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Re: [Qemu-devel] [PATCH 1/2] cpu: Provide vcpu throttling interface


From: Andrey Korolyov
Subject: Re: [Qemu-devel] [PATCH 1/2] cpu: Provide vcpu throttling interface
Date: Mon, 1 Jun 2015 18:23:58 +0300

On Mon, Jun 1, 2015 at 6:17 PM, Jason J. Herne
<address@hidden> wrote:
> Provide a method to throttle guest cpu execution. CPUState is augmented with
> timeout controls and throttle start/stop functions. To throttle the guest cpu
> the caller simply has to call the throttle start function and provide a ratio 
> of
> sleep time to normal execution time.
>
> Signed-off-by: Jason J. Herne <address@hidden>
> Reviewed-by: Matthew Rosato <address@hidden>
> ---
>  cpus.c            | 62 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  include/qom/cpu.h | 46 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 108 insertions(+)
>
> diff --git a/cpus.c b/cpus.c
> index de6469f..7568357 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -64,6 +64,9 @@
>
>  #endif /* CONFIG_LINUX */
>
> +/* Number of ms between cpu throttle operations */
> +#define CPU_THROTTLE_TIMESLICE 10
> +
>  static CPUState *next_cpu;
>  int64_t max_delay;
>  int64_t max_advance;
> @@ -919,6 +922,65 @@ static void qemu_kvm_wait_io_event(CPUState *cpu)
>      qemu_wait_io_event_common(cpu);
>  }
>
> +static void cpu_throttle_thread(void *opq)
> +{
> +    CPUState *cpu = (CPUState *)opq;
> +    long sleeptime_ms = (long)(cpu->throttle_ratio * CPU_THROTTLE_TIMESLICE);
> +
> +    /* Stop the timer if needed */
> +    if (cpu->throttle_timer_stop) {
> +        timer_del(cpu->throttle_timer);
> +        timer_free(cpu->throttle_timer);
> +        cpu->throttle_timer = NULL;
> +        return;
> +    }
> +
> +    qemu_mutex_unlock_iothread();
> +    g_usleep(sleeptime_ms * 1000); /* Convert ms to us for usleep call */
> +    qemu_mutex_lock_iothread();
> +
> +    timer_mod(cpu->throttle_timer, qemu_clock_get_ms(QEMU_CLOCK_REALTIME) +
> +                                   CPU_THROTTLE_TIMESLICE);
> +}
> +
> +static void cpu_throttle_timer_pop(void *opq)
> +{
> +    CPUState *cpu = (CPUState *)opq;
> +
> +    async_run_on_cpu(cpu, cpu_throttle_thread, cpu);
> +}
> +
> +void cpu_throttle_start(CPUState *cpu, float throttle_ratio)
> +{
> +    assert(throttle_ratio > 0);
> +    cpu->throttle_ratio = throttle_ratio;
> +
> +    if (!cpu_throttle_active(cpu)) {
> +        cpu->throttle_timer = timer_new_ms(QEMU_CLOCK_REALTIME,
> +                                           cpu_throttle_timer_pop, cpu);
> +        timer_mod(cpu->throttle_timer, 
> qemu_clock_get_ms(QEMU_CLOCK_REALTIME) +
> +                                       CPU_THROTTLE_TIMESLICE);
> +        cpu->throttle_timer_stop = false;
> +    }
> +}
> +
> +void cpu_throttle_stop(CPUState *cpu)
> +{
> +    assert(cpu_throttle_active(cpu));
> +    cpu->throttle_timer_stop = true;
> +}
> +
> +bool cpu_throttle_active(CPUState *cpu)
> +{
> +    return (cpu->throttle_timer != NULL);
> +}
> +
> +float cpu_throttle_get_ratio(CPUState *cpu)
> +{
> +    assert(cpu_throttle_active(cpu));
> +    return cpu->throttle_ratio;
> +}
> +
>  static void *qemu_kvm_cpu_thread_fn(void *arg)
>  {
>      CPUState *cpu = arg;
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index 39f0f19..9d16e6a 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -310,6 +310,11 @@ struct CPUState {
>      uint32_t can_do_io;
>      int32_t exception_index; /* used by m68k TCG */
>
> +    /* vcpu throttling controls */
> +    QEMUTimer *throttle_timer;
> +    bool throttle_timer_stop;
> +    float throttle_ratio;
> +
>      /* Note that this is accessed at the start of every TB via a negative
>         offset from AREG0.  Leave this field at the end so as to make the
>         (absolute value) offset as small as possible.  This reduces code
> @@ -553,6 +558,47 @@ CPUState *qemu_get_cpu(int index);
>   */
>  bool cpu_exists(int64_t id);
>
> +/**
> + * cpu_throttle_start:
> + * @cpu: The vcpu to throttle
> + *
> + * Throttles a vcpu by forcing it to sleep. The duration of the sleep is a
> + * ratio of sleep time to running time. A ratio of 1.0 corresponds to a 50%
> + * duty cycle (example: 10ms sleep for every 10ms awake).
> + *
> + * cpu_throttle_start can be called as needed to adjust the throttle ratio.
> + * Once the throttling starts, it will remain in effect until 
> cpu_throttle_stop
> + * is called.
> + */
> +void cpu_throttle_start(CPUState *cpu, float throttle_ratio);
> +
> +/**
> + * cpu_throttle_stop:
> + * @cpu: The vcpu to stop throttling
> + *
> + * Stops the vcpu throttling started by cpu_throttle_start.
> + */
> +void cpu_throttle_stop(CPUState *cpu);
> +
> +/**
> + * cpu_throttle_active:
> + * @cpu: The vcpu to check
> + *
> + * Returns %true if this vcpu is currently being throttled, %false otherwise.
> + */
> +bool cpu_throttle_active(CPUState *cpu);
> +
> +/**
> + * cpu_throttle_get_ratio:
> + * @cpu: The vcpu whose throttle ratio to return.
> + *
> + * Returns the ratio being used to throttle this vcpu. See cpu_throttle_start
> + * for details.
> + *
> + * Returns The ratio being used to throttle this vcpu.
> + */
> +float cpu_throttle_get_ratio(CPUState *cpu);
> +
>  #ifndef CONFIG_USER_ONLY
>
>  typedef void (*CPUInterruptHandler)(CPUState *, int);
> --
> 1.9.1
>
>

Thanks Jason, this patch would be quite interesting as it eliminates
slight overhead from scheduler when cgroups are actively used for same
task (~5% for per-vcpu cgroup layout simular to libvirt`s one for
guest perf numa bench). Are you planning to add wakeup frequency
throttler as well to same interface?



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