Re: [PATCH RFC v1 1/3] aio-poll: avoid unnecessary polling time computation

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Jan 13, 2026 at 11:48:22AM -0600, 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.

Do these two changes need to be made together? If not, please split them
into two commits. This will make the commit descriptions easier to
understand.

> 
> This avoids iterating over all nodes to compute max_ns unnecessarily
> when polling is disabled or timeout is 0.
> 
> Signed-off-by: Jaehoon Kim <jhkim@linux.ibm.com>
> ---
>  util/aio-posix.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/util/aio-posix.c b/util/aio-posix.c
> index e24b955fd9..7ddf92a25f 100644
> --- a/util/aio-posix.c
> +++ b/util/aio-posix.c
> @@ -306,9 +306,8 @@ static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
>       * fdmon_supports_polling(), but only until the fd fires for the first
>       * time.
>       */
> -    if (!QLIST_IS_INSERTED(node, node_deleted) &&
> -        !QLIST_IS_INSERTED(node, node_poll) &&
> -        node->io_poll) {
> +    if (ctx->poll_max_ns && !QLIST_IS_INSERTED(node, node_deleted) &&
> +        !QLIST_IS_INSERTED(node, node_poll) && node->io_poll) {
>          trace_poll_add(ctx, node, node->pfd.fd, revents);
>          if (ctx->poll_started && node->io_poll_begin) {
>              node->io_poll_begin(node->opaque);
> @@ -630,7 +629,7 @@ static void adjust_polling_time(AioContext *ctx, AioPolledEvent *poll,
>  bool aio_poll(AioContext *ctx, bool blocking)
>  {
>      AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
> -    bool progress;
> +    bool progress = false;
>      bool use_notify_me;
>      int64_t timeout;
>      int64_t start = 0;
> @@ -655,7 +654,9 @@ bool aio_poll(AioContext *ctx, bool blocking)
>      }
>  
>      timeout = blocking ? aio_compute_timeout(ctx) : 0;
> -    progress = try_poll_mode(ctx, &ready_list, &timeout);
> +    if ((ctx->poll_max_ns != 0) && (timeout != 0)) {
> +        progress = try_poll_mode(ctx, &ready_list, &timeout);
> +    }
>      assert(!(timeout && progress));

Can you walk me through the timeout == 0 case when polling is active?

Further down in aio_poll():

  /* If polling is allowed, non-blocking aio_poll does not need the
   * system call---a single round of run_poll_handlers_once suffices.
   */
  if (timeout || ctx->fdmon_ops->need_wait(ctx)) {

My concern is that aio_poll(timeout=0) could return without polling or
waiting for fds when polling is active. Maybe I've missed something that
prevents this?

Stefan

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