Re: [PATCH] main-loop: Avoid some unnecessary poll calls

[Ross Lagerwall <ross.lagerwall@citrix.com>]

On Mon, Feb 12, 2024 at 11:45 AM Ross Lagerwall
<ross.lagerwall@citrix.com> wrote:
>
> A common pattern is seen where a timer fires, the callback does some
> work, then rearms the timer which implicitly calls qemu_notify_event().
>
> qemu_notify_event() is supposed to interrupt the main loop's poll() by
> calling qemu_bh_schedule(). In the case that this is being called from a
> main loop callback, the main loop is already not waiting on poll() and
> instead it means the main loop does an addition iteration with a timeout
> of 0 to handle the bottom half wakeup, before once again polling with
> the expected timeout value.
>
> Detect this situation by skipping the qemu_bh_schedule() call if the
> default main context is currently owned by the caller. i.e. it is being
> called as part of a poll / timer callback. Adjust the scope of the main
> context acquire / release to cover the timer callbacks in
> qemu_clock_run_all_timers().
>
> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
> ---
>  util/main-loop.c | 24 +++++++++++++-----------
>  1 file changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/util/main-loop.c b/util/main-loop.c
> index a0386cfeb60c..a2afbb7d0e13 100644
> --- a/util/main-loop.c
> +++ b/util/main-loop.c
> @@ -145,10 +145,16 @@ AioContext *qemu_get_aio_context(void)
>
>  void qemu_notify_event(void)
>  {
> +    GMainContext *context;
> +
>      if (!qemu_aio_context) {
>          return;
>      }
> -    qemu_bh_schedule(qemu_notify_bh);
> +
> +    context = g_main_context_default();
> +    if (!g_main_context_is_owner(context)) {
> +        qemu_bh_schedule(qemu_notify_bh);
> +    }
>  }
>
>  static GArray *gpollfds;
> @@ -292,11 +298,8 @@ static void glib_pollfds_poll(void)
>
>  static int os_host_main_loop_wait(int64_t timeout)
>  {
> -    GMainContext *context = g_main_context_default();
>      int ret;
>
> -    g_main_context_acquire(context);
> -
>      glib_pollfds_fill(&timeout);
>
>      bql_unlock();
> @@ -309,8 +312,6 @@ static int os_host_main_loop_wait(int64_t timeout)
>
>      glib_pollfds_poll();
>
> -    g_main_context_release(context);
> -
>      return ret;
>  }
>  #else
> @@ -470,15 +471,12 @@ static int os_host_main_loop_wait(int64_t timeout)
>      fd_set rfds, wfds, xfds;
>      int nfds;
>
> -    g_main_context_acquire(context);
> -
>      /* XXX: need to suppress polling by better using win32 events */
>      ret = 0;
>      for (pe = first_polling_entry; pe != NULL; pe = pe->next) {
>          ret |= pe->func(pe->opaque);
>      }
>      if (ret != 0) {
> -        g_main_context_release(context);
>          return ret;
>      }
>
> @@ -538,8 +536,6 @@ static int os_host_main_loop_wait(int64_t timeout)
>          g_main_context_dispatch(context);
>      }
>
> -    g_main_context_release(context);
> -
>      return select_ret || g_poll_ret;
>  }
>  #endif
> @@ -559,6 +555,7 @@ void main_loop_poll_remove_notifier(Notifier *notify)
>
>  void main_loop_wait(int nonblocking)
>  {
> +    GMainContext *context = g_main_context_default();
>      MainLoopPoll mlpoll = {
>          .state = MAIN_LOOP_POLL_FILL,
>          .timeout = UINT32_MAX,
> @@ -586,7 +583,10 @@ void main_loop_wait(int nonblocking)
>                                        timerlistgroup_deadline_ns(
>                                            &main_loop_tlg));
>
> +    g_main_context_acquire(context);
> +
>      ret = os_host_main_loop_wait(timeout_ns);
> +
>      mlpoll.state = ret < 0 ? MAIN_LOOP_POLL_ERR : MAIN_LOOP_POLL_OK;
>      notifier_list_notify(&main_loop_poll_notifiers, &mlpoll);
>
> @@ -598,6 +598,8 @@ void main_loop_wait(int nonblocking)
>          icount_start_warp_timer();
>      }
>      qemu_clock_run_all_timers();
> +
> +    g_main_context_release(context);
>  }
>
>  /* Functions to operate on the main QEMU AioContext.  */
> --
> 2.43.0
>

Ping?

Ross