Re: [Qemu-devel] [PATCH] main-loop: Don't lock starve io-threads when main_loop_tlg has pending events

[Alex Bligh]

On 9 Oct 2013, at 19:28, Alex Bligh wrote:

>> 
>> static void audio_reset_timer (AudioState *s)
>> {
>>   if (audio_is_timer_needed ()) {
>>       timer_mod (s->ts, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + 1);
>>   }
>>   else {
>>       timer_del (s->ts);
>>   }
>> }
>> 
>> static void audio_timer (void *opaque)
>> {
>>   audio_run ("timer");
>>   audio_reset_timer (opaque);
>> }
>> 
>> Note how that is using a timer which expires every freaking nano second,
>> I think it is very likely that is the culprit.
> 
> Indeed. I am hoping that it is not my automated perl conversion code that
> did that, because if it is, it may have broken other stuff :-/
> 
> Thanks for finding this - let me see whether the bug existed before
> my automated changes commit.

I think this was broken prior to my changes. Here's audio/audio.c before
my changes:

static void audio_reset_timer (AudioState *s)
{
    if (audio_is_timer_needed ()) {
        qemu_mod_timer (s->ts, qemu_get_clock_ns (vm_clock) + 1);
    }
    else {
        qemu_del_timer (s->ts);
    }
}


Now qemu_get_clock_ns can only return something in nanoseconds, so it's
adding 1 nanosecond. This is thus either broken because:
  1. ts->scale is in nanoseconds, in which case that timer expires in
     one nano-second's time; or
  2. ts->scale is not in nanoseconds, in which case nanosecond VM clock
     is going to be a huge time in the future, and it's never going
     to expire.

I wonder whether it's meant to be 1 millisecond or 1 microsecond?

Whatever, it looks like this is suitable for an individual patch to
audio/audio.c

-- 
Alex Bligh