Re: [PATCH V5 1/6] Sched: Scheduler time slice extension

[Steven Rostedt]

On Mon, 9 Jun 2025 16:55:32 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> So I applied your patches and fixed up my "extend-sched.c" program to use
> your method. I booted on bare-metal PREEMPT_RT and ran:

In case anyone else wants to play, I'm attaching the source of extend-sched.c

I ran it with: sleep 5; ./extend-sched

Then switched over to cyclic test, counted to five and it was pretty
noticeable when it triggered.

To build, simply do:

  $ cd linux.git
  $ mkdir /tmp/extend
  $ cp tools/testing/selftests/rseq/rseq-abi.h /tmp/extend
  $ cd /tmp/extend

  [ download extend-sched.c here ]

  $ gcc extend-sched.c -o extend-sched


-- Steve

// Run with: GLIBC_TUNABLES=glibc.pthread.rseq=0 

#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/time.h>

#ifdef ENABLE_TRACEFS
#include <tracefs.h>
#else
static inline void tracefs_printf(void *inst, const char *fmt, ...) { }
static inline void tracefs_print_init(void *inst) { }
#endif

#include <sys/rseq.h>
#include "rseq-abi.h"

static bool no_rseq;
static bool extend_wait;

/* In case we want to play with priorities */
static int busy_prio = 0;
static int lock_prio = 0;

static int loop_spin = 15000;

//#define barrier() asm volatile ("" ::: "memory")
#define rmb() asm volatile ("lfence" ::: "memory")
#define wmb() asm volatile ("sfence" ::: "memory")

#define NR_BUSY_THREADS 5

static pthread_barrier_t pbarrier;

static __thread struct rseq_abi *rseq_map;

static void init_extend_map(void)
{
	if (no_rseq)
		return;

	rseq_map = (void *)__builtin_thread_pointer() + __rseq_offset;
}

struct data;

struct thread_data {
	unsigned long long			x_count;
	unsigned long long			total;
	unsigned long long			max;
	unsigned long long			min;
	unsigned long long			total_wait;
	unsigned long long			max_wait;
	unsigned long long			min_wait;
	unsigned long long			contention;
	unsigned long long			extended;
	struct data				*data;
	int					cpu;
};

struct data {
	unsigned long long		x;
	unsigned long			lock;
	struct thread_data		*tdata;
	bool				done;
};

static inline unsigned long
cmpxchg(volatile unsigned long *ptr, unsigned long old, unsigned long new)
{
        unsigned long prev;

	asm volatile("lock; cmpxchg %b1,%2"
		     : "=a"(prev)
		     : "q"(new), "m"(*(ptr)), "0"(old)
		     : "memory");
        return prev;
}

static void extend(void)
{
	if (no_rseq)
		return;

	rseq_map->flags |= 1 << 3;
}

static int unextend(void)
{
	int flags;
	if (no_rseq)
		return 0;

	flags = rseq_map->flags;
	rseq_map->flags &= ~((1 << 3) | (1 << 4));
	if (!(flags & (1 << 4)))
		return 0;

	tracefs_printf(NULL, "Yield!\n");
	sched_yield();
		return 1;
}

#define sec2usec(sec) (sec * 1000000ULL)
#define usec2sec(usec) (usec / 1000000ULL)

static unsigned long long get_time(void)
{
	struct timeval tv;
	unsigned long long time;

	gettimeofday(&tv, NULL);

	time = sec2usec(tv.tv_sec);
	time += tv.tv_usec;

	return time;
}

static void do_sleep(unsigned usecs)
{
	struct timespec ts;

	ts.tv_sec = 0;
	ts.tv_nsec = usecs * 1000;
	nanosleep(&ts, NULL);
}

static void grab_lock(struct thread_data *tdata, struct data *data)
{
	unsigned long long start_wait, start, end, delta;
	unsigned long long end_wait;
	unsigned long prev;
	bool contention = false;

	start_wait = get_time();

