Re: [PATCH] multifd: Make the main thread yield periodically to the main loop

[Peter Xu <peterx@redhat.com>]

On Thu, Aug 07, 2025 at 10:41:17AM +0800, yong.huang@smartx.com wrote:
> diff --git a/migration/multifd.c b/migration/multifd.c
> index b255778855..aca0aeb341 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -1228,6 +1228,16 @@ void multifd_recv_sync_main(void)
>              }
>          }
>          trace_multifd_recv_sync_main_signal(p->id);
> +        do {
> +            if (qemu_sem_timedwait(&multifd_recv_state->sem_sync, 10000) == 0) {
> +                break;
> +            }
> +            if (qemu_in_coroutine()) {
> +                aio_co_schedule(qemu_get_current_aio_context(),
> +                                qemu_coroutine_self());
> +                qemu_coroutine_yield();
> +            }
> +        } while (1);

I still think either yank or fixing migrate_cancel is the way to go, but
when staring at this change.. I don't think I understand this patch at all.

It timedwait()s on the sem_sync that we just consumed.  Do you at least
need to remove the ones above this piece of code to not hang forever?

    for (i = 0; i < thread_count; i++) {
        trace_multifd_recv_sync_main_wait(i);
        qemu_sem_wait(&multifd_recv_state->sem_sync);
    }

>          qemu_sem_post(&p->sem_sync);
>      }
>      trace_multifd_recv_sync_main(multifd_recv_state->packet_num);
> -- 
> 2.27.0
> 

-- 
Peter Xu