tick/sched: Ensure quiet_vmstat() is called when the idle tick was stopped too

From: Aaron Tomlin <[email protected]>

Add CPU-specific variable namely vmstat_dirty to indicate if a vmstat imbalance is present for a given CPU. Therefore, at the appropriate time, we can fold all the remaining differentials.

This speeds up quiet_vmstat in case no per-CPU differentials exist.

Based on
https://lore.kernel.org/lkml/[email protected]/

Signed-off-by Aaron Tomlin <[email protected]>

Signed-off-by Marcelo Tosatti <[email protected]>

---
 mm/vmstat.c |   54 ++++++++++++++++++++----------------------------------
 1 file changed, 20 insertions(+), 34 deletions(-)

Index: linux-2.6/mm/vmstat.c
===================================================================

mm/vmstat.c void fold_vm_numa_events(void) · line 203

Quoted 12 lines

mm/vmstat.c +6 -0

@@ -195,6 +195,12 @@ void fold_vm_numa_events(void)
 #endif
 
 #ifdef CONFIG_SMP
+static DEFINE_PER_CPU_ALIGNED(bool, vmstat_dirty);
+
+static inline void mark_vmstat_dirty(void)
+{
+	this_cpu_write(vmstat_dirty, true);
+}

If we're to have a helper for this then how about helpers for clearing it and reading it?

Also, vmstat_mark_dirty(), vmstat_clear_dirty() and vmstat_dirty() would be better identifiers.

Then those helper functions become good sites for comments explaining what's going on.

From: Aaron Tomlin <[email protected]>

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.

kernel/time/tick-sched.c +18 -0

@@ -26,6 +26,7 @@
 #include <linux/posix-timers.h>
 #include <linux/context_tracking.h>
 #include <linux/mm.h>
+#include <linux/rcupdate.h>
 
 #include <asm/irq_regs.h>
 

@@ -519,6 +520,20 @@ void __tick_nohz_task_switch(void)
 	}
 }
 
+void __tick_nohz_user_enter_prepare(void)
+{
+	struct tick_sched *ts;
+
+	if (tick_nohz_full_cpu(smp_processor_id())) {
+		ts = this_cpu_ptr(&tick_cpu_sched);
+
+		if (ts->tick_stopped)
+			quiet_vmstat();
+		rcu_nocb_flush_deferred_wakeup();
+	}
+}
+EXPORT_SYMBOL_GPL(__tick_nohz_user_enter_prepare);
+
 /* Get the boot-time nohz CPU list from the kernel parameters. */
 void __init tick_nohz_full_setup(cpumask_var_t cpumask)
 {

@@ -890,6 +905,9 @@ static void tick_nohz_stop_tick(struct t
 		ts->do_timer_last = 0;
 	}
 
+	/* Attempt to fold when the idle tick is stopped or not */
+	quiet_vmstat();
+
 	/* Skip reprogram of event if its not changed */
 	if (ts->tick_stopped && (expires == ts->next_tick)) {
 		/* Sanity check: make sure clockevent is actually programmed */

From: Aaron Tomlin <[email protected]>

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:

1. CPU Y migrated running task A to CPU X that was
in an idle state i.e. waiting for an IRQ - not polling; 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 from CPU Y;
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 Y, indirectly released a few pages (e.g. see __free_one_page()); CPU Y'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 Y's vmstat counters since reprogramming of the scheduling-clock tick is not required/or needed (see [2])

Index: linux-2.6/kernel/time/tick-sched.c
===================================================================

kernel/time/tick-sched.c +1 -0

@@ -26,6 +26,7 @@
 #include <linux/posix-timers.h>
 #include <linux/context_tracking.h>
 #include <linux/mm.h>
+#include <linux/rcupdate.h>
 
 #include <asm/irq_regs.h>
 

@@ -519,6 +520,20 @@ void __tick_nohz_task_switch(void)
 	}
 }

kernel/time/tick-sched.c +5 -0

+		rcu_nocb_flush_deferred_wakeup();
+	}
+}
 
+EXPORT_SYMBOL_GPL(__tick_nohz_user_enter_prepare);
+
 /* Get the boot-time nohz CPU list from the kernel parameters. */
 void __init tick_nohz_full_setup(cpumask_var_t cpumask)

{

kernel/time/tick-sched.c +0 -1

@@ -911,7 +929,6 @@ static void tick_nohz_stop_tick(struct t
 	 */
 	if (!ts->tick_stopped) {
 		calc_load_nohz_start();
-		quiet_vmstat();
 
 		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
 		ts->tick_stopped = 1;

Quoted 7 lines · Marcelo Tosatti

From: Aaron Tomlin <[email protected]>

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

Since quiet_vmstat() is only called when ts->tick_stopped = false, this can only happen if the idle loop did not enter into dynticks idle mode but the exiting idle task eventually stops the tick (tick_nohz_idle_update_tick()).

This can happen for example if we enter the idle loop with a timer callback pending in one jiffies, then once that timer fires, which wakes up a task, we exit the idle loop and then tick_nohz_idle_update_tick() doesn't see any timer callback pending left and the tick can be stopped.

Or am I missing something?

Quoted 21 lines

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()).

Quoted 8 lines

Consider the following theoretical scenario:

1. CPU Y migrated running task A to CPU X that was
in an idle state i.e. waiting for an IRQ - not polling; 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())

CPU Y is nohz_full right?

