Re: [PATCH RFC v2 1/3] aio-poll: avoid unnecessary polling time computation

[JAEHOON KIM <jhkim@linux.ibm.com>]

On 3/25/2026 12:22 PM, Stefan Hajnoczi wrote:
> On Mon, Mar 23, 2026 at 08:54:49AM -0500, Jaehoon Kim wrote:
>> Nodes are no longer added to poll_aio_handlers when adaptive polling is
>> disabled, preventing unnecessary try_poll_mode() calls. Additionally,
>> aio_poll() skips try_poll_mode() when timeout is 0.
> Skipping when timeout is 0 seems risky to me. VIRTIO devices disable
> guest kicks when polling mode is started. When aio_poll(ctx,
> blocking=false) is called, we will skip polling and
> ctx->fdmon_ops->need_wait(ctx) won't detect an event either. aio_poll()
> will return without noticing that the VIRTIO device's AioHandler is
> ready.
>
> Is skipping when timeout 0 necessary for performance or can it be
> dropped from the patch?
>
> Aside from this:
>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

Thank you for the review!
Removing the (timeout != 0) check does not cause any issue on my side,
so I plan to remove it in the next version.

However, I'd like to clarify: when timeout is 0, is there any concern
in the try_poll_mode function below that I might be missing?
I just want to make sure I understand correctly.

static bool try_poll_mode(AioContext *ctx, AioHandlerList *ready_list,
                           int64_t *timeout)
{
     AioHandler *node;
     int64_t max_ns;

     if (QLIST_EMPTY_RCU(&ctx->poll_aio_handlers)) {
         return false;
     }

     max_ns = 0;
     QLIST_FOREACH(node, &ctx->poll_aio_handlers, node_poll) {
         max_ns = MAX(max_ns, node->poll.ns);
     }
     max_ns = qemu_soonest_timeout(*timeout, max_ns);

     if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
         /*
          * Enable poll mode. It pairs with the poll_set_started() in
          * aio_poll() which disables poll mode.
          */
         poll_set_started(ctx, ready_list, true);


     .....


Regards,
Jaehoon Kim.