	rmb();
	while (data->lock && !data->done) {
		contention = true;
		rmb();
	}

	tracefs_printf(NULL, "Grab lock\n");
	if (extend_wait)
		extend();
	do {
		if (!extend_wait)
			extend();
		start = get_time();
		prev = cmpxchg(&data->lock, 0, 1);
		if (prev) {
			contention = true;
			if (!extend_wait && unextend())
				tdata->extended++;
			while (data->lock && !data->done)
				rmb();
		}
	} while (prev && !data->done);

	if (contention)
		tdata->contention++;

	if (data->done)
		return;

	end_wait = get_time();

	tracefs_printf(NULL, "Have lock!\n");

	delta = end_wait - start_wait;
	if (!tdata->total_wait || tdata->max_wait < delta)
		tdata->max_wait = delta;
	if (!tdata->total_wait || tdata->min_wait > delta)
		tdata->min_wait = delta;
	tdata->total_wait += delta;

	data->x++;

	if (data->lock != 1) {
		printf("Failed locking\n");
		exit(-1);
	}

	/* Loop */
	for (int i = 0; i < loop_spin; i++)
		wmb();

	prev = cmpxchg(&data->lock, 1, 0);
	end = get_time();
	tracefs_printf(NULL, "released lock!\n");
	if (unextend())
		tdata->extended++;
	if (prev != 1) {
		printf("Failed unlocking\n");
		exit(-1);
	}

	delta = end - start;
	if (!tdata->total || tdata->max < delta) {
		tracefs_printf(NULL, "New max: %lld\n", delta);
		tdata->max = delta;
	}

	if (!tdata->total || tdata->min > delta)
		tdata->min = delta;

	tdata->total += delta;
	tdata->x_count++;
}

static void *busy_thread(void *d)
{
	struct data *data = d;
	int i;

	nice(busy_prio);

	while (!data->done) {
		for (i = 0; i < 100; i++)
			wmb();
		do_sleep(10);
		rmb();
	}
	return NULL;
}

static void *run_thread(void *d)
{
	struct thread_data *tdata = d;
	struct data *data = tdata->data;

	init_extend_map();

	nice(lock_prio);

	pthread_barrier_wait(&pbarrier);

	while (!data->done) {
		grab_lock(tdata, data);
		/* Make slighty different waits */
		/* 100us + cpu * 27us */
		do_sleep(100 + tdata->cpu * 27);
		rmb();
	}
	return NULL;
}

int main (int argc, char **argv)
{
	unsigned long long total_wait = 0;
	unsigned long long total_held = 0;
	unsigned long long total_contention = 0;
	unsigned long long total_extended = 0;
	unsigned long long max_wait = 0;
	unsigned long long max = 0;
	unsigned long long secs;
	unsigned long long avg_wait;
	unsigned long long avg_secs;
	unsigned long long avg_held;
	unsigned long long avg_held_secs;
	unsigned long long total_count = 0;
	bool verbose = false;
	pthread_t *threads;
	cpu_set_t *save_affinity;
	cpu_set_t *set_affinity;
	size_t cpu_size;
	struct data data;
	int cpus;
	int ch;
	int i;

	while ((ch = getopt(argc, argv, "dwv")) >= 0) {
		switch (ch) {
			case 'd':
				no_rseq = true;
				break;
			case 'w':
				extend_wait = true;
				break;
			case 'v':
				verbose = true;
				break;
			default:
				fprintf(stderr, "usage: extend-sched [-d|-w|-v]\n"
						"  -d: disable rseq\n"
						"  -w: extend while trying to get lock\n"
						"  -v: verbose output\n");
				exit(-1);
		}
	}
	memset(&data, 0, sizeof(data));

	cpus = sysconf(_SC_NPROCESSORS_CONF);

	cpu_size = CPU_ALLOC_SIZE(cpus);
	save_affinity = CPU_ALLOC(cpus);
	set_affinity = CPU_ALLOC(cpus);
	if (!save_affinity || !set_affinity) {
		perror("Allocating CPU sets");
		exit(-1);
	}
	if (sched_getaffinity(0, cpu_size, save_affinity) < 0) {
		perror("Getting affinity");
		exit(-1);
	}

