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io_uring: reduce scheduling due to tw

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Every task_work will try to wake the task to be executed, which causes
excessive scheduling and additional 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 is not enough to satisfy the waiting condition. Task_work
that uses the optimisation should be simple enough and never post more
than one CQE. It's also ignored for non DEFER_TASKRUN rings.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d2b77e99d1e86624d8a69f7037d764b739dcd225.1680782017.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Pavel Begunkov 2023-04-06 14:20:12 +01:00 committed by Jens Axboe
parent 5150940079
commit 8751d15426
6 changed files with 61 additions and 25 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

3
include/linux/io_uring_types.h
View file

@ -296,7 +296,7 @@ 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
@ -566,6 +566,7 @@ struct io_kiocb {
atomic_t refs;
atomic_t poll_refs;
struct io_task_work io_task_work;
unsigned nr_tw;
/* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
union {
struct hlist_node hash_node;

68
io_uring/io_uring.c
View file

@ -1300,35 +1300,59 @@ 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;
unsigned nr_wait, nr_tw, nr_tw_prev;
struct llist_node *first;
if (req->flags & (REQ_F_LINK | REQ_F_HARDLINK))
flags &= ~IOU_F_TWQ_LAZY_WAKE;
first = READ_ONCE(ctx->work_llist.first);
do {
nr_tw_prev = 0;
if (first) {
struct io_kiocb *first_req = container_of(first,
struct io_kiocb,
io_task_work.node);
/*
* Might be executed at any moment, rely on
* SLAB_TYPESAFE_BY_RCU to keep it alive.
*/
nr_tw_prev = READ_ONCE(first_req->nr_tw);
}
nr_tw = nr_tw_prev + 1;
/* Large enough to fail the nr_wait comparison below */
if (!(flags & IOU_F_TWQ_LAZY_WAKE))
nr_tw = -1U;
req->nr_tw = nr_tw;
req->io_task_work.node.next = first;
} while (!try_cmpxchg(&ctx->work_llist.first, &first,
&req->io_task_work.node));
if (first)
return;
/* 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);
return;
if (!first) {
if (unlikely(atomic_read(&req->task->io_uring->in_cancel))) {
io_move_task_work_from_local(ctx);
return;
}
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 (READ_ONCE(ctx->cq_waiting))
wake_up_state(ctx->submitter_task, TASK_INTERRUPTIBLE);
nr_wait = atomic_read(&ctx->cq_wait_nr);
/* no one is waiting */
if (!nr_wait)
return;
/* either not enough or the previous add has already woken it up */
if (nr_wait > nr_tw || nr_tw_prev >= nr_wait)
return;
/* pairs with set_current_state() in io_cqring_wait() */
smp_mb__after_atomic();
wake_up_state(ctx->submitter_task, TASK_INTERRUPTIBLE);
}
void __io_req_task_work_add(struct io_kiocb *req, unsigned flags)
@ -1339,7 +1363,7 @@ 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)) {
rcu_read_lock();
io_req_local_work_add(req);
io_req_local_work_add(req, flags);
rcu_read_unlock();
return;
}
@ -2625,7 +2649,9 @@ 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 nr_wait = (int) iowq.cq_tail - READ_ONCE(ctx->rings->cq.tail);
atomic_set(&ctx->cq_wait_nr, nr_wait);
set_current_state(TASK_INTERRUPTIBLE);
} else {
prepare_to_wait_exclusive(&ctx->cq_wait, &iowq.wq,
@ -2634,7 +2660,7 @@ 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;
@ -4517,7 +4543,7 @@ static int __init io_uring_init(void)
io_uring_optable_init();
req_cachep = KMEM_CACHE(io_kiocb, SLAB_HWCACHE_ALIGN | SLAB_PANIC |
SLAB_ACCOUNT);
SLAB_ACCOUNT | SLAB_TYPESAFE_BY_RCU);
return 0;
};
__initcall(io_uring_init);

9
io_uring/io_uring.h
View file

@ -18,6 +18,15 @@
enum {
/* don't use deferred task_work */
IOU_F_TWQ_FORCE_NORMAL = 1,
/*
* A hint to not wake right away but delay until there are enough of
* tw's queued to match the number of CQEs the task is waiting for.
*
* Must not be used wirh requests generating more than one CQE.
* It's also ignored unless IORING_SETUP_DEFER_TASKRUN is set.
*/
IOU_F_TWQ_LAZY_WAKE = 2,
};
enum {

2
io_uring/notif.c
View file

@ -31,7 +31,7 @@ static void io_tx_ubuf_callback(struct sk_buff *skb, struct ubuf_info *uarg,
struct io_kiocb *notif = cmd_to_io_kiocb(nd);
if (refcount_dec_and_test(&uarg->refcnt))
io_req_task_work_add(notif);
__io_req_task_work_add(notif, IOU_F_TWQ_LAZY_WAKE);
}
static void io_tx_ubuf_callback_ext(struct sk_buff *skb, struct ubuf_info *uarg,

2
io_uring/notif.h
View file

@ -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_LAZY_WAKE);
}
static inline int io_notif_account_mem(struct io_kiocb *notif, unsigned len)

2
io_uring/rw.c
View file

@ -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_LAZY_WAKE);
}
static void io_complete_rw_iopoll(struct kiocb *kiocb, long res)