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io_uring: avoid ring quiesce while registering/unregistering eventfd

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This is done by creating a new RCU data structure (io_ev_fd) as part of
io_ring_ctx that holds the eventfd_ctx.

The function io_eventfd_signal is executed under rcu_read_lock with a
single rcu_dereference to io_ev_fd so that if another thread unregisters
the eventfd while io_eventfd_signal is still being executed, the
eventfd_signal for which io_eventfd_signal was called completes
successfully.

The process of registering/unregistering eventfd is already done under
uring_lock so multiple threads won't enter a race condition while
registering/unregistering eventfd.

With the above approach ring quiesce can be avoided which is much more
expensive then using RCU lock. On the system tested, io_uring_register
with IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared
to 15ms before with ring quiesce.

Signed-off-by: Usama Arif <usama.arif@bytedance.com>
Link: https://lore.kernel.org/r/20220204145117.1186568-3-usama.arif@bytedance.com
[axboe: long line fixups]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Usama Arif 2022-02-04 14:51:14 +00:00 committed by Jens Axboe
parent 2757be22c0
commit 77bc59b498
1 changed files with 68 additions and 21 deletions
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89
fs/io_uring.c
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@ -326,6 +326,11 @@ struct io_submit_state {
struct blk_plug plug;
};
struct io_ev_fd {
struct eventfd_ctx *cq_ev_fd;
struct rcu_head rcu;
};
struct io_ring_ctx {
/* const or read-mostly hot data */
struct {
@ -399,7 +404,7 @@ struct io_ring_ctx {
struct {
unsigned cached_cq_tail;
unsigned cq_entries;
struct eventfd_ctx *cq_ev_fd;
struct io_ev_fd __rcu *io_ev_fd;
struct wait_queue_head cq_wait;
unsigned cq_extra;
atomic_t cq_timeouts;
@ -1726,13 +1731,36 @@ static inline struct io_uring_cqe *io_get_cqe(struct io_ring_ctx *ctx)
return &rings->cqes[tail & mask];
}
static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
static void io_eventfd_signal(struct io_ring_ctx *ctx)
{
if (likely(!ctx->cq_ev_fd))
return false;
struct io_ev_fd *ev_fd;
/* Return quickly if ctx->io_ev_fd doesn't exist */
if (likely(!rcu_dereference_raw(ctx->io_ev_fd)))
return;
rcu_read_lock();
/*
* rcu_dereference ctx->io_ev_fd once and use it for both for checking
* and eventfd_signal
*/
ev_fd = rcu_dereference(ctx->io_ev_fd);
/*
* Check again if ev_fd exists incase an io_eventfd_unregister call
* completed between the NULL check of ctx->io_ev_fd at the start of
* the function and rcu_read_lock.
*/
if (unlikely(!ev_fd))
goto out;
if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
return false;
return !ctx->eventfd_async || io_wq_current_is_worker();
goto out;
if (!ctx->eventfd_async || io_wq_current_is_worker())
eventfd_signal(ev_fd->cq_ev_fd, 1);
out:
rcu_read_unlock();
}
/*
@ -1751,8 +1779,7 @@ static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
*/
if (wq_has_sleeper(&ctx->cq_wait))
wake_up_all(&ctx->cq_wait);
if (io_should_trigger_evfd(ctx))
eventfd_signal(ctx->cq_ev_fd, 1);
io_eventfd_signal(ctx);
}
static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
@ -1764,8 +1791,7 @@ static void io_cqring_ev_posted_iopoll(struct io_ring_ctx *ctx)
if (waitqueue_active(&ctx->cq_wait))
wake_up_all(&ctx->cq_wait);
}
if (io_should_trigger_evfd(ctx))
eventfd_signal(ctx->cq_ev_fd, 1);
io_eventfd_signal(ctx);
}
/* Returns true if there are no backlogged entries after the flush */
@ -9361,31 +9387,50 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
{
struct io_ev_fd *ev_fd;
__s32 __user *fds = arg;
int fd;
int fd, ret;
if (ctx->cq_ev_fd)
ev_fd = rcu_dereference_protected(ctx->io_ev_fd,
lockdep_is_held(&ctx->uring_lock));
if (ev_fd)
return -EBUSY;
if (copy_from_user(&fd, fds, sizeof(*fds)))
return -EFAULT;
ctx->cq_ev_fd = eventfd_ctx_fdget(fd);
if (IS_ERR(ctx->cq_ev_fd)) {
int ret = PTR_ERR(ctx->cq_ev_fd);
ev_fd = kmalloc(sizeof(*ev_fd), GFP_KERNEL);
if (!ev_fd)
return -ENOMEM;
ctx->cq_ev_fd = NULL;
ev_fd->cq_ev_fd = eventfd_ctx_fdget(fd);
if (IS_ERR(ev_fd->cq_ev_fd)) {
ret = PTR_ERR(ev_fd->cq_ev_fd);
kfree(ev_fd);
return ret;
}
return 0;
rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
return ret;
}
static void io_eventfd_put(struct rcu_head *rcu)
{
struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
eventfd_ctx_put(ev_fd->cq_ev_fd);
kfree(ev_fd);
}
static int io_eventfd_unregister(struct io_ring_ctx *ctx)
{
if (ctx->cq_ev_fd) {
eventfd_ctx_put(ctx->cq_ev_fd);
ctx->cq_ev_fd = NULL;
struct io_ev_fd *ev_fd;
ev_fd = rcu_dereference_protected(ctx->io_ev_fd,
lockdep_is_held(&ctx->uring_lock));
if (ev_fd) {
rcu_assign_pointer(ctx->io_ev_fd, NULL);
call_rcu(&ev_fd->rcu, io_eventfd_put);
return 0;
}
@ -9450,8 +9495,8 @@ static __cold void io_ring_ctx_free(struct io_ring_ctx *ctx)
__io_sqe_files_unregister(ctx);
if (ctx->rings)
__io_cqring_overflow_flush(ctx, true);
mutex_unlock(&ctx->uring_lock);
io_eventfd_unregister(ctx);
mutex_unlock(&ctx->uring_lock);
io_destroy_buffers(ctx);
if (ctx->sq_creds)
put_cred(ctx->sq_creds);
@ -10968,6 +11013,8 @@ static bool io_register_op_must_quiesce(int op)
case IORING_REGISTER_FILES:
case IORING_UNREGISTER_FILES:
case IORING_REGISTER_FILES_UPDATE:
case IORING_REGISTER_EVENTFD:
case IORING_UNREGISTER_EVENTFD:
case IORING_REGISTER_PROBE:
case IORING_REGISTER_PERSONALITY:
case IORING_UNREGISTER_PERSONALITY: