Re: [PATCH 4/4] io_uring: optimise compl locking for non-shared rings

[Pavel Begunkov <asml.silence@gmail.com>]

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On 3/18/22 15:21, Jens Axboe wrote:
> On 3/18/22 9:13 AM, Pavel Begunkov wrote:
>> On 3/18/22 14:54, Jens Axboe wrote:
>>> On 3/18/22 7:52 AM, Pavel Begunkov wrote:
>>>> When only one task submits requests, most of CQEs are expected to be
>>>> filled from that task context so we have natural serialisation. That
>>>> would mean that in those cases we don't need spinlocking around CQE
>>>> posting. One downside is that it also mean that io-wq workers can't emit
>>>> CQEs directly but should do it through the original task context using
>>>> task_works. That may hurt latency and performance and might matter much
>>>> to some workloads, but it's not a huge deal in general as io-wq is a
>>>> slow path and there is some additional merit from tw completion
>>>> batching.
>>>
>>> Not too worried about io-wq task_work for cq filling, it is the slower
>>> path after all. And I think we can get away with doing notifications as
>>> it's just for CQ filling. If the task is currently waiting in
>>> cqring_wait, then it'll get woken anyway and it will process task work.
>>> If it's in userspace, it doesn't need a notification. That should make
>>> it somewhat lighter than requiring using TIF_NOTIFY_SIGNAL for that.
>>>
>>>> The feature should be opted-in by the userspace by setting a new
>>>> IORING_SETUP_PRIVATE_CQ flag. It doesn't work with IOPOLL, and also for
>>>> now only the task that created a ring can submit requests to it.
>>>
>>> I know this is a WIP, but why do we need CQ_PRIVATE? And this needs to
>>
>> One reason is because of the io-wq -> tw punting, which is not optimal
>> for e.g. active users of IOSQE_ASYNC. The second is because the
>> fundamental requirement is that only one task should be submitting
>> requests. Was thinking about automating it, e.g. when we register
>> a second tctx we go through a slow path waiting for all current tw
>> to complete and then removing an internal and not userspace visible
>> CQ_PRIVATE flag.
> 
> Was thinking something along those lines too. The alternative is setting
> up the ring with SETUP_SINGLE_ISSUER or something like that, having the
> application tell us that it is a single issuer and no submits are
> shared across threads. Serves the same kind of purpose as CQ_PRIVATE,
> but enables us to simply fail things if the task violates those
> constraints. Would also be a better name I believe as it might enable
> further optimizations in the future, like for example the mutex
> reduction for submits.

That's exactly what it is, including the failing part. And I like
your name better, will take it

>> Also, as SQPOLL task is by definition the only one submitting SQEs,
>> was thinking about enabling it by default for them, but didn't do
>> because of the io-wq / IOSQE_ASYNC.
> 
> Gotcha.
> 
>>> work with registered files (and ring fd) as that is probably a bigger
>>> win than skipping the completion_lock if you're not shared anyway.
>>
>> It does work with fixed/registered files and registered io_uring fds.
> 
> t/io_uring fails for me with registered files or rings, getting EINVAL.
> Might be user error, but that's simply just setting CQ_PRIVATE for
> setup.

One thing I changed in the tool is that the ring should be created
by the submitter task, so move setup_ring into the submitter thread.
Plan to get rid of this restriction though.

Weird that it works only for you only without reg files/rings, will
take a look.

Attached io_uring.c that I used, it's based on some old version,
so do_nop can't be set in argv but should turned in the source code.
IORING_ENTER_REGISTERED_RING is always enabled.

