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

[Alex Bennée]

KONRAD Frederic <address@hidden> writes:

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

It becomes qemu_cpu_kick_rr_cpu() in later patches and joins the rest of
this code.

>
> 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);
>>
>>


--
Alex Bennée