On 8/15/22 14:09, Dylan Yudaken wrote:
Allow deferring async tasks until the user calls io_uring_enter(2) with
the IORING_ENTER_GETEVENTS flag.
Being able to hand pick when tasks are run prevents the problem where
there is current work to be done, however task work runs anyway.
For example, a common workload would obtain a batch of CQEs, and process
each one. Interrupting this to additional taskwork would add latency but
not gain anything. If instead task work is deferred to just before more
CQEs are obtained then no additional latency is added.
The way this is implemented is by trying to keep task work local to a
io_ring_ctx, rather than to the submission task. This is required, as the
application will want to wake up only a single io_ring_ctx at a time to
process work, and so the lists of work have to be kept separate.
This has some other benefits like not having to check the task continually
in handle_tw_list (and potentially unlocking/locking those), and reducing
locks in the submit & process completions path.
There are networking cases where using this option can reduce request
latency by 50%. For example a contrived example using [1] where the client
sends 2k data and receives the same data back while doing some system
calls (to trigger task work) shows this reduction. The reason ends up
being that if sending responses is delayed by processing task work, then
the client side sits idle. Whereas reordering the sends first means that
the client runs it's workload in parallel with the local task work.
[1]:
Using https://github.com/DylanZA/netbench/tree/defer_run
Client:
./netbench --client_only 1 --control_port 10000 --host <host> --tx "epoll --threads 16 --per_thread 1 --size 2048 --resp 2048 --workload 1000"
Server:
./netbench --server_only 1 --control_port 10000 --rx "io_uring --defer_taskrun 0 --workload 100" --rx "io_uring --defer_taskrun 1 --workload 100"
Signed-off-by: Dylan Yudaken <dylany@xxxxxx>
---
include/linux/io_uring_types.h | 2 +
include/uapi/linux/io_uring.h | 7 ++
io_uring/cancel.c | 2 +-
io_uring/io_uring.c | 125 ++++++++++++++++++++++++++++-----
io_uring/io_uring.h | 31 +++++++-
io_uring/rsrc.c | 2 +-
6 files changed, 148 insertions(+), 21 deletions(-)
[...]
-void io_req_task_work_add(struct io_kiocb *req)
+static void io_req_local_work_add(struct io_kiocb *req)
+{
+ struct io_ring_ctx *ctx = req->ctx;
+
+ if (!llist_add(&req->io_task_work.node, &ctx->work_llist))
+ return;
+
+ if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+ atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+
+ io_cqring_wake(ctx);
+}
+
+static inline void __io_req_task_work_add(struct io_kiocb *req, bool allow_local)
{
struct io_uring_task *tctx = req->task->io_uring;
struct io_ring_ctx *ctx = req->ctx;
struct llist_node *node;
+ if (allow_local && ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
+ io_req_local_work_add(req);
+ return;
+ }
+
/* task_work already pending, we're done */
if (!llist_add(&req->io_task_work.node, &tctx->task_list))
return;
@@ -1074,6 +1093,69 @@ void io_req_task_work_add(struct io_kiocb *req)
}
}
+void io_req_task_work_add(struct io_kiocb *req)
+{
+ __io_req_task_work_add(req, true);
+}
+
+static void __cold io_move_task_work_from_local(struct io_ring_ctx *ctx)
+{
+ struct llist_node *node;
+
+ node = llist_del_all(&ctx->work_llist);
+ while (node) {
+ struct io_kiocb *req = container_of(node, struct io_kiocb,
+ io_task_work.node);
+
+ node = node->next;
+ __io_req_task_work_add(req, false);
+ }
+}
+
+bool io_run_local_work(struct io_ring_ctx *ctx, bool locked)
+{
+ struct llist_node *node;
+ struct llist_node fake;
+ struct llist_node *current_final = NULL;
+ unsigned int count;
+
+ if (!locked)
+ locked = mutex_trylock(&ctx->uring_lock);
+
+ node = io_llist_xchg(&ctx->work_llist, &fake);
+ count = 0;
+again:
+ while (node != current_final) {
+ struct llist_node *next = node->next;
+ struct io_kiocb *req = container_of(node, struct io_kiocb,
+ io_task_work.node);
+ prefetch(container_of(next, struct io_kiocb, io_task_work.node));
+ if (unlikely(!same_thread_group(req->task, current))) {
Not same thread group, they have to be executed by the same thread.
One of the assumptions is that current->io_uring is the same
as the request was initialised with.
+ __io_req_task_work_add(req, false);
Why do we mix local and normal tw in the same ring? I think we
need to do either one or another. What is blocking it?