>> In regards of "a bigger win", probably in many cases, but if you submit
>> a good batch at once, and completion tw batching doesn't kick in (e.g.
>> direct bdev read of not too high intensity), it might save
>> N spinlock/unlock when registered ring fd would kill only one pair of
>> fdget/fdput.
> 
> Definitely, various cases where one would be a bigger win than the
> other, agree on that. But let's just ensure that both work together :-)

-- 
Pavel Begunkov

[-- Attachment #2: io_uring.c --]
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#include <stdio.h>
#include <errno.h>
#include <assert.h>
#include <stdlib.h>
#include <stddef.h>
#include <signal.h>
#include <inttypes.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <sys/resource.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <linux/fs.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <pthread.h>
#include <sched.h>

#include "../arch/arch.h"
#include "../lib/types.h"
#include "../os/linux/io_uring.h"

#define min(a, b)		((a < b) ? (a) : (b))

struct io_sq_ring {
	unsigned *head;
	unsigned *tail;
	unsigned *ring_mask;
	unsigned *ring_entries;
	unsigned *flags;
	unsigned *array;
};


#define IORING_SETUP_PRIVATE_CQ	(1U << 8)

enum {
	IORING_REGISTER_RING_FDS		= 20,
};

#define IORING_ENTER_REGISTERED_RING	(1U << 4)

struct io_cq_ring {
	unsigned *head;
	unsigned *tail;
	unsigned *ring_mask;
	unsigned *ring_entries;
	struct io_uring_cqe *cqes;
};

#define DEPTH			128
#define BATCH_SUBMIT		32
#define BATCH_COMPLETE		32
#define BS			4096

#define MAX_FDS			16

static unsigned sq_ring_mask, cq_ring_mask;

struct file {
	unsigned long max_blocks;
	unsigned pending_ios;
	int real_fd;
	int fixed_fd;
};

struct submitter {
	pthread_t thread;
	int ring_fd;
	struct io_sq_ring sq_ring;
	struct io_uring_sqe *sqes;
	struct io_cq_ring cq_ring;
	int inflight;
	unsigned long reaps;
	unsigned long done;
	unsigned long calls;
	volatile int finish;

	__s32 *fds;

	struct file files[MAX_FDS];
	unsigned nr_files;
	unsigned cur_file;
	struct iovec iovecs[];
};

static struct submitter *submitter;
static volatile int finish;

static int depth = DEPTH;
static int batch_submit = BATCH_SUBMIT;
static int batch_complete = BATCH_COMPLETE;
static int bs = BS;
static int polled = 1;		/* use IO polling */
static int fixedbufs = 1;	/* use fixed user buffers */
static int register_files = 1;	/* use fixed files */
static int buffered = 0;	/* use buffered IO, not O_DIRECT */
static int sq_thread_poll = 0;	/* use kernel submission/poller thread */
static int sq_thread_cpu = -1;	/* pin above thread to this CPU */
static int do_nop = 1;		/* no-op SQ ring commands */

struct io_uring_rsrc_update {
	__u32 offset;
	__u32 resv;
	__aligned_u64 data;
};

static int vectored = 1;

static int setup_ring(struct submitter *s);

static int io_uring_register_buffers(struct submitter *s)
{
	if (do_nop)
		return 0;

	return syscall(__NR_io_uring_register, s->ring_fd,
			IORING_REGISTER_BUFFERS, s->iovecs, depth);
}

static int io_uring_register_io_uring_fd(struct submitter *s)
{
	struct io_uring_rsrc_update up = {};

	up.offset = 0;
	up.data = s->ring_fd;

	return syscall(__NR_io_uring_register, s->ring_fd,
			IORING_REGISTER_RING_FDS, &up, 1);
}

static int io_uring_register_files(struct submitter *s)
{
	int i;

	if (do_nop)
		return 0;

	s->fds = calloc(s->nr_files, sizeof(__s32));
	for (i = 0; i < s->nr_files; i++) {
		s->fds[i] = s->files[i].real_fd;
		s->files[i].fixed_fd = i;
	}

	return syscall(__NR_io_uring_register, s->ring_fd,
			IORING_REGISTER_FILES, s->fds, s->nr_files);
}

static int io_uring_setup(unsigned entries, struct io_uring_params *p)
{
	return syscall(__NR_io_uring_setup, entries, p);
}

static void io_uring_probe(int fd)
{
	struct io_uring_probe *p;
	int ret;