Quoted 18 lines

2. CPU X acknowledged the reschedule IPI from CPU Y;
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 Y, indirectly released a few pages (e.g. see __free_one_page()); CPU Y's 'vm_stat_diff[NR_FREE_PAGES]' was updated and zone specific 'vm_stat[]' update was deferred as per the CPU-specific stat threshold

Quoted 5 lines

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())

This happens on CPU X, right?

Quoted 1 line

Quoted 6 lines

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 Y's vmstat counters since reprogramming of the scheduling-clock tick is not required/or needed (see [2])

And now back to CPU Y, confused...

[...]

Quoted 16 lines

Index: linux-2.6/kernel/time/tick-sched.c
===================================================================

kernel/time/tick-sched.c +1 -0

@@ -26,6 +26,7 @@
 #include <linux/posix-timers.h>
 #include <linux/context_tracking.h>
 #include <linux/mm.h>
+#include <linux/rcupdate.h>
 
 #include <asm/irq_regs.h>
 

@@ -519,6 +520,20 @@ void __tick_nohz_task_switch(void)
 	}
 }
 

kernel/time/tick-sched.c void __tick_nohz_task_switch(void) · line 531

Quoted 9 lines

kernel/time/tick-sched.c +9 -0

+void __tick_nohz_user_enter_prepare(void)
+{
+	struct tick_sched *ts;
+
+	if (tick_nohz_full_cpu(smp_processor_id())) {
+		ts = this_cpu_ptr(&tick_cpu_sched);
+
+		if (ts->tick_stopped)
+			quiet_vmstat();

Wasn't it supposed to be part of the quiescing in task isolation mode?

Because currently vmstat is a deferrable timer but that deferrability may not apply to nohz_full anymore (outside idle). And quiet_vmstat() doesn't cancel the timer so you'll still get the disturbance.

See this patch: https://lore.kernel.org/lkml/20220725104356.GA2950296@lothringen/

Quoted 10 lines

kernel/time/tick-sched.c +5 -0

+		rcu_nocb_flush_deferred_wakeup();
+	}
+}
 
+EXPORT_SYMBOL_GPL(__tick_nohz_user_enter_prepare);
+
 /* Get the boot-time nohz CPU list from the kernel parameters. */
 void __init tick_nohz_full_setup(cpumask_var_t cpumask)

{

kernel/time/tick-sched.c static void tick_nohz_stop_tick(struct t · line 913

Quoted 9 lines

kernel/time/tick-sched.c +3 -0

@@ -890,6 +905,9 @@ static void tick_nohz_stop_tick(struct t
 		ts->do_timer_last = 0;
 	}
 
+	/* Attempt to fold when the idle tick is stopped or not */
+	quiet_vmstat();
+
 	/* Skip reprogram of event if its not changed */
 	if (ts->tick_stopped && (expires == ts->next_tick)) {
 		/* Sanity check: make sure clockevent is actually programmed */

But that chunk looks good.

Thanks.

Quoted 10 lines

kernel/time/tick-sched.c +0 -1

@@ -911,7 +929,6 @@ static void tick_nohz_stop_tick(struct t
 	 */
 	if (!ts->tick_stopped) {
 		calc_load_nohz_start();
-		quiet_vmstat();
 
 		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
 		ts->tick_stopped = 1;

For the scenario where we re-enter idle without calling quiet_vmstat:

CPU-0 CPU-1

0) vmstat_shepherd notices its necessary to queue vmstat work to remote CPU, queues deferrable timer into timer wheel, and calls trigger_dyntick_cpu (target_cpu == cpu-1).

1) Stop the tick (get_next_timer_interrupt will not take deferrable timers into account), calls quiet_vmstat, which keeps the vmstat work (vmstat_update function) queued.
2) Idle
3) Idle exit
4) Run thread on CPU, some activity marks vmstat dirty
5) Idle
6) Goto 3

At 5, since the tick is already stopped, the deferrable timer for the delayed work item will not execute, and vmstat_shepherd will consider

static void vmstat_shepherd(struct work_struct *w)

{
        int cpu;

        cpus_read_lock();
        /* Check processors whose vmstat worker threads have been disabled */
        for_each_online_cpu(cpu) {
                struct delayed_work *dw = &per_cpu(vmstat_work, cpu);

                if (!delayed_work_pending(dw) && need_update(cpu))
                        queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);

                cond_resched();
        }
        cpus_read_unlock();

        schedule_delayed_work(&shepherd,
                round_jiffies_relative(sysctl_stat_interval));
}

As far as i can tell...

Quoted 45 lines

Quoted 8 lines

Consider the following theoretical scenario:

1. CPU Y migrated running task A to CPU X that was
in an idle state i.e. waiting for an IRQ - not polling; 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())

CPU Y is nohz_full right?

Quoted 18 lines

2. CPU X acknowledged the reschedule IPI from CPU Y;
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 Y, indirectly released a few pages (e.g. see __free_one_page()); CPU Y's 'vm_stat_diff[NR_FREE_PAGES]' was updated and zone specific 'vm_stat[]' update was deferred as per the CPU-specific stat threshold

Quoted 5 lines

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())

This happens on CPU X, right?

Quoted 7 lines

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 Y's vmstat counters since reprogramming of the scheduling-clock tick is not required/or needed (see [2])

Quoted 1 line

And now back to CPU Y, confused...

Aaron, can you explain the diagram above?

On Fri 2022-09-09 16:35 -0300, Marcelo Tosatti wrote:
Hi Frederic,

Sorry about that. How about the following:

- 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])

Kind regards,

Signature · 1 line

Aaron Tomlin

From the vmstat shepherd, for CPUs that have the tick stopped,