+ } else {
+ req->io_task_work.func(req, &locked);
+ count++;
+ }
+ node = next;
+ }
+
+ if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+ atomic_andnot(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+
+ node = io_llist_cmpxchg(&ctx->work_llist, &fake, NULL);
+ if (node != &fake) {
+ current_final = &fake;
+ node = io_llist_xchg(&ctx->work_llist, &fake);
+ goto again;
+ }
+
+ if (locked) {
+ io_submit_flush_completions(ctx);
+ mutex_unlock(&ctx->uring_lock);
+ }
+ return count > 0;
+}
+
static void io_req_tw_post(struct io_kiocb *req, bool *locked)
{
io_req_complete_post(req);
@@ -1284,8 +1366,7 @@ static int io_iopoll_check(struct io_ring_ctx *ctx, long min)
if (wq_list_empty(&ctx->iopoll_list)) {
u32 tail = ctx->cached_cq_tail;
- mutex_unlock(&ctx->uring_lock);
- io_run_task_work();
+ io_run_task_work_unlock_ctx(ctx);
mutex_lock(&ctx->uring_lock);
/* some requests don't go through iopoll_list */
@@ -2146,7 +2227,9 @@ struct io_wait_queue {
static inline bool io_has_work(struct io_ring_ctx *ctx)
{
- return test_bit(IO_CHECK_CQ_OVERFLOW_BIT, &ctx->check_cq);
+ return test_bit(IO_CHECK_CQ_OVERFLOW_BIT, &ctx->check_cq) ||
+ ((ctx->flags & IORING_SETUP_DEFER_TASKRUN) &&
+ !llist_empty(&ctx->work_llist));
}
static inline bool io_should_wake(struct io_wait_queue *iowq)
@@ -2178,9 +2261,9 @@ static int io_wake_function(struct wait_queue_entry *curr, unsigned int mode,
return -1;
}
-int io_run_task_work_sig(void)
+int io_run_task_work_sig(struct io_ring_ctx *ctx)
{
- if (io_run_task_work())
+ if (io_run_task_work_ctx(ctx, true))
return 1;
if (task_sigpending(current))
return -EINTR;
@@ -2196,7 +2279,7 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,
unsigned long check_cq;
/* make sure we run task_work before checking for signals */
- ret = io_run_task_work_sig();
+ ret = io_run_task_work_sig(ctx);
if (ret || io_should_wake(iowq))
return ret;
@@ -2230,7 +2313,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
io_cqring_overflow_flush(ctx);
if (io_cqring_events(ctx) >= min_events)
return 0;
- if (!io_run_task_work())
+ if (!io_run_task_work_ctx(ctx, false))
break;
} while (1);
@@ -2768,13 +2851,14 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
}
}
+ io_move_task_work_from_local(ctx);
Why do we even need to move them? Isn't it easier to just
execute them here?
ret |= io_cancel_defer_files(ctx, task, cancel_all);
mutex_lock(&ctx->uring_lock);
ret |= io_poll_remove_all(ctx, task, cancel_all);
mutex_unlock(&ctx->uring_lock);
ret |= io_kill_timeouts(ctx, task, cancel_all);
if (task)
- ret |= io_run_task_work();
+ ret |= io_run_task_work_ctx(ctx, true);
return ret;
}
@@ -2837,7 +2921,7 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
}
prepare_to_wait(&tctx->wait, &wait, TASK_INTERRUPTIBLE);
- io_run_task_work();
+ io_run_task_work_ctx(ctx, true);
io_uring_drop_tctx_refs(current);
/*
@@ -3055,12 +3139,15 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
mutex_unlock(&ctx->uring_lock);
goto out;
}
- if ((flags & IORING_ENTER_GETEVENTS) && ctx->syscall_iopoll)
+
+ if (!(flags & IORING_ENTER_GETEVENTS))
+ mutex_unlock(&ctx->uring_lock);
+ else if (ctx->syscall_iopoll)
goto iopoll_locked;
- mutex_unlock(&ctx->uring_lock);
- io_run_task_work();
+ else
+ io_run_task_work_unlock_ctx(ctx);
Let's unroll this function and get rid of conditional
locking, especially since you don't need the io_run_task_work()
part here.
} else {
- io_run_task_work();
+ io_run_task_work_ctx(ctx, false);
}
...
--- a/io_uring/io_uring.h
+++ b/io_uring/io_uring.h
@@ -26,7 +26,8 @@ enum {
...
+static inline bool io_run_task_work_ctx(struct io_ring_ctx *ctx, bool all)
+{
+ bool ret = false;
+
+ if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
+ ret = io_run_local_work(ctx, false);
+ if (!all)
+ return ret;
+ }
+ ret |= io_run_task_work();
+ return ret;
+}
+
+static inline bool io_run_task_work_unlock_ctx(struct io_ring_ctx *ctx)
+{
+ bool ret;
+
+ if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
+ ret = io_run_local_work(ctx, true);
+ mutex_unlock(&ctx->uring_lock);
If I read it right, io_run_local_work(locked=true) will
release the lock, and then we unlock it a second time
here. I'd suggest moving conditional locking out
of io_run_local_work().
+ } else {
+ mutex_unlock(&ctx->uring_lock);
+ ret = io_run_task_work();
+ }
+
+ return ret;
+}
+
--
Pavel Begunkov