	p = malloc(sizeof(*p) + 256 * sizeof(struct io_uring_probe_op));
	if (!p)
		return;

	memset(p, 0, sizeof(*p) + 256 * sizeof(struct io_uring_probe_op));
	ret = syscall(__NR_io_uring_register, fd, IORING_REGISTER_PROBE, p, 256);
	if (ret < 0)
		goto out;

	if (IORING_OP_READ > p->ops_len)
		goto out;

	if ((p->ops[IORING_OP_READ].flags & IO_URING_OP_SUPPORTED))
		vectored = 0;
out:
	free(p);
}

static int io_uring_enter(struct submitter *s, unsigned int to_submit,
			  unsigned int min_complete, unsigned int flags)
{
	return syscall(__NR_io_uring_enter, 0, to_submit, min_complete,
			flags | IORING_ENTER_REGISTERED_RING, NULL, 0);
}

#ifndef CONFIG_HAVE_GETTID
static int gettid(void)
{
	return syscall(__NR_gettid);
}
#endif

static unsigned file_depth(struct submitter *s)
{
	return (depth + s->nr_files - 1) / s->nr_files;
}

static void init_io(struct submitter *s, unsigned index)
{
	struct io_uring_sqe *sqe = &s->sqes[index];
	unsigned long offset;
	struct file *f;
	long r;

	if (do_nop) {
		sqe->opcode = IORING_OP_NOP;
		return;
	}

	if (s->nr_files == 1) {
		f = &s->files[0];
	} else {
		f = &s->files[s->cur_file];
		if (f->pending_ios >= file_depth(s)) {
			s->cur_file++;
			if (s->cur_file == s->nr_files)
				s->cur_file = 0;
			f = &s->files[s->cur_file];
		}
	}
	f->pending_ios++;

	r = lrand48();
	offset = (r % (f->max_blocks - 1)) * bs;

	if (register_files) {
		sqe->flags = IOSQE_FIXED_FILE;
		sqe->fd = f->fixed_fd;
	} else {
		sqe->flags = 0;
		sqe->fd = f->real_fd;
	}
	if (fixedbufs) {
		sqe->opcode = IORING_OP_READ_FIXED;
		sqe->addr = (unsigned long) s->iovecs[index].iov_base;
		sqe->len = bs;
		sqe->buf_index = index;
	} else if (!vectored) {
		sqe->opcode = IORING_OP_READ;
		sqe->addr = (unsigned long) s->iovecs[index].iov_base;
		sqe->len = bs;
		sqe->buf_index = 0;
	} else {
		sqe->opcode = IORING_OP_READV;
		sqe->addr = (unsigned long) &s->iovecs[index];
		sqe->len = 1;
		sqe->buf_index = 0;
	}
	sqe->ioprio = 0;
	sqe->off = offset;
	sqe->user_data = (unsigned long) f;
}

static int prep_more_ios(struct submitter *s, int max_ios)
{
	struct io_sq_ring *ring = &s->sq_ring;
	unsigned index, tail, next_tail, prepped = 0;

	next_tail = tail = *ring->tail;
	do {
		next_tail++;
		if (next_tail == atomic_load_acquire(ring->head))
			break;

		index = tail & sq_ring_mask;
		init_io(s, index);
		ring->array[index] = index;
		prepped++;
		tail = next_tail;
	} while (prepped < max_ios);

	if (prepped)
		atomic_store_release(ring->tail, tail);
	return prepped;
}

static int get_file_size(struct file *f)
{
	struct stat st;

	if (fstat(f->real_fd, &st) < 0)
		return -1;
	if (S_ISBLK(st.st_mode)) {
		unsigned long long bytes;

		if (ioctl(f->real_fd, BLKGETSIZE64, &bytes) != 0)
			return -1;

		f->max_blocks = bytes / bs;
		return 0;
	} else if (S_ISREG(st.st_mode)) {
		f->max_blocks = st.st_size / bs;
		return 0;
	}

