Re: io_uring_prep_openat_direct() and link/drain

[Miklos Szeredi <miklos@szeredi.hu>]

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On Fri, 1 Apr 2022 at 17:36, Jens Axboe <axboe@kernel.dk> wrote:

> I take it you're continually reusing those slots?

Yes.

>  If you have a test
> case that'd be ideal. Agree that it sounds like we just need an
> appropriate breather to allow fput/task_work to run. Or it could be the
> deferral free of the fixed slot.

Adding a breather could make the worst case latency be large.  I think
doing the fput synchronously would be better in general.

I test this on an VM with 8G of memory and run the following:

./forkbomb 14 &
# wait till 16k processes are forked
for i in `seq 1 100`; do ./procreads u; done

You can compare performance with plain reads (./procreads p), the
other tests don't work on public kernels.

Thanks,
Miklos

[-- Attachment #2: procreads.c --]
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#define _GNU_SOURCE

#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <stdlib.h>
#include <dirent.h>
#include <unistd.h>
#include <err.h>
#include "liburing.h"

#define CHECK_NEGERR(_expr) \
	({ typeof(_expr) _ret = (_expr); if (_ret < 0) { errno = -_ret; err(1, #_expr); } _ret; })
#define CHECK_NULL(_expr) \
	({ typeof(_expr) _ret = (_expr); if (_ret == NULL) { errx(1, #_expr " returned NULL"); } _ret; })
#define CHECK_ERR(_expr) \
	({ typeof(_expr) _ret = (_expr); if (_ret == -1) { err(1, #_expr); } _ret; })


struct name_val {
	char *name;		/* in */
	struct iovec value_in;	/* in */
	struct iovec value_out;	/* out */
	uint32_t error;		/* out */
	uint32_t reserved;
};

static bool debug;
static const char *proc_list[] = { "stat", "status", "cmdline", "cgroup" };
#define proc_num (sizeof(proc_list)/sizeof(proc_list[0]))
#define batch 10

int getvalues(int dfd, const char *path, struct name_val *vec, size_t num,
	      unsigned int flags)
{
	return syscall(451, dfd, path, vec, num, flags);
}

static void print_val(const char *name, struct name_val *nv)
{
	const char *s = nv->value_out.iov_base;
	size_t len = nv->value_out.iov_len;
	const size_t prmax = 40;
	int prlen = len < prmax ? len : prmax;
	const char *cont = len < prmax ? "" : "...";

	if (nv->error)
		printf("/proc/%s/%s = ERROR %s (%i)\n",
		       name, nv->name, strerror(nv->error), nv->error);
	else if (debug)
		printf("/proc/%s/%s = \"%.*s\"%s (len=%zi)\n",
		       name, nv->name, prlen, s, cont, len);
}

static void print_values(const char *name, struct name_val *vec, size_t num,
			 ssize_t ret)
{
	int i;

	if (ret < 0) {
		errno = -ret; warn("getvalues failed");
	} else {
		if ((size_t) ret < num)
			warnx("%zi values read out of %zi", ret, num);
		for (i = 0; i < ret; i++)
			print_val(name, &vec[i]);
	}
}

static ssize_t readfile_plain(int dfd, const char *path, char *buf, size_t size)
{
	int fd;
	ssize_t ret;

	fd = openat(dfd, path, O_RDONLY);
	if (fd == -1)
		return -errno;

	ret = read(fd, buf, size);
	if (ret == -1)
		ret = -errno;
	else if ((size_t) ret == size)
		ret = -EOVERFLOW;

	close(fd);

	return ret;
}

static int readfiles_plain(int dfd, const char *path, struct name_val *vec,
			   size_t num, int mode)
{
	struct name_val *nv;
	ssize_t ret;
	size_t i;

	if (path[0])
		dfd = CHECK_ERR(openat(dfd, path, O_PATH));

	for (i = 0; i < num; i++) {
		nv = &vec[i];
		if (mode) {
			CHECK_ERR(getvalues(dfd, "", nv, 1, mode == 2));
		} else {
			ret = readfile_plain(dfd, nv->name,
					     nv->value_in.iov_base,
					     nv->value_in.iov_len);
			if (ret < 0) {
				nv->error = -ret;
			} else {
				nv->error = 0;
				nv->value_out.iov_base = nv->value_in.iov_base;
				nv->value_out.iov_len = ret;
			}
		}
	}
	if (path[0])
		close(dfd);

	return num;
}

static int readfiles_uring(struct io_uring *ring, int dfd, const char *path,
			   struct name_val *vec, size_t num)
{
	struct io_uring_sqe *sqe;
	struct io_uring_cqe *cqe;
	size_t slot;
	int ret, i;
	static int seq = 1;
	struct name_val *nv;

	if (path[0])
		dfd = CHECK_ERR(openat(dfd, path, O_PATH));

	for (slot = 0; slot < num; slot++) {
		nv = &vec[slot];
		sqe = io_uring_get_sqe(ring);
		io_uring_prep_openat_direct(sqe, dfd, nv->name, O_RDONLY, 0,
					    slot);
		sqe->flags = IOSQE_IO_LINK | IOSQE_CQE_SKIP_SUCCESS;
		sqe->user_data = seq + slot * 2;

