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

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/10/19 19:40, Alex Bennée wrote:
> 
> Paolo Bonzini <pbonzini@redhat.com> writes:
> 
>> 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.
> 
> Are you going to take this via your queue?

I wasn't, since we have had a proper TCG maintainer for a while. :)

Paolo

> 
>>
>> 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()) {
>>>              /*
>>>
> 
> 
> --
> Alex Bennée
>