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io_uring: provide FIFO ordering for task_work
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task_work is a LIFO list, due to how it's implemented as a lockless
list. For long chains of task_work, this can be problematic as the
first entry added is the last one processed. Similarly, we'd waste
a lot of CPU cycles reversing this list.

Wrap the task_work so we have a single task_work entry per task per
ctx, and use that to run it in the right order.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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Jens Axboe committed Feb 10, 2021
1 parent 1b4c351 commit 7cbf172
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Showing 3 changed files with 108 additions and 16 deletions.
9 changes: 0 additions & 9 deletions fs/io-wq.h
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Original file line number Diff line number Diff line change
Expand Up @@ -27,15 +27,6 @@ enum io_wq_cancel {
IO_WQ_CANCEL_NOTFOUND, /* work not found */
};

struct io_wq_work_node {
struct io_wq_work_node *next;
};

struct io_wq_work_list {
struct io_wq_work_node *first;
struct io_wq_work_node *last;
};

static inline void wq_list_add_after(struct io_wq_work_node *node,
struct io_wq_work_node *pos,
struct io_wq_work_list *list)
Expand Down
101 changes: 94 additions & 7 deletions fs/io_uring.c
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Original file line number Diff line number Diff line change
Expand Up @@ -721,6 +721,11 @@ struct async_poll {
struct io_poll_iocb *double_poll;
};

struct io_task_work {
struct io_wq_work_node node;
task_work_func_t func;
};

/*
* NOTE! Each of the iocb union members has the file pointer
* as the first entry in their struct definition. So you can
Expand Down Expand Up @@ -779,7 +784,10 @@ struct io_kiocb {
* 2. to track reqs with ->files (see io_op_def::file_table)
*/
struct list_head inflight_entry;
struct callback_head task_work;
union {
struct io_task_work io_task_work;
struct callback_head task_work;
};
/* for polled requests, i.e. IORING_OP_POLL_ADD and async armed poll */
struct hlist_node hash_node;
struct async_poll *apoll;
Expand Down Expand Up @@ -2129,6 +2137,81 @@ static inline struct io_kiocb *io_req_find_next(struct io_kiocb *req)
return __io_req_find_next(req);
}

static bool __tctx_task_work(struct io_uring_task *tctx)
{
struct io_wq_work_list list;
struct io_wq_work_node *node;

if (wq_list_empty(&tctx->task_list))
return false;

spin_lock(&tctx->task_lock);
list = tctx->task_list;
INIT_WQ_LIST(&tctx->task_list);
spin_unlock(&tctx->task_lock);

node = list.first;
while (node) {
struct io_wq_work_node *next = node->next;
struct io_kiocb *req;

req = container_of(node, struct io_kiocb, io_task_work.node);
req->task_work.func(&req->task_work);
node = next;
}

return list.first != NULL;
}

static void tctx_task_work(struct callback_head *cb)
{
struct io_uring_task *tctx = container_of(cb, struct io_uring_task, task_work);

while (__tctx_task_work(tctx))
cond_resched();

clear_bit(0, &tctx->task_state);
}

static int io_task_work_add(struct task_struct *tsk, struct io_kiocb *req,
enum task_work_notify_mode notify)
{
struct io_uring_task *tctx = tsk->io_uring;
struct io_wq_work_node *node, *prev;
int ret;

WARN_ON_ONCE(!tctx);

spin_lock(&tctx->task_lock);
wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
spin_unlock(&tctx->task_lock);

/* task_work already pending, we're done */
if (test_bit(0, &tctx->task_state) ||
test_and_set_bit(0, &tctx->task_state))
return 0;

if (!task_work_add(tsk, &tctx->task_work, notify))
return 0;

/*
* Slow path - we failed, find and delete work. if the work is not
* in the list, it got run and we're fine.
*/
ret = 0;
spin_lock(&tctx->task_lock);
wq_list_for_each(node, prev, &tctx->task_list) {
if (&req->io_task_work.node == node) {
wq_list_del(&tctx->task_list, node, prev);
ret = 1;
break;
}
}
spin_unlock(&tctx->task_lock);
clear_bit(0, &tctx->task_state);
return ret;
}

static int io_req_task_work_add(struct io_kiocb *req)
{
struct task_struct *tsk = req->task;
Expand All @@ -2149,15 +2232,15 @@ static int io_req_task_work_add(struct io_kiocb *req)
if (!(ctx->flags & IORING_SETUP_SQPOLL))
notify = TWA_SIGNAL;

ret = task_work_add(tsk, &req->task_work, notify);
ret = io_task_work_add(tsk, req, notify);
if (!ret)
wake_up_process(tsk);

return ret;
}

static void io_req_task_work_add_fallback(struct io_kiocb *req,
void (*cb)(struct callback_head *))
task_work_func_t cb)
{
struct task_struct *tsk = io_wq_get_task(req->ctx->io_wq);

Expand Down Expand Up @@ -2216,7 +2299,7 @@ static void io_req_task_queue(struct io_kiocb *req)
{
int ret;

init_task_work(&req->task_work, io_req_task_submit);
req->task_work.func = io_req_task_submit;
percpu_ref_get(&req->ctx->refs);

ret = io_req_task_work_add(req);
Expand Down Expand Up @@ -2347,7 +2430,7 @@ static void io_free_req_deferred(struct io_kiocb *req)
{
int ret;

init_task_work(&req->task_work, io_put_req_deferred_cb);
req->task_work.func = io_put_req_deferred_cb;
ret = io_req_task_work_add(req);
if (unlikely(ret))
io_req_task_work_add_fallback(req, io_put_req_deferred_cb);
Expand Down Expand Up @@ -3392,7 +3475,7 @@ static int io_async_buf_func(struct wait_queue_entry *wait, unsigned mode,
req->rw.kiocb.ki_flags &= ~IOCB_WAITQ;
list_del_init(&wait->entry);

init_task_work(&req->task_work, io_req_task_submit);
req->task_work.func = io_req_task_submit;
percpu_ref_get(&req->ctx->refs);

/* submit ref gets dropped, acquire a new one */
Expand Down Expand Up @@ -5083,7 +5166,7 @@ static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,
list_del_init(&poll->wait.entry);

req->result = mask;
init_task_work(&req->task_work, func);
req->task_work.func = func;
percpu_ref_get(&req->ctx->refs);

/*
Expand Down Expand Up @@ -8086,6 +8169,10 @@ static int io_uring_alloc_task_context(struct task_struct *task)
io_init_identity(&tctx->__identity);
tctx->identity = &tctx->__identity;
task->io_uring = tctx;
spin_lock_init(&tctx->task_lock);
INIT_WQ_LIST(&tctx->task_list);
tctx->task_state = 0;
init_task_work(&tctx->task_work, tctx_task_work);
return 0;
}

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14 changes: 14 additions & 0 deletions include/linux/io_uring.h
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,15 @@ struct io_identity {
refcount_t count;
};

struct io_wq_work_node {
struct io_wq_work_node *next;
};

struct io_wq_work_list {
struct io_wq_work_node *first;
struct io_wq_work_node *last;
};

struct io_uring_task {
/* submission side */
struct xarray xa;
Expand All @@ -32,6 +41,11 @@ struct io_uring_task {
struct io_identity *identity;
atomic_t in_idle;
bool sqpoll;

spinlock_t task_lock;
struct io_wq_work_list task_list;
unsigned long task_state;
struct callback_head task_work;
};

#if defined(CONFIG_IO_URING)
Expand Down

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