	/* Create two threads for ever CPU. One grabbing the lock, and a busy task */
	threads = calloc(cpus * (NR_BUSY_THREADS + 1), sizeof(*threads));
	if (!threads) {
		perror("threads");
		exit(-1);
	}

	/* Allocate the data for the lock grabbers */
	data.tdata = calloc(cpus, sizeof(*data.tdata));
	if (!data.tdata) {
		perror("Allocating tdata");
		exit(-1);
	}

	tracefs_print_init(NULL);
	pthread_barrier_init(&pbarrier, NULL, cpus + 1);

	/* Save current affinity */
	for (i = 0; i < cpus; i++) {
		int ret;

		/* Set the affinity to this CPU as threads will inherit it */
		CPU_ZERO_S(cpu_size, set_affinity);
		CPU_SET_S(i, cpu_size, set_affinity);
		if (sched_setaffinity(0, cpu_size, set_affinity) < 0) {
			perror("Setting affinity");
			fprintf(stderr, " Setting cpu %d\n", i);
			exit(-1);
		}

		data.tdata[i].data = &data;
		data.tdata[i].cpu = i;

		ret = pthread_create(&threads[i], NULL, run_thread, &data.tdata[i]);
		if (ret < 0) {
			perror("creating lock threads");
			exit(-1);
		}

		for (int n = 1; n <= NR_BUSY_THREADS; n++) {
			ret = pthread_create(&threads[i + cpus * n], NULL, busy_thread, &data);
			if (ret < 0) {
				perror("creating busy threads");
				exit(-1);
			}
		}
	}

	if (sched_setaffinity(0, cpu_size, save_affinity) < 0) {
		perror("Setting saved affinity");
		exit(-1);
	}

	pthread_barrier_wait(&pbarrier);
	sleep(5);

	printf("Finish up\n");
	data.done = true;
	wmb();

	for (i = 0; i < cpus; i++) {
		for (int n = 1; n <= NR_BUSY_THREADS; n++)
			pthread_join(threads[i + cpus * n], NULL);
	}

	for (i = 0; i < cpus; i++) {
		pthread_join(threads[i], NULL);
		if (verbose) {
			printf("thread %i:\n", i);
			printf("   count:\t%lld\n", data.tdata[i].x_count);
			printf("   total:\t%lld\n", data.tdata[i].total);
			printf("     max:\t%lld\n", data.tdata[i].max);
			printf("     min:\t%lld\n", data.tdata[i].min);
			printf("   total wait:\t%lld\n", data.tdata[i].total_wait);
			printf("     max wait:\t%lld\n", data.tdata[i].max_wait);
			printf("     min wait:\t%lld\n", data.tdata[i].min_wait);
			printf("   contention:\t%lld\n", data.tdata[i].contention);
			printf("     extended:\t%lld\n", data.tdata[i].extended);
		}
		total_count += data.tdata[i].x_count;
		total_wait += data.tdata[i].total_wait;
		total_contention += data.tdata[i].contention;
		total_held += data.tdata[i].total;
		total_extended += data.tdata[i].extended;
		if (data.tdata[i].max_wait > max_wait)
			max_wait = data.tdata[i].max_wait;
		if (data.tdata[i].max > max)
			max = data.tdata[i].max;
	}

	secs = usec2sec(total_wait);
	avg_wait = total_count ? total_wait / total_count : 0;
	avg_secs = usec2sec(avg_wait);
	avg_held = total_count ? total_held / total_count : 0;
	avg_held_secs = usec2sec(avg_held);

	printf("Ran for %lld times\n", data.x);
	printf("Total wait time: %llu.%06llu  (avg: %llu.%06llu)\n", secs, total_wait - sec2usec(secs),
				avg_secs, avg_wait - sec2usec(avg_secs));
	printf("Total contetion: %lld\n", total_contention);
	printf("Total extended: %lld\n", total_extended);
	printf("      max wait: %lld\n", max_wait);
	printf("           max: %lld (avg: %llu.%06llu)\n", max, avg_held_secs, avg_held - sec2usec(avg_held_secs));
	return 0;
}