	return -1;
}

static int reap_events(struct submitter *s)
{
	struct io_cq_ring *ring = &s->cq_ring;
	struct io_uring_cqe *cqe;
	unsigned head, reaped = 0;

	head = *ring->head;
	do {
		struct file *f;

		read_barrier();
		if (head == atomic_load_acquire(ring->tail))
			break;
		cqe = &ring->cqes[head & cq_ring_mask];
		if (!do_nop) {
			f = (struct file *) (uintptr_t) cqe->user_data;
			f->pending_ios--;
			if (cqe->res != bs) {
				printf("io: unexpected ret=%d\n", cqe->res);
				if (polled && cqe->res == -EOPNOTSUPP)
					printf("Your filesystem/driver/kernel doesn't support polled IO\n");
				return -1;
			}
		}
		reaped++;
		head++;
	} while (1);

	if (reaped) {
		s->inflight -= reaped;
		atomic_store_release(ring->head, head);
	}
	return reaped;
}

static void *submitter_fn(void *data)
{
	struct submitter *s = data;
	struct io_sq_ring *ring = &s->sq_ring;
	int ret, prepped;

	ret = setup_ring(s);
	if (ret) {
		printf("ring setup failed: %s, %d\n", strerror(errno), ret);
		return NULL;
	}

	printf("submitter=%d\n", gettid());

	srand48(pthread_self());

	prepped = 0;
	do {
		int to_wait, to_submit, this_reap, to_prep;
		unsigned ring_flags = 0;

		if (!prepped && s->inflight < depth) {
			to_prep = min(depth - s->inflight, batch_submit);
			prepped = prep_more_ios(s, to_prep);
		}
		s->inflight += prepped;
submit_more:
		to_submit = prepped;
submit:
		if (to_submit && (s->inflight + to_submit <= depth))
			to_wait = 0;
		else
			to_wait = min(s->inflight + to_submit, batch_complete);

		/*
		 * Only need to call io_uring_enter if we're not using SQ thread
		 * poll, or if IORING_SQ_NEED_WAKEUP is set.
		 */
		if (sq_thread_poll)
			ring_flags = atomic_load_acquire(ring->flags);
		if (!sq_thread_poll || ring_flags & IORING_SQ_NEED_WAKEUP) {
			unsigned flags = 0;

			if (to_wait)
				flags = IORING_ENTER_GETEVENTS;
			if (ring_flags & IORING_SQ_NEED_WAKEUP)
				flags |= IORING_ENTER_SQ_WAKEUP;
			ret = io_uring_enter(s, to_submit, to_wait, flags);
			s->calls++;
		} else {
			/* for SQPOLL, we submitted it all effectively */
			ret = to_submit;
		}

		/*
		 * For non SQ thread poll, we already got the events we needed
		 * through the io_uring_enter() above. For SQ thread poll, we
		 * need to loop here until we find enough events.
		 */
		this_reap = 0;
		do {
			int r;
			r = reap_events(s);
			if (r == -1) {
				s->finish = 1;
				break;
			} else if (r > 0)
				this_reap += r;
		} while (sq_thread_poll && this_reap < to_wait);
		s->reaps += this_reap;

		if (ret >= 0) {
			if (!ret) {
				to_submit = 0;
				if (s->inflight)
					goto submit;
				continue;
			} else if (ret < to_submit) {
				int diff = to_submit - ret;

				s->done += ret;
				prepped -= diff;
				goto submit_more;
			}
			s->done += ret;
			prepped = 0;
			continue;
		} else if (ret < 0) {
			if (errno == EAGAIN) {
				if (s->finish)
					break;
				if (this_reap)
					goto submit;
				to_submit = 0;
				goto submit;
			}
			printf("io_submit: %s\n", strerror(errno));
			break;
		}
	} while (!s->finish);

	finish = 1;
	return NULL;
}

static void sig_int(int sig)
{
	printf("Exiting on signal %d\n", sig);
	submitter->finish = 1;
	finish = 1;
}

