Re: [Qemu-devel] [PATCH v5 14/33] tcg: add kick timer for single-threaded vCPU emulation

[KONRAD Frederic]

Hi Alex,

This is nice! Do we actually need to do qemu_cpu_kick_no_halt() in the
timer handler?

Thanks,
Fred

Le 27/10/2016 à 17:10, Alex Bennée a écrit :
> Currently we rely on the side effect of the main loop grabbing the
> iothread_mutex to give any long running basic block chains a kick to
> ensure the next vCPU is scheduled. As this code is being re-factored and
> rationalised we now do it explicitly here.
>
> Signed-off-by: Alex Bennée <address@hidden>
>
> ---
> v2
>   - re-base fixes
>   - get_ticks_per_sec() -> NANOSECONDS_PER_SEC
> v3
>   - add define for TCG_KICK_FREQ
>   - fix checkpatch warning
> v4
>   - wrap next calc in inline qemu_tcg_next_kick() instead of macro
> v5
>   - move all kick code into own section
>   - use global for timer
>   - add helper functions to start/stop timer
>   - stop timer when all cores paused
> ---
>  cpus.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 60 insertions(+)
>
> diff --git a/cpus.c b/cpus.c
> index aedec7c..ad4ab68 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -735,6 +735,52 @@ void configure_icount(QemuOpts *opts, Error **errp)
>  }
>
>  /***********************************************************/
> +/* TCG vCPU kick timer
> + *
> + * The kick timer is responsible for moving single threaded vCPU
> + * emulation on to the next vCPU. If more than one vCPU is running a
> + * timer event with force a cpu->exit so the next vCPU can get
> + * scheduled.
> + *
> + * The timer is removed if all vCPUs are idle and restarted again once
> + * idleness is complete.
> + */
> +
> +static QEMUTimer *tcg_kick_vcpu_timer;
> +
> +static void qemu_cpu_kick_no_halt(void);
> +
> +#define TCG_KICK_PERIOD (NANOSECONDS_PER_SECOND / 10)
> +
> +static inline int64_t qemu_tcg_next_kick(void)
> +{
> +    return qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + TCG_KICK_PERIOD;
> +}
> +
> +static void kick_tcg_thread(void *opaque)
> +{
> +    timer_mod(tcg_kick_vcpu_timer, qemu_tcg_next_kick());
> +    qemu_cpu_kick_no_halt();
> +}
> +
> +static void start_tcg_kick_timer(void)
> +{
> +    if (!tcg_kick_vcpu_timer && CPU_NEXT(first_cpu)) {
> +        tcg_kick_vcpu_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,  
> kick_tcg_thread, NULL);
> +        timer_mod(tcg_kick_vcpu_timer, qemu_tcg_next_kick());
> +    }
> +}
> +
> +static void stop_tcg_kick_timer(void)
> +{
> +    if (tcg_kick_vcpu_timer) {
> +        timer_del(tcg_kick_vcpu_timer);
> +        tcg_kick_vcpu_timer = NULL;
> +    }
> +}
> +
> +
> +/***********************************************************/
>  void hw_error(const char *fmt, ...)
>  {
>      va_list ap;
> @@ -988,9 +1034,12 @@ static void qemu_wait_io_event_common(CPUState *cpu)
>  static void qemu_tcg_wait_io_event(CPUState *cpu)
>  {
>      while (all_cpu_threads_idle()) {
> +        stop_tcg_kick_timer();
>          qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
>      }
>
> +    start_tcg_kick_timer();
> +
>      while (iothread_requesting_mutex) {
>          qemu_cond_wait(&qemu_io_proceeded_cond, &qemu_global_mutex);
>      }
> @@ -1178,6 +1227,15 @@ static void deal_with_unplugged_cpus(void)
>      }
>  }
>
> +/* Single-threaded TCG
> + *
> + * In the single-threaded case each vCPU is simulated in turn. If
> + * there is more than a single vCPU we create a simple timer to kick
> + * the vCPU and ensure we don't get stuck in a tight loop in one vCPU.
> + * This is done explicitly rather than relying on side-effects
> + * elsewhere.
> + */
> +
>  static void *qemu_tcg_cpu_thread_fn(void *arg)
>  {
>      CPUState *cpu = arg;
> @@ -1204,6 +1262,8 @@ static void *qemu_tcg_cpu_thread_fn(void *arg)
>          }
>      }
>
> +    start_tcg_kick_timer();
> +
>      /* process any pending work */
>      atomic_mb_set(&exit_request, 1);