io_uring: remove unconditional looping in local task_work handling

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.

Fixes: c0e0d6ba25f1 ("io_uring: add IORING_SETUP_DEFER_TASKRUN")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 007cef9738eeeab97703a821ea3d5f7b98708ebe..d31e8b110de91292d3ba5e8d5ea6dd5da4623724 100644 (file)
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1386,7 +1386,20 @@ 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;
@@ -1414,18 +1427,20 @@ again:
        }
        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;
@@ -1433,20 +1448,20 @@ 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);
 
@@ -1642,7 +1657,7 @@ 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)) {
@@ -2486,7 +2501,7 @@ 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)
@@ -2554,7 +2569,7 @@ 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 */
@@ -2592,11 +2607,10 @@ 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 {
@@ -2615,7 +2629,7 @@ 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
@@ -3270,7 +3284,7 @@ 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);
@@ -3632,7 +3646,7 @@ 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);
        }