Re: [PATCH 13/18] io_uring: add file set registration
From: Jann Horn <jannh@google.com>
Date: 2019-01-30 01:29:35
Also in:
linux-api, linux-fsdevel
On Tue, Jan 29, 2019 at 8:27 PM Jens Axboe [off-list ref] wrote:
We normally have to fget/fput for each IO we do on a file. Even with the batching we do, the cost of the atomic inc/dec of the file usage count adds up. This adds IORING_REGISTER_FILES, and IORING_UNREGISTER_FILES opcodes for the io_uring_register(2) system call. The arguments passed in must be an array of __s32 holding file descriptors, and nr_args should hold the number of file descriptors the application wishes to pin for the duration of the io_uring context (or until IORING_UNREGISTER_FILES is called). When used, the application must set IOSQE_FIXED_FILE in the sqe->flags member. Then, instead of setting sqe->fd to the real fd, it sets sqe->fd to the index in the array passed in to IORING_REGISTER_FILES. Files are automatically unregistered when the io_uring context is torn down. An application need only unregister if it wishes to register a new set of fds.
Crazy idea:
Taking a step back, at a high level, basically this patch creates sort
of the same difference that you get when you compare the following
scenarios for normal multithreaded I/O in userspace:
===========================================================
~/tests/fdget_perf$ cat fdget_perf.c
#define _GNU_SOURCE
#include <sys/wait.h>
#include <sched.h>
#include <unistd.h>
#include <stdbool.h>
#include <string.h>
#include <err.h>
#include <signal.h>
#include <sys/eventfd.h>
#include <stdio.h>
// two different physical processors on my machine
#define CORE_A 0
#define CORE_B 14
static void pin_to_core(int coreid) {
cpu_set_t set;
CPU_ZERO(&set);
CPU_SET(coreid, &set);
if (sched_setaffinity(0, sizeof(cpu_set_t), &set))
err(1, "sched_setaffinity");
}
static int fd = -1;
static volatile int time_over = 0;
static void alarm_handler(int sig) { time_over = 1; }
static void run_stuff(void) {
unsigned long long iterations = 0;
if (signal(SIGALRM, alarm_handler) == SIG_ERR) err(1, "signal");
alarm(10);
while (1) {
uint64_t val;
read(fd, &val, sizeof(val));
if (time_over) {
printf("iterations = 0x%llx\n", iterations);
return;
}
iterations++;
}
}
static int child_fn(void *dummy) {
pin_to_core(CORE_B);
run_stuff();
return 0;
}
static char child_stack[1024*1024];
int main(int argc, char **argv) {
fd = eventfd(0, EFD_NONBLOCK);
if (fd == -1) err(1, "eventfd");
if (argc != 2) errx(1, "bad usage");
int flags = SIGCHLD;
if (strcmp(argv[1], "shared") == 0) {
flags |= CLONE_FILES;
} else if (strcmp(argv[1], "cloned") == 0) {
/* nothing */
} else {
errx(1, "bad usage");
}
pid_t child = clone(child_fn, child_stack+sizeof(child_stack), flags, NULL);
if (child == -1) err(1, "clone");
pin_to_core(CORE_A);
run_stuff();
int status;
if (wait(&status) != child) err(1, "wait");
return 0;
}
~/tests/fdget_perf$ gcc -Wall -o fdget_perf fdget_perf.c
~/tests/fdget_perf$ ./fdget_perf shared
iterations = 0x8d3010
iterations = 0x92d894
~/tests/fdget_perf$ ./fdget_perf cloned
iterations = 0xad3bbd
iterations = 0xb08838
~/tests/fdget_perf$ ./fdget_perf shared
iterations = 0x8cc340
iterations = 0x8e4e64
~/tests/fdget_perf$ ./fdget_perf cloned
iterations = 0xada5f3
iterations = 0xb04b6f
===========================================================
This kinda makes me wonder whether this is really something that
should be implemented specifically for the io_uring API, or whether it
would make sense to somehow handle part of this in the generic VFS
code and give the user the ability to prepare a new files_struct that
can then be transferred to the worker thread, or something like
that... I'm not sure whether there's a particularly clean way to do
that though.
