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

Hi Frederic, Marcelo,

This patch series addresses the following two problems:

1. 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())

2. With a SCHED_FIFO task that busy loops on a given CPU, and kworker for that CPU at SCHED_OTHER priority, queuing work to sync per-vmstats will either cause that work to never execute, or stalld (i.e. stall daemon) boosts kworker priority which causes a latency violation

As seen previously, the trivial test program (i.e. attached at the end of this cover letter) executed inside a KVM VM, was used to determine the somewhat impact under vanilla and with the proposed changes. Firstly, the mlock(2) and munlock(2) system calls was used solely to modify vmstat item 'NR_MLOCK'. In another scenario, 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 across the aforementioned system calls and the idle loop, respectively. I believe these results are negligible:

Vanilla Modified

  Cycles per idle loop            151858                  153258  (+1.0%)
  Cycles per syscall              8461                    8690    (+2.6%)

Any feedback would be appreciated. Thanks.

Changes since v7 [1]
- Added trivial helpers for modification and testing (Andrew Morton)
- Modified comment since we do now cancel any delayed work if the tick is stopped in quiet_vmstat()
- Moved check to ensure vmstat differentials do not remain if the tick is stopped on exiting to user-mode into a separate patch
(Frederic Weisbecker)

Changes since v6 [2]:
- Clean vmstat_dirty before differential sync loop
- Cancel pending work if tick stopped
- Do not queue work to remote CPU if tick stopped

Changes since v5 [3]:

- Introduced __tick_nohz_user_enter_prepare()
- Switched to EXPORT_SYMBOL_GPL()

Changes since v4 [4]:

- Moved vmstat_dirty specific changes into a separate patch (Marcelo Tosatti)

Changes since v3 [5]:

- Used EXPORT_SYMBOL() on tick_nohz_user_enter_prepare()
- Replaced need_update()
- Introduced CPU-specific variable namely vmstat_dirty and mark_vmstat_dirty()

[1]: https://lore.kernel.org/lkml/[email protected]/
[2]: https://lore.kernel.org/linux-mm/[email protected]/
[3]: https://lore.kernel.org/lkml/[email protected]/
[4]: https://lore.kernel.org/lkml/[email protected]/
[5]: https://lore.kernel.org/lkml/[email protected]/

Aaron Tomlin (4):
mm/vmstat: Add CPU-specific variable to track a vmstat discrepancy mm/vmstat: Use vmstat_dirty to track CPU-specific vmstat discrepancies tick/nohz_full: Ensure quiet_vmstat() is called on exit to user-mode
when the idle tick is stopped
tick/sched: Ensure quiet_vmstat() is called when the idle tick was
stopped too

Marcelo Tosatti (1):
mm/vmstat: Do not queue vmstat_update if tick is stopped

 include/linux/tick.h     |  5 ++-
 kernel/time/tick-sched.c | 18 ++++++++-
 mm/vmstat.c              | 80 +++++++++++++++++++++-------------------
 3 files changed, 62 insertions(+), 41 deletions(-)

2.37.1

--- test-vmstat-overhead.c ---

#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>
#include <string.h>

typedef unsigned long long cycles_t;
typedef unsigned long long usecs_t;
typedef unsigned long long u64;

#ifdef __x86_64__
#define DECLARE_ARGS(val, low, high)    unsigned long low, high
#define EAX_EDX_VAL(val, low, high)     ((low) | ((u64)(high) << 32))
#define EAX_EDX_ARGS(val, low, high)    "a" (low), "d" (high)
#define EAX_EDX_RET(val, low, high)     "=a" (low), "=d" (high)
#else
#define DECLARE_ARGS(val, low, high)    unsigned long long val
#define EAX_EDX_VAL(val, low, high)     (val)
#define EAX_EDX_ARGS(val, low, high)    "A" (val)
#define EAX_EDX_RET(val, low, high)     "=A" (val)
#endif

static inline unsigned long long __rdtscll(void)
{
        DECLARE_ARGS(val, low, high);

        asm volatile("cpuid; rdtsc" : EAX_EDX_RET(val, low, high));

        return EAX_EDX_VAL(val, low, high);
}

#define rdtscll(val) do { (val) = __rdtscll(); } while (0)

#define NRSYSCALLS 30000
#define NRSLEEPS   100000

void main(int argc, char *argv[])
{
	unsigned long a, b, cycles;
	int i, syscall = 0;
	void *page = malloc(4096);

	if (mlock(page, 4096))
		perror("mlock");
	if (munlock(page, 4096))
		perror("munlock");

	if (argc != 2) {
		printf("usage: %s {idle,syscall}\n", argv[0]);
		exit(1);
	}

        rdtscll(a);

	if (strncmp("idle", argv[1], 4) == 0)
		syscall = 0;
	else if (strncmp("syscall", argv[1], 7) == 0)
		syscall = 1;
	else {
		printf("usage: %s {idle,syscall}\n", argv[0]);
		exit(1);
	}
	
	if (syscall == 1) {
        	for (i = 0; i < NRSYSCALLS; i++) {
			if (mlock(page, 4096))
				perror("mlock");
			if (munlock(page, 4096))
				perror("munlock");
		}
	} else {
        	for (i = 0; i < NRSLEEPS; i++)
		 	usleep(10);
	}

        rdtscll(b);

        cycles = b - a;

	if (syscall == 1)
        	printf("cycles per syscall: %d\n", (b-a)/(NRSYSCALLS*2));
	else
		printf("cycles per idle loop: %d\n", (b-a)/NRSLEEPS);
}