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

["Alex Bennée" <alex.bennee@linaro.org>]

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?

>
> 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