		sqe = io_uring_get_sqe(ring);
		io_uring_prep_read(sqe, slot, nv->value_in.iov_base,
				   nv->value_in.iov_len, 0);
		sqe->flags = IOSQE_FIXED_FILE;
		sqe->user_data = seq + slot * 2 + 1;
	}

	ret = CHECK_NEGERR(io_uring_submit_and_wait(ring, num));
	ret /= 2;
	for (i = 0; i < ret; i++) {
		CHECK_NEGERR(io_uring_wait_cqe(ring, &cqe));
		slot = (cqe->user_data - seq) / 2;
		nv = &vec[slot];
		if (cqe->res < 0) {
			nv->error = -cqe->res;
		} else if ((size_t) cqe->res < nv->value_in.iov_len) {
			nv->error = 0;
			nv->value_out.iov_base = nv->value_in.iov_base;
			nv->value_out.iov_len = cqe->res;
		} else {
			nv->error = EOVERFLOW;
		}
		io_uring_cqe_seen(ring, cqe);
	}
	seq += 2 * num;
	if (path[0])
		close(dfd);

	return ret;
}

static const char *next_name(DIR *dp)
{
	const char *name;
	struct dirent *de;

	while ((de = readdir(dp))) {
		name = de->d_name;
		if (name[0] > '0' && name[0] <= '9')
			return name;
	}
	return NULL;
}

static size_t next_batch(DIR *dp, struct name_val *vec, size_t num,
			 const char **namep)
{
	const char *name;
	size_t i;

	if (batch == 1) {
		name = next_name(dp);
		if (!name)
			return 0;
		*namep = name;
		return 1;
	}

	*namep = "";
	for (i = 0; i < num; i++) {
		if (i % proc_num == 0 && (name = next_name(dp)) == NULL)
			break;
		free(vec[i].name);
		vec[i].name = CHECK_NULL(malloc(128));
		sprintf(vec[i].name, "%s/%s", name, proc_list[i % proc_num]);
	}
	return i;
}

static void test_uring(DIR *dp, struct name_val *vec, size_t num)
{
	int fds[proc_num * batch];
	const size_t numslots = sizeof(fds)/sizeof(fds[0]);
	struct io_uring ring;
	const char *name;
	ssize_t ret;

	memset(fds, -1, sizeof(fds));
	CHECK_NEGERR(io_uring_queue_init(num * 2, &ring, 0));
	CHECK_NEGERR(io_uring_register_files(&ring, fds, numslots));

	while ((num = next_batch(dp, vec, num, &name))) {
		ret = readfiles_uring(&ring, dirfd(dp), name, vec, num);
		print_values(name, vec, num, ret);
	}
	io_uring_queue_exit(&ring);
}

static void test_plain(DIR *dp, struct name_val *vec, size_t num, int mode)
{
	const char *name;
	ssize_t ret;

	while ((num = next_batch(dp, vec, num, &name))) {
		ret = readfiles_plain(dirfd(dp), name, vec, num, mode);
		print_values(name, vec, num, ret);
	}
}

static void test_values(DIR *dp, struct name_val *vec, size_t num, bool rf)
{
	const char *name;
	ssize_t ret;

	while ((num = next_batch(dp, vec, num, &name))) {
		ret = getvalues(dirfd(dp), name, vec, num, rf);
		print_values(name, vec, num, ret);
	}
}

int main(int argc, char *argv[])
{
	const size_t num = proc_num * batch;
	char buf[num][4096];
	struct name_val vec[num];
	DIR *dp;
	size_t i;
	char type = 'p';

	if (argc > 1)
		type = argv[1][0];

	if (argc > 2)
		debug = true;

	for (i = 0; i < num; i++) {
		vec[i].value_in.iov_base = (type != 'w' || !i) ? buf[i] : NULL;
		vec[i].value_in.iov_len = sizeof(buf[i]);
	}

	dp = CHECK_NULL(opendir("/proc"));
	switch (type) {
	case 'p':
		test_plain(dp, vec, num, 0);
		break;
	case 'r':
		test_plain(dp, vec, num, 1);
		break;
	case 's':
		test_plain(dp, vec, num, 2);
		break;
	case 'u':
		test_uring(dp, vec, num);
		break;
	case 'w':
		vec[0].value_in.iov_len = sizeof(buf[0]) * num;
		/* fallthrough */
	case 'v':
	case 'z':
		test_values(dp, vec, num, type == 'z');
		break;
	}
	closedir(dp);
	return 0;
}

[-- Attachment #3: forkbomb.c --]
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#include <unistd.h>
#include <stdio.h>
#include <err.h>
#include <pthread.h>
#include <stdlib.h>

static void *run(void *)
{
	sleep(1000);
	return NULL;
}

int main(int argc, char *argv[])
{
	int pid, level, i;
	pthread_t thr;
	int maxlevel = atoi(argv[1]);

	for (level = 0; level < maxlevel; level++) {
		pid = fork();
		if (pid == -1)
			err(1, "fork");

		fprintf(stderr, ".");
		#if 0
		if (pid == 0) {
			for (i = 0; i < 4; i++)
				pthread_create(&thr, NULL, run, NULL);

		}
		#endif
	}
	sleep(1000);
}