Re: [Qemu-devel] [PATCH] Limit ptimer rate to something achievable

[Anthony Liguori]

On 04/20/2012 12:32 AM, Peter Chubb wrote:
> If a guest sets very short timeouts, and asks for a timer to be reloaded on
> timeout, QEMU can go to 100%CPU utilisation and become unresponsive,
> as it is spending all its time generating timeout interrupts.  On real
> hardware this doesn't matter, as the interrupts are just coalesced,
> and the effect is to have the interrupt asserted all the time.
>
> This patch is a band-aid, that prevents timeouts less than 10
> microseconds from being set.  10 microseconds is a limit that was
> determined empirically on a variety of machines as the shortest that
> allowed QEMU to pick up a control-a c sequence to get at the monitor.
>
> Reported-by: Anna Lyons<address@hidden>
> Signed-off-by: Peter Chubb<address@hidden>

Applied.  Thanks.

Regards,

Anthony Liguori

> ---
>   hw/ptimer.c |   13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> Index: qemu-working/hw/ptimer.c
> ===================================================================
> --- qemu-working.orig/hw/ptimer.c       2012-04-20 15:09:09.317922659 +1000
> +++ qemu-working/hw/ptimer.c    2012-04-20 15:30:42.108486207 +1000
> @@ -180,6 +180,19 @@ void ptimer_set_freq(ptimer_state *s, ui
>      count = limit.  */
>   void ptimer_set_limit(ptimer_state *s, uint64_t limit, int reload)
>   {
> +    /*
> +     * Artificially limit timeout rate to something
> +     * achievable under QEMU.  Otherwise, QEMU spends all
> +     * its time generating timer interrupts, and there
> +     * is no forward progress.
> +     * About ten microseconds is the fastest that really works
> +     * on the current generation of host machines.
> +     */
> +
> +    if (limit * s->period<  10000&&  s->period) {
> +        limit = 10000 / s->period;
> +    }
> +
>       s->limit = limit;
>       if (reload)
>           s->delta = limit;
> --
> Dr Peter Chubb                                  peter.chubb AT nicta.com.au
> http://www.ssrg.nicta.com.au          Software Systems Research Group/NICTA