Or perhaps you could add a userspace API for marking file descriptor
table entries as "has percpu refcounting" somehow, with one percpu
refcount per files_struct and one bit per fd, allocated when percpu
refcounting is activated for the files_struct the first time, or
something like that...
quoted hunk ↗ jump to hunk
Signed-off-by: Jens Axboe <axboe@kernel.dk> --- fs/io_uring.c | 138 +++++++++++++++++++++++++++++----- include/uapi/linux/io_uring.h | 9 ++- 2 files changed, 127 insertions(+), 20 deletions(-)diff --git a/fs/io_uring.c b/fs/io_uring.c index 17c869f3ea2f..13c3f8212815 100644 --- a/fs/io_uring.c +++ b/fs/io_uring.c@@ -100,6 +100,14 @@ struct io_ring_ctx { struct fasync_struct *cq_fasync; } ____cacheline_aligned_in_smp; + /* + * If used, fixed file set. Writers must ensure that ->refs is dead, + * readers must ensure that ->refs is alive as long as the file* is + * used. Only updated through io_uring_register(2). + */ + struct file **user_files; + unsigned nr_user_files; + /* if used, fixed mapped user buffers */ unsigned nr_user_bufs; struct io_mapped_ubuf *user_bufs;@@ -136,6 +144,7 @@ struct io_kiocb { unsigned int flags; #define REQ_F_FORCE_NONBLOCK 1 /* inline submission attempt */ #define REQ_F_IOPOLL_COMPLETED 2 /* polled IO has completed */ +#define REQ_F_FIXED_FILE 4 /* ctx owns file */ u64 user_data; u64 error;@@ -350,15 +359,17 @@ static void io_iopoll_complete(struct io_ring_ctx *ctx, unsigned int *nr_events, * Batched puts of the same file, to avoid dirtying the * file usage count multiple times, if avoidable. */ - if (!file) { - file = req->rw.ki_filp; - file_count = 1; - } else if (file == req->rw.ki_filp) { - file_count++; - } else { - fput_many(file, file_count); - file = req->rw.ki_filp; - file_count = 1; + if (!(req->flags & REQ_F_FIXED_FILE)) { + if (!file) { + file = req->rw.ki_filp; + file_count = 1; + } else if (file == req->rw.ki_filp) { + file_count++; + } else { + fput_many(file, file_count); + file = req->rw.ki_filp; + file_count = 1; + } } if (to_free == ARRAY_SIZE(reqs))@@ -491,13 +502,19 @@ static void kiocb_end_write(struct kiocb *kiocb) } } +static void io_fput(struct io_kiocb *req) +{ + if (!(req->flags & REQ_F_FIXED_FILE)) + fput(req->rw.ki_filp); +} + static void io_complete_rw(struct kiocb *kiocb, long res, long res2) { struct io_kiocb *req = container_of(kiocb, struct io_kiocb, rw); kiocb_end_write(kiocb); - fput(kiocb->ki_filp); + io_fput(req); io_cqring_add_event(req->ctx, req->user_data, res, 0); io_free_req(req); }@@ -596,11 +613,22 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe, { struct io_ring_ctx *ctx = req->ctx; struct kiocb *kiocb = &req->rw; - unsigned ioprio; + unsigned ioprio, flags; int fd, ret; + flags = READ_ONCE(sqe->flags); fd = READ_ONCE(sqe->fd); - kiocb->ki_filp = io_file_get(state, fd); + + if (flags & IOSQE_FIXED_FILE) { + if (unlikely(!ctx->user_files || + (unsigned) fd >= ctx->nr_user_files)) + return -EBADF; + kiocb->ki_filp = ctx->user_files[fd]; + req->flags |= REQ_F_FIXED_FILE; + } else { + kiocb->ki_filp = io_file_get(state, fd); + } + if (unlikely(!kiocb->ki_filp)) return -EBADF; kiocb->ki_pos = READ_ONCE(sqe->off);@@ -641,7 +669,8 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe, } return 0; out_fput: - io_file_put(state, kiocb->ki_filp); + if (!