This is done by creating a new RCU data structure (io_ev_fd) as part of
io_ring_ctx that holds the eventfd_ctx, with reads to the structure protected
by rcu_read_lock and writes (register/unregister calls) protected by a mutex.
With the above approach ring quiesce can be avoided which is much more
expensive then using RCU lock. On the system tested, io_uring_reigster with
IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to 15ms
before with ring quiesce.
The first patch creates the RCU protected data structure and removes ring
quiesce for IORING_REGISTER_EVENTFD and IORING_UNREGISTER_EVENTFD.
The second patch builds on top of the first patch and removes ring quiesce
for IORING_REGISTER_EVENTFD_ASYNC.
---
v3->v4:
- Switch back to call_rcu and use rcu_barrier incase io_eventfd_register fails
to make sure all rcu callbacks have finished.
v2->v3:
- Switched to using synchronize_rcu from call_rcu in io_eventfd_unregister.
v1->v2:
- Added patch to remove eventfd from tracepoint (Patch 1) (Jens Axboe)
- Made the code of io_should_trigger_evfd as part of io_eventfd_signal (Jens Axboe)
Usama Arif (3):
io_uring: remove trace for eventfd
io_uring: avoid ring quiesce while registering/unregistering eventfd
io_uring: avoid ring quiesce for IORING_REGISTER_EVENTFD_ASYNC
fs/io_uring.c | 127 +++++++++++++++++++++++---------
include/trace/events/io_uring.h | 13 ++--
2 files changed, 96 insertions(+), 44 deletions(-)
--
2.25.1
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 done under a lock
so multiple threads don'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_reigster with
IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to 15ms
before with ring quiesce.
Signed-off-by: Usama Arif <[email protected]>
---
fs/io_uring.c | 104 ++++++++++++++++++++++++++++++++++++++------------
1 file changed, 79 insertions(+), 25 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 21531609a9c6..7a8f4ac7a785 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -326,6 +326,12 @@ struct io_submit_state {
struct blk_plug plug;
};
+struct io_ev_fd {
+ struct eventfd_ctx *cq_ev_fd;
+ struct io_ring_ctx *ctx;
+ struct rcu_head rcu;
+};
+
struct io_ring_ctx {
/* const or read-mostly hot data */
struct {
@@ -399,7 +405,8 @@ 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 mutex ev_fd_lock;
struct wait_queue_head cq_wait;
unsigned cq_extra;
atomic_t cq_timeouts;
@@ -1448,6 +1455,7 @@ static __cold struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
xa_init_flags(&ctx->io_buffers, XA_FLAGS_ALLOC1);
xa_init_flags(&ctx->personalities, XA_FLAGS_ALLOC1);
mutex_init(&ctx->uring_lock);
+ mutex_init(&ctx->ev_fd_lock);
init_waitqueue_head(&ctx->cq_wait);
spin_lock_init(&ctx->completion_lock);
spin_lock_init(&ctx->timeout_lock);
@@ -1726,13 +1734,24 @@ 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;
+
+ 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);
+
+ if (likely(!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 +1770,7 @@ 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 +1782,7 @@ 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 */
@@ -9353,35 +9370,70 @@ 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)
- return -EBUSY;
+ mutex_lock(&ctx->ev_fd_lock);
+ ret = -EBUSY;
+ if (rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock))) {
+ rcu_barrier();
+ if(rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock)))
+ goto out;
+ }
+ ret = -EFAULT;
if (copy_from_user(&fd, fds, sizeof(*fds)))
- return -EFAULT;
+ goto out;
- ctx->cq_ev_fd = eventfd_ctx_fdget(fd);
- if (IS_ERR(ctx->cq_ev_fd)) {
- int ret = PTR_ERR(ctx->cq_ev_fd);
+ ret = -ENOMEM;
+ ev_fd = kmalloc(sizeof(*ev_fd), GFP_KERNEL);
+ if (!ev_fd)
+ goto out;
- ctx->cq_ev_fd = NULL;
- return ret;
+ 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);
+ goto out;
}
+ ev_fd->ctx = ctx;
- return 0;
+ rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
+ ret = 0;
+
+out:
+ mutex_unlock(&ctx->ev_fd_lock);
+ 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);
+ struct io_ring_ctx *ctx = ev_fd->ctx;
+
+ eventfd_ctx_put(ev_fd->cq_ev_fd);
+ kfree(ev_fd);
+ rcu_assign_pointer(ctx->io_ev_fd, NULL);
}
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;
- return 0;
+ struct io_ev_fd *ev_fd;
+ int ret;
+
+ mutex_lock(&ctx->ev_fd_lock);
+ ev_fd = rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock));
+ if (ev_fd) {
+ call_rcu(&ev_fd->rcu, io_eventfd_put);
+ ret = 0;
+ goto out;
}
+ ret = -ENXIO;
- return -ENXIO;
+out:
+ mutex_unlock(&ctx->ev_fd_lock);
+ return ret;
}
static void io_destroy_buffers(struct io_ring_ctx *ctx)
@@ -10960,6 +11012,8 @@ 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:
--
2.25.1
On 2/3/22 11:24 AM, Usama Arif wrote:
> -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;
> +
> + 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);
> +
> + if (likely(!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();
> }
This still needs what we discussed in v3, something ala:
/*
* This will potential race with eventfd registration, but that's
* always going to be the case if there is IO inflight while an eventfd
* descriptor is being registered.
