Re: [Qemu-devel] [PATCH] mainloop.c: Keep unlocking BQL during busy-wait spin-out

[Peter Crosthwaite]

On Mon, Apr 15, 2013 at 11:08 PM, Anthony Liguori <address@hidden> wrote:
> Peter Crosthwaite <address@hidden> writes:
>
>> Modify Anthony's starvation detection logic to keep the BQL unlocked
>> until the starvation condition goes away. Otherwise the counter has to
>> count up to 1000 for each needed iteration until the busy-wait is
>> lifted.
>>
>> Reset the counter back to zero once glib_pollfds_fill returns with a
>> non-zero timout, (indicating a return to normality). The 1000 iteration
>> wait now only happens once on the transition from normal operation to
>> busy-wait starvation.
>>
>> Anthony's original patch fixed the serial paste bug, but this patch is
>> also needed to restore performance.
>>
>> Signed-off-by: Peter Crosthwaite <address@hidden>
>
> I'm going through patches for 1.5 candidates.
>
> I believe the paste performance issue has been resolved now and this
> patch is no longer needed.  I can't find a definitive statement on the
> list for that though.
>
> Peter, can you confirm?
>

Paos patches cleared it all up for me, so this patch is not needed for
the paste bug specifically.

Regards,
Peter

> Regards,
>
> Anthony Liguori
>
>> ---
>>  main-loop.c |   20 ++++++++++----------
>>  1 files changed, 10 insertions(+), 10 deletions(-)
>>
>> diff --git a/main-loop.c b/main-loop.c
>> index f46aece..93d2917 100644
>> --- a/main-loop.c
>> +++ b/main-loop.c
>> @@ -200,10 +200,13 @@ static int os_host_main_loop_wait(uint32_t 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.
>> +     * print a message to the screen.  If we run into this condition, unlock
>> +     * the BQL until a non-zero timout is given (indicating the starvation
>> +     * issue has gome away).
>>       */
>> -    if (spin_counter > MAX_MAIN_LOOP_SPIN) {
>> +    if (timeout > 0) {
>> +        spin_counter = 0;
>> +    } else if (spin_counter > MAX_MAIN_LOOP_SPIN) {
>>          static bool notified;
>>
>>          if (!notified) {
>> @@ -212,21 +215,18 @@ static int os_host_main_loop_wait(uint32_t timeout)
>>                      MAX_MAIN_LOOP_SPIN);
>>              notified = true;
>>          }
>> -
>> -        timeout = 1;
>>      }
>>
>> -    if (timeout > 0) {
>> -        spin_counter = 0;
>> +    if (timeout > 0 || spin_counter > MAX_MAIN_LOOP_SPIN) {
>>          qemu_mutex_unlock_iothread();
>> -    } else {
>> -        spin_counter++;
>>      }
>>
>>      ret = g_poll((GPollFD *)gpollfds->data, gpollfds->len, timeout);
>>
>> -    if (timeout > 0) {
>> +    if (timeout > 0 || spin_counter > MAX_MAIN_LOOP_SPIN) {
>>          qemu_mutex_lock_iothread();
>> +    } else {
>> +        spin_counter++;
>>      }
>>
>>      glib_pollfds_poll();
>> --
>> 1.7.0.4
>
>