(flags & IOSQE_FIXED_FILE)) + io_file_put(state, kiocb->ki_filp); return ret; }@@ -765,7 +794,7 @@ static ssize_t io_read(struct io_kiocb *req, const struct io_uring_sqe *sqe, kfree(iovec); out_fput: if (unlikely(ret)) - fput(file); + io_fput(req); return ret; }@@ -820,7 +849,7 @@ static ssize_t io_write(struct io_kiocb *req, const struct io_uring_sqe *sqe, kfree(iovec); out_fput: if (unlikely(ret)) - fput(file); + io_fput(req); return ret; }@@ -846,7 +875,7 @@ static int io_fsync(struct io_kiocb *req, const struct io_uring_sqe *sqe, loff_t sqe_off = READ_ONCE(sqe->off); loff_t sqe_len = READ_ONCE(sqe->len); loff_t end = sqe_off + sqe_len; - unsigned fsync_flags; + unsigned fsync_flags, flags; struct file *file; int ret, fd;@@ -864,14 +893,23 @@ static int io_fsync(struct io_kiocb *req, const struct io_uring_sqe *sqe, return -EINVAL; fd = READ_ONCE(sqe->fd); - file = fget(fd); + flags = READ_ONCE(sqe->flags); + + if (flags & IOSQE_FIXED_FILE) { + if (unlikely(!ctx->user_files || fd >= ctx->nr_user_files)) + return -EBADF; + file = ctx->user_files[fd]; + } else { + file = fget(fd); + } if (unlikely(!file)) return -EBADF; ret = vfs_fsync_range(file, sqe_off, end > 0 ? end : LLONG_MAX, fsync_flags & IORING_FSYNC_DATASYNC); - fput(file); + if (!(flags & IOSQE_FIXED_FILE)) + fput(file); io_cqring_add_event(ctx, sqe->user_data, ret, 0); io_free_req(req); return 0;@@ -1002,7 +1040,7 @@ static int io_submit_sqe(struct io_ring_ctx *ctx, const struct sqe_submit *s, ssize_t ret; /* enforce forwards compatibility on users */ - if (unlikely(s->sqe->flags)) + if (unlikely(s->sqe->flags & ~IOSQE_FIXED_FILE)) return -EINVAL; req = io_get_req(ctx, state);@@ -1220,6 +1258,58 @@ static int __io_uring_enter(struct io_ring_ctx *ctx, unsigned to_submit, return submitted ? submitted : ret; } +static int io_sqe_files_unregister(struct io_ring_ctx *ctx) +{ + int i; + + if (!ctx->user_files) + return -ENXIO; + + for (i = 0; i < ctx->nr_user_files; i++) + fput(ctx->user_files[i]); + + kfree(ctx->user_files); + ctx->user_files = NULL; + ctx->nr_user_files = 0; + return 0; +} + +static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg, + unsigned nr_args) +{ + __s32 __user *fds = (__s32 __user *) arg; + int fd, ret = 0; + unsigned i; + + if (ctx->user_files) + return -EBUSY; + if (!nr_args) + return -EINVAL; + + ctx->user_files = kcalloc(nr_args, sizeof(struct file *), GFP_KERNEL); + if (!ctx->user_files) + return -ENOMEM; + + for (i = 0; i < nr_args; i++) { + ret = -EFAULT; + if (copy_from_user(&fd, &fds[i], sizeof(fd))) + break; + + ctx->user_files[i] = fget(fd); + + ret = -EBADF; + if (!ctx->user_files[i]) + break; + ctx->nr_user_files++; + ret = 0; + } + + if (ret) + io_sqe_files_unregister(ctx); + + return ret; +} + static int io_sq_offload_start(struct io_ring_ctx *ctx) { int ret;@@ -1509,6 +1599,7 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx) io_iopoll_reap_events(ctx); io_sqe_buffer_unregister(ctx); + io_sqe_files_unregister(ctx); io_mem_free(ctx->sq_ring); io_mem_free(ctx->sq_sqes);@@ -1806,6 +1897,15 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode, break; ret = io_sqe_buffer_unregister(ctx); break; + case IORING_REGISTER_FILES: + ret = io_sqe_files_register(ctx, arg, nr_args); + break; + case IORING_UNREGISTER_FILES: + ret = -EINVAL; + if (arg || nr_args) + break; + ret = io_sqe_files_unregister(ctx); + break; default: ret = -EINVAL; break;diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h index 16c423d74f2e..3e79feb34a9c 100644 --- a/include/uapi/linux/io_uring.h +++ b/include/uapi/linux/io_uring.h@@ -16,7 +16,7 @@ */ struct io_uring_sqe { __u8 opcode; /* type of operation for this sqe */ - __u8 flags; /* as of now unused */ + __u8 flags; /* IOSQE_ flags */ __u16 ioprio; /* ioprio for the request */ __s32 fd; /* file descriptor to do IO on */ __u64 off; /* offset into file */@@ -33,6 +33,11 @@ struct io_uring_sqe { }; }; +/* + * sqe->flags + */ +#define IOSQE_FIXED_FILE (1U << 0) /* use fixed fileset */ + /* * io_uring_setup() flags */@@ -112,5 +117,7 @@ struct io_uring_params { */ #define IORING_REGISTER_BUFFERS 0 #define IORING_UNREGISTER_BUFFERS 1 +#define IORING_REGISTER_FILES 2 +#define IORING_UNREGISTER_FILES 3 #endif --2.17.1