Re: [Qemu-block] [PATCH] main-loop: drop spin_counter

[Paolo Bonzini]

On 30/05/2018 21:42, Stefan Hajnoczi wrote:
> Commit d759c951f3287fad04210a52f2dc93f94cf58c7f ("replay: push
> replay_mutex_lock up the call tree") removed the !timeout lock
> optimization in the main loop.
> 
> The idea of the optimization was to avoid ping-pongs between threads by
> keeping the Big QEMU Lock held across non-blocking (!timeout) main loop
> iterations.
> 
> A warning is printed when the main loop spins without releasing BQL for
> long periods of time.  These warnings were supposed to aid debugging but
> in practice they just alarm users.  They are considered noise because
> the cause of spinning is not shown and is hard to find.
> 
> Now that the lock optimization has been removed, there is no danger of
> hogging the BQL.  Drop the spin counter and the infamous warning.
> 
> Signed-off-by: Stefan Hajnoczi <address@hidden>

Very good idea, at last! :)

Paolo

> ---
>  util/main-loop.c                 | 25 -------------------------
>  tests/qemu-iotests/common.filter |  1 -
>  2 files changed, 26 deletions(-)
> 
> diff --git a/util/main-loop.c b/util/main-loop.c
> index 992f9b0f34..affe0403c5 100644
> --- a/util/main-loop.c
> +++ b/util/main-loop.c
> @@ -222,36 +222,11 @@ static int os_host_main_loop_wait(int64_t timeout)
>  {
>      GMainContext *context = g_main_context_default();
>      int ret;
> -    static int spin_counter;
>  
>      g_main_context_acquire(context);
>  
>      glib_pollfds_fill(&timeout);
>  
> -    /* If the I/O thread is very busy or we are incorrectly busy waiting in
> -     * the I/O thread, this can lead to starvation of the BQL such that the
> -     * VCPU threads never run.  To make sure we can detect the later case,
> -     * print a message to the screen.  If we run into this condition, create
> -     * a fake timeout in order to give the VCPU threads a chance to run.
> -     */
> -    if (!timeout && (spin_counter > MAX_MAIN_LOOP_SPIN)) {
> -        static bool notified;
> -
> -        if (!notified && !qtest_enabled() && !qtest_driver()) {
> -            warn_report("I/O thread spun for %d iterations",
> -                        MAX_MAIN_LOOP_SPIN);
> -            notified = true;
> -        }
> -
> -        timeout = SCALE_MS;
> -    }
> -
> -
> -    if (timeout) {
> -        spin_counter = 0;
> -    } else {
> -        spin_counter++;
> -    }
>      qemu_mutex_unlock_iothread();
>      replay_mutex_unlock();
>  
> diff --git a/tests/qemu-iotests/common.filter 
> b/tests/qemu-iotests/common.filter
> index f08ee55046..2031e353a5 100644
> --- a/tests/qemu-iotests/common.filter
> +++ b/tests/qemu-iotests/common.filter
> @@ -77,7 +77,6 @@ _filter_qemu()
>  {
>      sed -e "s#\\(^\\|(qemu) \\)$(basename $QEMU_PROG):#\1QEMU_PROG:#" \
>          -e 's#^QEMU [0-9]\+\.[0-9]\+\.[0-9]\+ monitor#QEMU X.Y.Z monitor#' \
> -        -e '/main-loop: WARNING: I\/O thread spun for [0-9]\+ iterations/d' \
>          -e $'s#\r##' # QEMU monitor uses \r\n line endings
>  }
>  
>