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io_uring: remove looping around handling traditional task_work

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A previous commit added looping around handling traditional task_work
as an optimization, and while that may seem like a good idea, it's also
possible to run into application starvation doing so. If the task_work
generation is bursty, we can get very deep task_work queues, and we can
end up looping in here for a very long time.

One immediately observable problem with that is handling network traffic
using provided buffers, where flooding incoming traffic and looping
task_work handling will very quickly lead to buffer starvation as we
keep running task_work rather than returning to the application so it
can handle the associated CQEs and also provide buffers back.

Fixes: 3a0c037b0e ("io_uring: batch task_work")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2024-01-30 07:00:47 -07:00
parent 8435c6f380
commit 592b480543
1 changed files with 7 additions and 38 deletions
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45
io_uring/io_uring.c
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@ -1175,12 +1175,11 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx, struct io_tw_state *ts)
static unsigned int handle_tw_list(struct llist_node *node,
struct io_ring_ctx **ctx,
struct io_tw_state *ts,
struct llist_node *last)
struct io_tw_state *ts)
{
unsigned int count = 0;
while (node && node != last) {
do {
struct llist_node *next = node->next;
struct io_kiocb *req = container_of(node, struct io_kiocb,
io_task_work.node);
@ -1204,7 +1203,7 @@ static unsigned int handle_tw_list(struct llist_node *node,
*ctx = NULL;
cond_resched();
}
}
} while (node);
return count;
}
@ -1223,22 +1222,6 @@ static inline struct llist_node *io_llist_xchg(struct llist_head *head,
return xchg(&head->first, new);
}
/**
* io_llist_cmpxchg - possibly swap all entries in a lock-less list
* @head: the head of lock-less list to delete all entries
* @old: expected old value of the first entry of the list
* @new: new entry as the head of the list
*
* perform a cmpxchg on the first entry of the list.
*/
static inline struct llist_node *io_llist_cmpxchg(struct llist_head *head,
struct llist_node *old,
struct llist_node *new)
{
return cmpxchg(&head->first, old, new);
}
static __cold void io_fallback_tw(struct io_uring_task *tctx, bool sync)
{
struct llist_node *node = llist_del_all(&tctx->task_list);
@ -1273,9 +1256,7 @@ 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 fake = {};
struct llist_node *node;
unsigned int loops = 0;
unsigned int count = 0;
if (unlikely(current->flags & PF_EXITING)) {
@ -1283,21 +1264,9 @@ void tctx_task_work(struct callback_head *cb)
return;
}
do {
loops++;
node = io_llist_xchg(&tctx->task_list, &fake);
count += handle_tw_list(node, &ctx, &ts, &fake);
/* skip expensive cmpxchg if there are items in the list */
if (READ_ONCE(tctx->task_list.first) != &fake)
continue;
if (ts.locked && !wq_list_empty(&ctx->submit_state.compl_reqs)) {
io_submit_flush_completions(ctx);
if (READ_ONCE(tctx->task_list.first) != &fake)
continue;
}
node = io_llist_cmpxchg(&tctx->task_list, &fake, NULL);
} while (node != &fake);
node = llist_del_all(&tctx->task_list);
if (node)
count = handle_tw_list(node, &ctx, &ts);
ctx_flush_and_put(ctx, &ts);
@ -1305,7 +1274,7 @@ void tctx_task_work(struct callback_head *cb)
if (unlikely(atomic_read(&tctx->in_cancel)))
io_uring_drop_tctx_refs(current);
trace_io_uring_task_work_run(tctx, count, loops);
trace_io_uring_task_work_run(tctx, count, 1);
}
static inline void io_req_local_work_add(struct io_kiocb *req, unsigned flags)