* Attempt to fold when the idle tick is stopped or not */
+ quiet_vmstat(false);
ts->next_tick = tick;
/*
