Re: [Qemu-devel] [PATCH v4 0/3] High downtime with 95+ throttle pct

[Paolo Bonzini <pbonzini@redhat.com>]

On 19/08/19 18:39, Paolo Bonzini wrote:
> On 15/08/19 11:13, Yury Kotov wrote:
>> Ping ping
> 
> Hi,
> 
> sorry for the delay, I was waiting for the 4.1 release.
> 
> I would like to make a small change so that preemption of QEMU does not
> result in overly long sleeps.  The following patch on top of yours computes
> the throttle-end time just once.  Of course you can still be unlucky if
> you are preempted at the wrong time, but the window becomes much smaller.

The new unit test is hanging for me on aarch64-softmmu.

Paolo

> diff --git a/cpus.c b/cpus.c
> index d091dbd..d7e2145 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -781,7 +781,7 @@ static void cpu_throttle_thread(CPUState *cpu, run_on_cpu_data opaque)
>  {
>      double pct;
>      double throttle_ratio;
> -    int64_t sleeptime_ns;
> +    int64_t sleeptime_ns, end;
>  
>      if (!cpu_throttle_get_percentage()) {
>          return;
> @@ -792,18 +792,17 @@ static void cpu_throttle_thread(CPUState *cpu, run_on_cpu_data opaque)
>      /* Add 1ns to fix double's rounding error (like 0.9999999...) */
>      sleeptime_ns = (int64_t)(throttle_ratio * CPU_THROTTLE_TIMESLICE_NS + 1);
>  
> -    while (sleeptime_ns >= SCALE_MS && !cpu->stop) {
> -        int64_t start, end;
> -        start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> -        qemu_cond_timedwait(cpu->halt_cond, &qemu_global_mutex,
> -                            sleeptime_ns / SCALE_MS);
> -        end = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
> -        sleeptime_ns -= end - start;
> -    }
> -    if (sleeptime_ns >= SCALE_US && !cpu->stop) {
> -        qemu_mutex_unlock_iothread();
> -        g_usleep(sleeptime_ns / SCALE_US);
> -        qemu_mutex_lock_iothread();
> +    end = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + sleeptime_ns;
> +    while (sleeptime_ns > 0 && !cpu->stop) {
> +        if (sleeptime_ns > SCALE_MS) {
> +            qemu_cond_timedwait(cpu->halt_cond, &qemu_global_mutex,
> +                                sleeptime_ns / SCALE_MS);
> +        } else {
> +            qemu_mutex_unlock_iothread();
> +            g_usleep(sleeptime_ns / SCALE_US);
> +            qemu_mutex_lock_iothread();
> +        }
> +        sleeptime_ns = end - qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
>      }
>      atomic_set(&cpu->throttle_thread_scheduled, 0);
>  }
>