*/
if (!rcu_dereference_raw(ctx->io_ev_fd))
return;
rcu_read_lock();
...
which I think is cheap enough and won't hit sparse complaints. The
> @@ -9353,35 +9370,70 @@ 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)
> - return -EBUSY;
> + mutex_lock(&ctx->ev_fd_lock);
> + ret = -EBUSY;
> + if (rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock))) {
> + rcu_barrier();
> + if(rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock)))
> + goto out;
> + }
I wonder if we can get away with assigning ctx->io_ev_fd to NULL when we
do the call_rcu(). The struct itself will remain valid as long as we're
under rcu_read_lock() protection, so I think we'd be fine? If we do
that, then we don't need any rcu_barrier() or synchronize_rcu() calls,
as we can register a new one while the previous one is still being
killed.
Hmm?
> 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;
> - return 0;
> + struct io_ev_fd *ev_fd;
> + int ret;
> +
> + mutex_lock(&ctx->ev_fd_lock);
> + ev_fd = rcu_dereference_protected(ctx->io_ev_fd, lockdep_is_held(&ctx->ev_fd_lock));
> + if (ev_fd) {
> + call_rcu(&ev_fd->rcu, io_eventfd_put);
> + ret = 0;
> + goto out;
> }
> + ret = -ENXIO;
>
> - return -ENXIO;
> +out:
> + mutex_unlock(&ctx->ev_fd_lock);
> + return ret;
> }
I also think that'd be cleaner without the goto:
{
struct io_ev_fd *ev_fd;
int ret;
mutex_lock(&ctx->ev_fd_lock);
ev_fd = rcu_dereference_protected(ctx->io_ev_fd,
lockdep_is_held(&ctx->ev_fd_lock));
if (ev_fd) {
call_rcu(&ev_fd->rcu, io_eventfd_put);
mutex_unlock(&ctx->ev_fd_lock);
return 0;
}
mutex_unlock(&ctx->ev_fd_lock);
return -ENXIO;
}
--
Jens Axboe
This is done using the RCU data structure (io_ev_fd). eventfd_async
is moved from io_ring_ctx to io_ev_fd which is RCU protected hence
avoiding ring quiesce which is much more expensive than an RCU lock.
io_should_trigger_evfd is already under rcu_read_lock so there is
no extra RCU read-side critical section needed.
Signed-off-by: Usama Arif <[email protected]>
---
fs/io_uring.c | 22 ++++++++++++----------
1 file changed, 12 insertions(+), 10 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 7a8f4ac7a785..e287fc61879f 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -329,6 +329,7 @@ struct io_submit_state {
struct io_ev_fd {
struct eventfd_ctx *cq_ev_fd;
struct io_ring_ctx *ctx;
+ unsigned int eventfd_async: 1;
struct rcu_head rcu;
};
@@ -341,7 +342,6 @@ struct io_ring_ctx {
unsigned int flags;
unsigned int compat: 1;
unsigned int drain_next: 1;
- unsigned int eventfd_async: 1;
unsigned int restricted: 1;
unsigned int off_timeout_used: 1;
unsigned int drain_active: 1;
@@ -1747,7 +1747,7 @@ static void io_eventfd_signal(struct io_ring_ctx *ctx)
if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
goto out;
- if (!ctx->eventfd_async || io_wq_current_is_worker())
+ if (!ev_fd->eventfd_async || io_wq_current_is_worker())
eventfd_signal(ev_fd->cq_ev_fd, 1);
out:
@@ -9368,7 +9368,8 @@ static int __io_sqe_buffers_update(struct io_ring_ctx *ctx,
return done ? done : err;
}
-static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
+static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg,
+ unsigned int eventfd_async)
{
struct io_ev_fd *ev_fd;
__s32 __user *fds = arg;
@@ -9398,6 +9399,7 @@ static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg)
goto out;
}
ev_fd->ctx = ctx;
+ ev_fd->eventfd_async = eventfd_async;
rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
ret = 0;
@@ -11013,6 +11015,7 @@ static bool io_register_op_must_quiesce(int op)
case IORING_UNREGISTER_FILES:
