Re: [RFC] io_uring CQ ring backpressure

[Jann Horn <jannh@google.com>]

On Wed, Nov 6, 2019 at 5:23 PM Jens Axboe <axboe@kernel.dk> wrote:
> Currently we drop completion events, if the CQ ring is full. That's fine
> for requests with bounded completion times, but it may make it harder to
> use io_uring with networked IO where request completion times are
> generally unbounded. Or with POLL, for example, which is also unbounded.
>
> This patch adds IORING_SETUP_CQ_NODROP, which changes the behavior a bit
> for CQ ring overflows. First of all, it doesn't overflow the ring, it
> simply stores backlog of completions that we weren't able to put into
> the CQ ring. To prevent the backlog from growing indefinitely, if the
> backlog is non-empty, we apply back pressure on IO submissions. Any
> attempt to submit new IO with a non-empty backlog will get an -EBUSY
> return from the kernel.
>
> I think that makes for a pretty sane API in terms of how the application
> can handle it. With CQ_NODROP enabled, we'll never drop a completion
> event (well unless we're totally out of memory...), but we'll also not
> allow submissions with a completion backlog.
[...]
> +static void io_cqring_overflow(struct io_ring_ctx *ctx, u64 ki_user_data,
> +                              long res)
> +       __must_hold(&ctx->completion_lock)
> +{
> +       struct cqe_drop *drop;
> +
> +       if (!(ctx->flags & IORING_SETUP_CQ_NODROP)) {
> +log_overflow:
> +               WRITE_ONCE(ctx->rings->cq_overflow,
> +                               atomic_inc_return(&ctx->cached_cq_overflow));
> +               return;
> +       }
> +
> +       drop = kmalloc(sizeof(*drop), GFP_ATOMIC);
> +       if (!drop)
> +               goto log_overflow;
> +
> +       drop->user_data = ki_user_data;
> +       drop->res = res;
> +       list_add_tail(&drop->list, &ctx->cq_overflow_list);
> +}

This could potentially consume moderately large amounts of atomic
memory quickly and without any guarantee that the memory will be freed
anytime soon, right? That seems moderately bad. Is there no way to
e.g. pre-reserve memory for completion events, or something like that?