static void arm_sig_int(void)
{
	struct sigaction act;

	memset(&act, 0, sizeof(act));
	act.sa_handler = sig_int;
	act.sa_flags = SA_RESTART;
	sigaction(SIGINT, &act, NULL);
}

static int setup_ring(struct submitter *s)
{
	struct io_sq_ring *sring = &s->sq_ring;
	struct io_cq_ring *cring = &s->cq_ring;
	struct io_uring_params p;
	int ret, fd;
	void *ptr;

	memset(&p, 0, sizeof(p));

	if (polled && !do_nop)
		p.flags |= IORING_SETUP_IOPOLL;
	if (sq_thread_poll) {
		p.flags |= IORING_SETUP_SQPOLL;
		if (sq_thread_cpu != -1) {
			p.flags |= IORING_SETUP_SQ_AFF;
			p.sq_thread_cpu = sq_thread_cpu;
		}
	}
	p.flags |= IORING_SETUP_PRIVATE_CQ;

	fd = io_uring_setup(depth, &p);
	if (fd < 0) {
		perror("io_uring_setup");
		return 1;
	}
	s->ring_fd = fd;

	io_uring_probe(fd);

	ret = io_uring_register_io_uring_fd(s);
	if (ret < 0) {
		perror("io_uring_register_io_uring_fd");
		return 1;
	}

	if (fixedbufs) {
		ret = io_uring_register_buffers(s);
		if (ret < 0) {
			perror("io_uring_register_buffers");
			return 1;
		}
	}

	if (register_files) {
		ret = io_uring_register_files(s);
		if (ret < 0) {
			perror("io_uring_register_files");
			return 1;
		}
	}

	ptr = mmap(0, p.sq_off.array + p.sq_entries * sizeof(__u32),
			PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE, fd,
			IORING_OFF_SQ_RING);
	printf("sq_ring ptr = 0x%p\n", ptr);
	sring->head = ptr + p.sq_off.head;
	sring->tail = ptr + p.sq_off.tail;
	sring->ring_mask = ptr + p.sq_off.ring_mask;
	sring->ring_entries = ptr + p.sq_off.ring_entries;
	sring->flags = ptr + p.sq_off.flags;
	sring->array = ptr + p.sq_off.array;
	sq_ring_mask = *sring->ring_mask;

	s->sqes = mmap(0, p.sq_entries * sizeof(struct io_uring_sqe),
			PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE, fd,
			IORING_OFF_SQES);
	printf("sqes ptr    = 0x%p\n", s->sqes);

	ptr = mmap(0, p.cq_off.cqes + p.cq_entries * sizeof(struct io_uring_cqe),
			PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE, fd,
			IORING_OFF_CQ_RING);
	printf("cq_ring ptr = 0x%p\n", ptr);
	cring->head = ptr + p.cq_off.head;
	cring->tail = ptr + p.cq_off.tail;
	cring->ring_mask = ptr + p.cq_off.ring_mask;
	cring->ring_entries = ptr + p.cq_off.ring_entries;
	cring->cqes = ptr + p.cq_off.cqes;
	cq_ring_mask = *cring->ring_mask;
	return 0;
}

static void file_depths(char *buf)
{
	struct submitter *s = submitter;
	char *p;
	int i;

	buf[0] = '\0';
	p = buf;
	for (i = 0; i < s->nr_files; i++) {
		struct file *f = &s->files[i];

		if (i + 1 == s->nr_files)
			p += sprintf(p, "%d", f->pending_ios);
		else
			p += sprintf(p, "%d, ", f->pending_ios);
	}
}

static void usage(char *argv)
{
	printf("%s [options] -- [filenames]\n"
		" -d <int> : IO Depth, default %d\n"
		" -s <int> : Batch submit, default %d\n"
		" -c <int> : Batch complete, default %d\n"
		" -b <int> : Block size, default %d\n"
		" -p <bool> : Polled IO, default %d\n",
		argv, DEPTH, BATCH_SUBMIT, BATCH_COMPLETE, BS, polled);
	exit(0);
}

