[RFC 2/2] io_uring: reduce sheduling due to tw

Pavel Begunkov
2023-03-10T19:05:42Z

Every task_work will try to wake the task to be executed, which causes
excessive scheduling with corresponding overhead. For some tw it's
justified, but others won't do much but post a single CQE.

When a task waits for multiple cqes, every such task_work will wake it
up. Instead, the task may give a hint about how many cqes it waits for,
io_req_local_work_add() will compare against it and skip wake ups
if #cqes + #tw items is not enough to satisfy the task. The optimisation
is used only for simple enough tws, more complex and/or urgent items
will force wake up. It's also limited to DEFER_TASKRUN.

The trade-off is having extra atomics in io_req_local_work_add() but
saving more on rescheduling the task..

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
include/linux/io_uring_types.h | 2 +-
io_uring/io_uring.c | 41 +++++++++++++++++++++-------------
io_uring/io_uring.h | 1 +
io_uring/notif.h | 2 +-
io_uring/rw.c | 2 +-
5 files changed, 29 insertions(+), 19 deletions(-)

diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
index 00689c12f6ab..fdf0ae28023d 100644
--- a/include/linux/io_uring_types.h
+++ b/include/linux/io_uring_types.h
@@ -295,7 +295,7 @@ struct io_ring_ctx {
spinlock_t completion_lock;

bool poll_multi_queue;
- bool cq_waiting;
+ atomic_t cq_wait_nr;

/*
* ->iopoll_list is protected by the ctx->uring_lock for
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 42ada470845f..0fa4dee8dcf4 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1279,31 +1279,38 @@ static __cold void io_fallback_tw(struct io_uring_task *tctx)
}
}

-static void io_req_local_work_add(struct io_kiocb *req)
+static void io_req_local_work_add(struct io_kiocb *req, unsigned flags)
{
struct io_ring_ctx *ctx = req->ctx;
+ bool first;

percpu_ref_get(&ctx->refs);

- if (!llist_add(&req->io_task_work.node, &ctx->work_llist))
- goto put_ref;
-
+ first = llist_add(&req->io_task_work.node, &ctx->work_llist);
/* needed for the following wake up */
smp_mb__after_atomic();

- if (unlikely(atomic_read(&req->task->io_uring->in_cancel))) {
- io_move_task_work_from_local(ctx);
- goto put_ref;
+ if (first) {
+ if (unlikely(atomic_read(&req->task->io_uring->in_cancel))) {
+ io_move_task_work_from_local(ctx);
+ goto put_ref;
+ }
+
+ if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+ atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+ if (ctx->has_evfd)
+ io_eventfd_signal(ctx);
}

- if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
- atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
- if (ctx->has_evfd)
- io_eventfd_signal(ctx);
+ if (atomic_read(&ctx->cq_wait_nr) <= 0)
+ goto put_ref;

- if (READ_ONCE(ctx->cq_waiting))
- wake_up_state(ctx->submitter_task, TASK_INTERRUPTIBLE);
+ if (!(flags & IOU_F_TWQ_FACILE))
+ atomic_set(&ctx->cq_wait_nr, 0);
+ else if (atomic_dec_return(&ctx->cq_wait_nr) > 0)
+ goto put_ref;

+ wake_up_state(ctx->submitter_task, TASK_INTERRUPTIBLE);
put_ref:
percpu_ref_put(&ctx->refs);
}
@@ -1315,7 +1322,7 @@ void __io_req_task_work_add(struct io_kiocb *req, unsigned flags)

if (!(flags & IOU_F_TWQ_FORCE_NORMAL) &&
(ctx->flags & IORING_SETUP_DEFER_TASKRUN)) {
- io_req_local_work_add(req);
+ io_req_local_work_add(req, flags);
return;
}

@@ -2601,7 +2608,9 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
unsigned long check_cq;

if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
- WRITE_ONCE(ctx->cq_waiting, 1);
+ int to_wait = (int) iowq.cq_tail - READ_ONCE(ctx->rings->cq.tail);
+
+ atomic_set(&ctx->cq_wait_nr, to_wait);
set_current_state(TASK_INTERRUPTIBLE);
} else {
prepare_to_wait_exclusive(&ctx->cq_wait, &iowq.wq,
@@ -2610,7 +2619,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,

ret = io_cqring_wait_schedule(ctx, &iowq);
__set_current_state(TASK_RUNNING);
- WRITE_ONCE(ctx->cq_waiting, 0);
+ atomic_set(&ctx->cq_wait_nr, 0);

if (ret < 0)
break;
diff --git a/io_uring/io_uring.h b/io_uring/io_uring.h
index cd2e702f206c..98ff9b71d498 100644
--- a/io_uring/io_uring.h
+++ b/io_uring/io_uring.h
@@ -18,6 +18,7 @@
enum {
/* don't use deferred task_work */
IOU_F_TWQ_FORCE_NORMAL = 1,
+ IOU_F_TWQ_FACILE = 2,
};

enum {
diff --git a/io_uring/notif.h b/io_uring/notif.h
index c88c800cd89d..ec9998fb0be6 100644
--- a/io_uring/notif.h
+++ b/io_uring/notif.h
@@ -33,7 +33,7 @@ static inline void io_notif_flush(struct io_kiocb *notif)

/* drop slot's master ref */
if (refcount_dec_and_test(&nd->uarg.refcnt))
- io_req_task_work_add(notif);
+ __io_req_task_work_add(notif, IOU_F_TWQ_FACILE);
}

static inline int io_notif_account_mem(struct io_kiocb *notif, unsigned len)
diff --git a/io_uring/rw.c b/io_uring/rw.c
index 4c233910e200..a4578c120973 100644
--- a/io_uring/rw.c
+++ b/io_uring/rw.c
@@ -304,7 +304,7 @@ static void io_complete_rw(struct kiocb *kiocb, long res)
return;
io_req_set_res(req, io_fixup_rw_res(req, res), 0);
req->io_task_work.func = io_req_rw_complete;
- io_req_task_work_add(req);
+ __io_req_task_work_add(req, IOU_F_TWQ_FACILE);
}

static void io_complete_rw_iopoll(struct kiocb *kiocb, long res)
--
2.39.1