unimore/kernel-hacking-2024-linux-steffo
unimore/kernel-hacking-2024-linux-steffo
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io_uring: lockless task list

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With networking use cases we see contention on the spinlock used to
protect the task_list when multiple threads try and add completions at once.
Instead we can use a lockless list, and assume that the first caller to
add to the list is responsible for kicking off task work.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220622134028.2013417-4-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Dylan Yudaken 2022-06-22 06:40:23 -07:00 committed by Jens Axboe
parent c34398a8c0
commit f88262e60b
4 changed files with 14 additions and 35 deletions
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2
include/linux/io_uring_types.h
View file

@ -428,7 +428,7 @@ typedef void (*io_req_tw_func_t)(struct io_kiocb *req, bool *locked);
struct io_task_work {
union {
struct io_wq_work_node node;
struct llist_node node;
struct llist_node fallback_node;
};
io_req_tw_func_t func;

38
io_uring/io_uring.c
View file

@ -986,11 +986,12 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx, bool *locked)
percpu_ref_put(&ctx->refs);
}
static void handle_tw_list(struct io_wq_work_node *node,
static void handle_tw_list(struct llist_node *node,
struct io_ring_ctx **ctx, bool *locked)
{
do {
struct io_wq_work_node *next = node->next;
struct llist_node *next = node->next;
struct io_kiocb *req = container_of(node, struct io_kiocb,
io_task_work.node);
@ -1014,23 +1015,11 @@ void tctx_task_work(struct callback_head *cb)
struct io_ring_ctx *ctx = NULL;
struct io_uring_task *tctx = container_of(cb, struct io_uring_task,
task_work);
struct llist_node *node = llist_del_all(&tctx->task_list);
while (1) {
struct io_wq_work_node *node;
spin_lock_irq(&tctx->task_lock);
node = tctx->task_list.first;
INIT_WQ_LIST(&tctx->task_list);
if (!node)
tctx->task_running = false;
spin_unlock_irq(&tctx->task_lock);
if (!node)
break;
if (node) {
handle_tw_list(node, &ctx, &uring_locked);
cond_resched();
if (data_race(!tctx->task_list.first) && uring_locked)
io_submit_flush_completions(ctx);
}
ctx_flush_and_put(ctx, &uring_locked);
@ -1044,16 +1033,10 @@ void io_req_task_work_add(struct io_kiocb *req)
{
struct io_uring_task *tctx = req->task->io_uring;
struct io_ring_ctx *ctx = req->ctx;
struct io_wq_work_node *node;
unsigned long flags;
struct llist_node *node;
bool running;
spin_lock_irqsave(&tctx->task_lock, flags);
wq_list_add_tail(&req->io_task_work.node, &tctx->task_list);
running = tctx->task_running;
if (!running)
tctx->task_running = true;
spin_unlock_irqrestore(&tctx->task_lock, flags);
running = !llist_add(&req->io_task_work.node, &tctx->task_list);
/* task_work already pending, we're done */
if (running)
@ -1065,11 +1048,8 @@ void io_req_task_work_add(struct io_kiocb *req)
if (likely(!task_work_add(req->task, &tctx->task_work, ctx->notify_method)))
return;
spin_lock_irqsave(&tctx->task_lock, flags);
tctx->task_running = false;
node = tctx->task_list.first;
INIT_WQ_LIST(&tctx->task_list);
spin_unlock_irqrestore(&tctx->task_lock, flags);
node = llist_del_all(&tctx->task_list);
while (node) {
req = container_of(node, struct io_kiocb, io_task_work.node);

3
io_uring/tctx.c
View file

@ -86,8 +86,7 @@ __cold int io_uring_alloc_task_context(struct task_struct *task,
atomic_set(&tctx->in_idle, 0);
atomic_set(&tctx->inflight_tracked, 0);
task->io_uring = tctx;
spin_lock_init(&tctx->task_lock);
INIT_WQ_LIST(&tctx->task_list);
init_llist_head(&tctx->task_list);
init_task_work(&tctx->task_work, tctx_task_work);
return 0;
}

6
io_uring/tctx.h
View file

@ -1,5 +1,7 @@
// SPDX-License-Identifier: GPL-2.0
#include <linux/llist.h>
/*
* Arbitrary limit, can be raised if need be
*/
@ -19,9 +21,7 @@ struct io_uring_task {
struct percpu_counter inflight;
struct { /* task_work */
spinlock_t task_lock;
bool task_running;
struct io_wq_work_list task_list;
struct llist_head task_list;
struct callback_head task_work;
} ____cacheline_aligned_in_smp;
};