Re: [PATCH v3 7/7] sched/eevdf: Move to a single runqueue

["Jakub Okoński" <jakub@okonski.org>]

Hi Peter,

On 6/5/26 14:40, Peter Zijlstra wrote:

> Change fair/cgroup to a single runqueue.
>
> [...]
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9916,7 +9837,6 @@ static void wakeup_preempt_fair(struct r
>  	if (!sched_feat(WAKEUP_PREEMPTION))
>  		return;
>  
> -	find_matching_se(&se, &pse);
>  	WARN_ON_ONCE(!pse);
>  
>  	cse_is_idle = se_is_idle(se);

I think this changes how cpu.idle affects wakeup preemption.

With the single runqueue, find_matching_se() is gone and se_is_idle()
reduces to task_has_idle_policy(). Wouldn't this mean that SCHED_NORMAL
tasks in a cpu.idle cgroup are no longer treated as idle for
wakeup_preempt_fair()?

I tested my existing 7.2.9 desktop against 7.3-rc6 with the setup below.
cyclictest doesn't reproduce it because it just sleeps and wakes, no CPU
burn and therefore its accrued EEVDF lag keeps the latency low?

cc -O2 -o wakelat wakelat.c
systemd-run --user --collect --unit=spin -p AllowedCPUs=7 -p CPUWeight=idle sh -c 'while :; do :; done'
systemd-run --user --collect --wait --pipe --same-dir -p AllowedCPUs=7 ./wakelat 1000 15 200 |
  mlr --inidx --ifs space --opprint --barred --right label lat \
    then stats1 -a count,p50,p90,p99,max -f lat then rename -r '^lat_,' \
    then put -q 'emit mapsum({"kernel": system("uname -r")}, $*)'
systemctl --user stop spin

wakelat wakes on a fixed 1 ms schedule, records how late each wakeup
was, then busy-loops for 200 micros (wall clock).

+-----------+-------+-----+-----+------+------+
|    kernel | count | p50 | p90 |  p99 |  max |
+-----------+-------+-----+-----+------+------+
|     7.2.9 | 14998 |  11 |  11 |   12 | 1869 |
| 7.3.0-rc6 | 13853 |  11 |  12 | 2029 | 2116 |
+-----------+-------+-----+-----+------+------+

count is out of 15000 periods. rc6 had 1147 wakeups delayed by over 1 ms.


wakelat.c:

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <sys/prctl.h>
#include <time.h>

static long long now_ns(void)
{
	struct timespec t;
	clock_gettime(CLOCK_MONOTONIC, &t);
	return t.tv_sec * 1000000000LL + t.tv_nsec;
}

int main(int argc, char **argv)
{
	if (argc != 4)
		return 2;
	long long interval = atoll(argv[1]) * 1000, dur = atoll(argv[2]) * 1000000000LL;
	long long work = atoll(argv[3]) * 1000;
	long max = dur / interval + 1, n = 0;
	long *lat = malloc(max * sizeof(*lat));

	prctl(PR_SET_TIMERSLACK, 1UL);
	long long start = now_ns(), next = start + interval;
	while (next - start < dur && n < max) {
		struct timespec ts = { next / 1000000000LL, next % 1000000000LL };
		clock_nanosleep(CLOCK_MONOTONIC, TIMER_ABSTIME, &ts, NULL);
		long long now = now_ns();
		lat[n++] = (now - next) / 1000;
		while (now_ns() - now < work)
			;
		do next += interval; while (next <= now);
	}
	for (long i = 0; i < n; i++)
		printf("%ld\n", lat[i]);
	return 0;
}

Regards,
Jakub