io_uring: flag SQPOLL busy condition to userspace

Now that we have backpressure, for SQPOLL, we have one more condition
that warrants flagging that the application needs to enter the kernel:
we failed to submit IO due to backpressure. Make sure we catch that
and flag it appropriately.

If we run into backpressure issues with the SQPOLL thread, flag it
as such to the application by setting IORING_SQ_NEED_WAKEUP. This will
cause the application to enter the kernel, and that will flush the
backlog and clear the condition.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index b709825023360e79edbde79c33b6b8d95d5098b4..912d2648f8db7ef0f8526bc8a9a945e9b7a68e82 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3118,16 +3118,16 @@ static int io_sq_thread(void *data)
        DEFINE_WAIT(wait);
        unsigned inflight;
        unsigned long timeout;
+       int ret;
 
        complete(&ctx->completions[1]);
 
        old_fs = get_fs();
        set_fs(USER_DS);
 
-       timeout = inflight = 0;
+       ret = timeout = inflight = 0;
        while (!kthread_should_park()) {
                unsigned int to_submit;
-               int ret;
 
                if (inflight) {
                        unsigned nr_events = 0;
@@ -3161,13 +3161,21 @@ static int io_sq_thread(void *data)
                }
 
                to_submit = io_sqring_entries(ctx);
-               if (!to_submit) {
+
+               /*
+                * If submit got -EBUSY, flag us as needing the application
+                * to enter the kernel to reap and flush events.
+                */
+               if (!to_submit || ret == -EBUSY) {
                        /*
                         * We're polling. If we're within the defined idle
                         * period, then let us spin without work before going
-                        * to sleep.
+                        * to sleep. The exception is if we got EBUSY doing
+                        * more IO, we should wait for the application to
+                        * reap events and wake us up.
                         */
-                       if (inflight || !time_after(jiffies, timeout)) {
+                       if (inflight ||
+                           (!time_after(jiffies, timeout) && ret != -EBUSY)) {
                                cond_resched();
                                continue;
                        }
@@ -3193,7 +3201,7 @@ static int io_sq_thread(void *data)
                        smp_mb();
 
                        to_submit = io_sqring_entries(ctx);
-                       if (!to_submit) {
+                       if (!to_submit || ret == -EBUSY) {
                                if (kthread_should_park()) {
                                        finish_wait(&ctx->sqo_wait, &wait);
                                        break;
@@ -4351,6 +4359,8 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
         */
        ret = 0;
        if (ctx->flags & IORING_SETUP_SQPOLL) {
+               if (!list_empty_careful(&ctx->cq_overflow_list))
+                       io_cqring_overflow_flush(ctx, false);
                if (flags & IORING_ENTER_SQ_WAKEUP)
                        wake_up(&ctx->sqo_wait);
                submitted = to_submit;