case IORING_REGISTER_FILES_UPDATE:
case IORING_REGISTER_EVENTFD:
+ case IORING_REGISTER_EVENTFD_ASYNC:
case IORING_UNREGISTER_EVENTFD:
case IORING_REGISTER_PROBE:
case IORING_REGISTER_PERSONALITY:
@@ -11113,17 +11116,16 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
ret = io_register_files_update(ctx, arg, nr_args);
break;
case IORING_REGISTER_EVENTFD:
- case IORING_REGISTER_EVENTFD_ASYNC:
ret = -EINVAL;
if (nr_args != 1)
break;
- ret = io_eventfd_register(ctx, arg);
- if (ret)
+ ret = io_eventfd_register(ctx, arg, 0);
+ break;
+ case IORING_REGISTER_EVENTFD_ASYNC:
+ ret = -EINVAL;
+ if (nr_args != 1)
break;
- if (opcode == IORING_REGISTER_EVENTFD_ASYNC)
- ctx->eventfd_async = 1;
- else
- ctx->eventfd_async = 0;
+ ret = io_eventfd_register(ctx, arg, 1);
break;
case IORING_UNREGISTER_EVENTFD:
ret = -EINVAL;
--
2.25.1
The information on whether eventfd is registered is not very useful
and would result in the tracepoint being enclosed in an rcu_readlock
in a later patch that tries to avoid ring quiesce for registering
eventfd.
Suggested-by: Jens Axboe <[email protected]>
Signed-off-by: Usama Arif <[email protected]>
---
fs/io_uring.c | 3 +--
include/trace/events/io_uring.h | 13 +++++--------
2 files changed, 6 insertions(+), 10 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 2e04f718319d..21531609a9c6 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -11171,8 +11171,7 @@ SYSCALL_DEFINE4(io_uring_register, unsigned int, fd, unsigned int, opcode,
mutex_lock(&ctx->uring_lock);
ret = __io_uring_register(ctx, opcode, arg, nr_args);
mutex_unlock(&ctx->uring_lock);
- trace_io_uring_register(ctx, opcode, ctx->nr_user_files, ctx->nr_user_bufs,
- ctx->cq_ev_fd != NULL, ret);
+ trace_io_uring_register(ctx, opcode, ctx->nr_user_files, ctx->nr_user_bufs, ret);
out_fput:
fdput(f);
return ret;
diff --git a/include/trace/events/io_uring.h b/include/trace/events/io_uring.h
index 7346f0164cf4..098beda7601a 100644
--- a/include/trace/events/io_uring.h
+++ b/include/trace/events/io_uring.h
@@ -57,10 +57,9 @@ TRACE_EVENT(io_uring_create,
* @opcode: describes which operation to perform
* @nr_user_files: number of registered files
* @nr_user_bufs: number of registered buffers
- * @cq_ev_fd: whether eventfs registered or not
* @ret: return code
*
- * Allows to trace fixed files/buffers/eventfds, that could be registered to
+ * Allows to trace fixed files/buffers, that could be registered to
* avoid an overhead of getting references to them for every operation. This
* event, together with io_uring_file_get, can provide a full picture of how
* much overhead one can reduce via fixing.
@@ -68,16 +67,15 @@ TRACE_EVENT(io_uring_create,
TRACE_EVENT(io_uring_register,
TP_PROTO(void *ctx, unsigned opcode, unsigned nr_files,
- unsigned nr_bufs, bool eventfd, long ret),
+ unsigned nr_bufs, long ret),
- TP_ARGS(ctx, opcode, nr_files, nr_bufs, eventfd, ret),
+ TP_ARGS(ctx, opcode, nr_files, nr_bufs, ret),
TP_STRUCT__entry (
__field( void *, ctx )
__field( unsigned, opcode )
__field( unsigned, nr_files )
__field( unsigned, nr_bufs )
- __field( bool, eventfd )
__field( long, ret )
),
@@ -86,14 +84,13 @@ TRACE_EVENT(io_uring_register,
__entry->opcode = opcode;
__entry->nr_files = nr_files;
__entry->nr_bufs = nr_bufs;
- __entry->eventfd = eventfd;
__entry->ret = ret;
),
TP_printk("ring %p, opcode %d, nr_user_files %d, nr_user_bufs %d, "
- "eventfd %d, ret %ld",
+ "ret %ld",
__entry->ctx, __entry->opcode, __entry->nr_files,
- __entry->nr_bufs, __entry->eventfd, __entry->ret)
+ __entry->nr_bufs, __entry->ret)
);
/**
--
2.25.1