Re: [PATCH] cpus: kick all vCPUs when running thread=single

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/10/19 18:04, Alex Bennée wrote:
> qemu_cpu_kick is used for a number of reasons including to indicate
> there is work to be done. However when thread=single the old
> qemu_cpu_kick_rr_cpu only advanced the vCPU to the next executing one
> which can lead to a hang in the case that:
> 
>   a) the kick is from outside the vCPUs (e.g. iothread)
>   b) the timers are paused (i.e. iothread calling run_on_cpu)
> 
> To avoid this lets split qemu_cpu_kick_rr into two functions. One for
> the timer which continues to advance to the next timeslice and another
> for all other kicks.
> 
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: Doug Gale <doug16k@gmail.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Peter Maydell <peter.maydell@linaro.org>
> ---
>  cpus.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)

Looks good to me.  Single-threaded TCG is not going to have high vCPU
counts anyway.

Paolo

> diff --git a/cpus.c b/cpus.c
> index d2c61ff155..bee7209134 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -949,8 +949,8 @@ static inline int64_t qemu_tcg_next_kick(void)
>      return qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + TCG_KICK_PERIOD;
>  }
>  
> -/* Kick the currently round-robin scheduled vCPU */
> -static void qemu_cpu_kick_rr_cpu(void)
> +/* Kick the currently round-robin scheduled vCPU to next */
> +static void qemu_cpu_kick_rr_next_cpu(void)
>  {
>      CPUState *cpu;
>      do {
> @@ -961,6 +961,16 @@ static void qemu_cpu_kick_rr_cpu(void)
>      } while (cpu != atomic_mb_read(&tcg_current_rr_cpu));
>  }
>  
> +/* Kick all RR vCPUs */
> +static void qemu_cpu_kick_rr_cpus(void)
> +{
> +    CPUState *cpu;
> +
> +    CPU_FOREACH(cpu) {
> +        cpu_exit(cpu);
> +    };
> +}
> +
>  static void do_nothing(CPUState *cpu, run_on_cpu_data unused)
>  {
>  }
> @@ -993,7 +1003,7 @@ void qemu_timer_notify_cb(void *opaque, QEMUClockType type)
>  static void kick_tcg_thread(void *opaque)
>  {
>      timer_mod(tcg_kick_vcpu_timer, qemu_tcg_next_kick());
> -    qemu_cpu_kick_rr_cpu();
> +    qemu_cpu_kick_rr_next_cpu();
>  }
>  
>  static void start_tcg_kick_timer(void)
> @@ -1828,9 +1838,11 @@ void qemu_cpu_kick(CPUState *cpu)
>  {
>      qemu_cond_broadcast(cpu->halt_cond);
>      if (tcg_enabled()) {
> -        cpu_exit(cpu);
> -        /* NOP unless doing single-thread RR */
> -        qemu_cpu_kick_rr_cpu();
> +        if (qemu_tcg_mttcg_enabled()) {
> +            cpu_exit(cpu);
> +        } else {
> +            qemu_cpu_kick_rr_cpus();
> +        }
>      } else {
>          if (hax_enabled()) {
>              /*
>