int main(int argc, char *argv[])
{
	struct submitter *s;
	unsigned long done, calls, reap;
	int i, flags, fd, opt;
	char *fdepths;
	void *ret;

	if (!do_nop && argc < 2) {
		printf("%s: filename [options]\n", argv[0]);
		return 1;
	}

	while ((opt = getopt(argc, argv, "d:s:c:b:p:B:F:h?")) != -1) {
		switch (opt) {
		case 'd':
			depth = atoi(optarg);
			break;
		case 's':
			batch_submit = atoi(optarg);
			break;
		case 'c':
			batch_complete = atoi(optarg);
			break;
		case 'b':
			bs = atoi(optarg);
			break;
		case 'p':
			polled = !!atoi(optarg);
			break;
		case 'B':
			fixedbufs = !!atoi(optarg);
			break;
		case 'F':
			register_files = !!atoi(optarg);
			break;
		case 'h':
		case '?':
		default:
			usage(argv[0]);
			break;
		}
	}

	submitter = malloc(sizeof(*submitter) + depth * sizeof(struct iovec));
	memset(submitter, 0, sizeof(*submitter) + depth * sizeof(struct iovec));
	s = submitter;

	flags = O_RDONLY | O_NOATIME;
	if (!buffered)
		flags |= O_DIRECT;

	i = optind;
	while (!do_nop && i < argc) {
		struct file *f;

		if (s->nr_files == MAX_FDS) {
			printf("Max number of files (%d) reached\n", MAX_FDS);
			break;
		}
		fd = open(argv[i], flags);
		if (fd < 0) {
			perror("open");
			return 1;
		}

		f = &s->files[s->nr_files];
		f->real_fd = fd;
		if (get_file_size(f)) {
			printf("failed getting size of device/file\n");
			return 1;
		}
		if (f->max_blocks <= 1) {
			printf("Zero file/device size?\n");
			return 1;
		}
		f->max_blocks--;

		printf("Added file %s\n", argv[i]);
		s->nr_files++;
		i++;
	}

	if (fixedbufs) {
		struct rlimit rlim;

		rlim.rlim_cur = RLIM_INFINITY;
		rlim.rlim_max = RLIM_INFINITY;
		if (setrlimit(RLIMIT_MEMLOCK, &rlim) < 0) {
			perror("setrlimit");
			return 1;
		}
	}

	arm_sig_int();

	for (i = 0; i < depth; i++) {
		void *buf;

		if (posix_memalign(&buf, bs, bs)) {
			printf("failed alloc\n");
			return 1;
		}
		s->iovecs[i].iov_base = buf;
		s->iovecs[i].iov_len = bs;
	}

	printf("polled=%d, fixedbufs=%d, register_files=%d, buffered=%d", polled, fixedbufs, register_files, buffered);
	printf(" QD=%d, sq_ring=%d, cq_ring=%d\n", depth, 0, 0);

	pthread_create(&s->thread, NULL, submitter_fn, s);

	fdepths = malloc(8 * s->nr_files);
	reap = calls = done = 0;
	do {
		unsigned long this_done = 0;
		unsigned long this_reap = 0;
		unsigned long this_call = 0;
		unsigned long rpc = 0, ipc = 0;

		sleep(1);
		this_done += s->done;
		this_call += s->calls;
		this_reap += s->reaps;
		if (this_call - calls) {
			rpc = (this_done - done) / (this_call - calls);
			ipc = (this_reap - reap) / (this_call - calls);
		} else
			rpc = ipc = -1;
		file_depths(fdepths);
		printf("IOPS=%lu, IOS/call=%ld/%ld, inflight=%u (%s)\n",
				this_done - done, rpc, ipc, s->inflight,
				fdepths);
		done = this_done;
		calls = this_call;
		reap = this_reap;
	} while (!finish);

	pthread_join(s->thread, &ret);
	close(s->ring_fd);
	free(fdepths);
	return 0;
}