Re: [PATCH 6.8 015/715] io_uring: remove unconditional looping in local task_work handling
From: Jiri Slaby
Date: Tue Apr 02 2024 - 04:12:20 EST
On 24. 03. 24, 23:23, Sasha Levin wrote:
From: Jens Axboe <axboe@xxxxxxxxx>
[ Upstream commit 9fe3eaea4a3530ca34a8d8ff00b1848c528789ca ]
If we have a ton of notifications coming in, we can be looping in here
for a long time. This can be problematic for various reasons, mostly
because we can starve userspace. If the application is waiting on N
events, then only re-run if we need more events.
This commit breaks test/recv-multishot.c from liburing:
early error: res 4
test stream=1 wait_each=0 recvmsg=0 early_error=0 defer=1 failed
The behaviour is the same in 6.9-rc2 (which contains the commit too).
Reverting the commit on the top of 6.8.2 makes it pass again.
Should the test be updated or is the commit wrong?
Thanks.
Fixes: c0e0d6ba25f1 ("io_uring: add IORING_SETUP_DEFER_TASKRUN")
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
io_uring/io_uring.c | 44 +++++++++++++++++++++++++++++---------------
1 file changed, 29 insertions(+), 15 deletions(-)
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 5233a20d01b54..39dfb83dc9fc4 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1389,7 +1389,20 @@ static void __cold io_move_task_work_from_local(struct io_ring_ctx *ctx)
}
}
-static int __io_run_local_work(struct io_ring_ctx *ctx, struct io_tw_state *ts)
+static bool io_run_local_work_continue(struct io_ring_ctx *ctx, int events,
+ int min_events)
+{
+ if (llist_empty(&ctx->work_llist))
+ return false;
+ if (events < min_events)
+ return true;
+ if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
+ atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
+ return false;
+}
+
+static int __io_run_local_work(struct io_ring_ctx *ctx, struct io_tw_state *ts,
+ int min_events)
{
struct llist_node *node;
unsigned int loops = 0;
@@ -1418,18 +1431,20 @@ static int __io_run_local_work(struct io_ring_ctx *ctx, struct io_tw_state *ts)
}
loops++;
- if (!llist_empty(&ctx->work_llist))
+ if (io_run_local_work_continue(ctx, ret, min_events))
goto again;
if (ts->locked) {
io_submit_flush_completions(ctx);
- if (!llist_empty(&ctx->work_llist))
+ if (io_run_local_work_continue(ctx, ret, min_events))
goto again;
}
+
trace_io_uring_local_work_run(ctx, ret, loops);
return ret;
}
-static inline int io_run_local_work_locked(struct io_ring_ctx *ctx)
+static inline int io_run_local_work_locked(struct io_ring_ctx *ctx,
+ int min_events)
{
struct io_tw_state ts = { .locked = true, };
int ret;
@@ -1437,20 +1452,20 @@ static inline int io_run_local_work_locked(struct io_ring_ctx *ctx)
if (llist_empty(&ctx->work_llist))
return 0;
- ret = __io_run_local_work(ctx, &ts);
+ ret = __io_run_local_work(ctx, &ts, min_events);
/* shouldn't happen! */
if (WARN_ON_ONCE(!ts.locked))
mutex_lock(&ctx->uring_lock);
return ret;
}
-static int io_run_local_work(struct io_ring_ctx *ctx)
+static int io_run_local_work(struct io_ring_ctx *ctx, int min_events)
{
struct io_tw_state ts = {};
int ret;
ts.locked = mutex_trylock(&ctx->uring_lock);
- ret = __io_run_local_work(ctx, &ts);
+ ret = __io_run_local_work(ctx, &ts, min_events);
if (ts.locked)
mutex_unlock(&ctx->uring_lock);
@@ -1646,7 +1661,7 @@ static int io_iopoll_check(struct io_ring_ctx *ctx, long min)
io_task_work_pending(ctx)) {
u32 tail = ctx->cached_cq_tail;
- (void) io_run_local_work_locked(ctx);
+ (void) io_run_local_work_locked(ctx, min);
if (task_work_pending(current) ||
wq_list_empty(&ctx->iopoll_list)) {
@@ -2489,7 +2504,7 @@ int io_run_task_work_sig(struct io_ring_ctx *ctx)
{
if (!llist_empty(&ctx->work_llist)) {
__set_current_state(TASK_RUNNING);
- if (io_run_local_work(ctx) > 0)
+ if (io_run_local_work(ctx, INT_MAX) > 0)
return 0;
}
if (io_run_task_work() > 0)
@@ -2557,7 +2572,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
if (!io_allowed_run_tw(ctx))
return -EEXIST;
if (!llist_empty(&ctx->work_llist))
- io_run_local_work(ctx);
+ io_run_local_work(ctx, min_events);
io_run_task_work();
io_cqring_overflow_flush(ctx);
/* if user messes with these they will just get an early return */
@@ -2595,11 +2610,10 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
trace_io_uring_cqring_wait(ctx, min_events);
do {
+ int nr_wait = (int) iowq.cq_tail - READ_ONCE(ctx->rings->cq.tail);
unsigned long check_cq;
if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
- 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 {
@@ -2618,7 +2632,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
*/
io_run_task_work();
if (!llist_empty(&ctx->work_llist))
- io_run_local_work(ctx);
+ io_run_local_work(ctx, nr_wait);
/*
* Non-local task_work will be run on exit to userspace, but
@@ -3273,7 +3287,7 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
if ((ctx->flags & IORING_SETUP_DEFER_TASKRUN) &&
io_allowed_defer_tw_run(ctx))
- ret |= io_run_local_work(ctx) > 0;
+ ret |= io_run_local_work(ctx, INT_MAX) > 0;
ret |= io_cancel_defer_files(ctx, task, cancel_all);
mutex_lock(&ctx->uring_lock);
ret |= io_poll_remove_all(ctx, task, cancel_all);
@@ -3635,7 +3649,7 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
* it should handle ownership problems if any.
*/
if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
- (void)io_run_local_work_locked(ctx);
+ (void)io_run_local_work_locked(ctx, min_complete);
}
mutex_unlock(&ctx->uring_lock);
}
--
js
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