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 <[email protected]>
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.
