[Qemu-devel] Re: [PATCH] fix smp with tcg mode and --enable-io-thread

[Jan Kiszka <jan.kiszka@web.de>]

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Jan Kiszka wrote:
> Marcelo Tosatti wrote:
>> Clear exit_request when iothread grabs the global lock. 
>>
>> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>>
>> diff --git a/cpu-exec.c b/cpu-exec.c
>> index 026980a..74cb973 100644
>> --- a/cpu-exec.c
>> +++ b/cpu-exec.c
>> @@ -236,10 +236,8 @@ int cpu_exec(CPUState *env1)
>>      asm("");
>>      env = env1;
>>  
>> -    if (exit_request) {
>> +    if (exit_request)
>>          env->exit_request = 1;
>> -        exit_request = 0;
>> -    }
> 
> Coding style...
> 
>>  
>>  #if defined(TARGET_I386)
>>      if (!kvm_enabled()) {
>> diff --git a/cpus.c b/cpus.c
>> index fcd0f09..ef1ab22 100644
>> --- a/cpus.c
>> +++ b/cpus.c
>> @@ -598,6 +598,7 @@ void qemu_mutex_lock_iothread(void)
>>          }
>>          qemu_mutex_unlock(&qemu_fair_mutex);
>>      }
>> +   exit_request = 0;
>>  }
>>  
>>  void qemu_mutex_unlock_iothread(void)
>>
>>
> 
> I looked into this a bit as well, and that's what I also have in my
> queue.
> 
> But things are still widely broken: pause_all_vcpus and run_on_cpu as
> there is no guarantee that all VCPUs regularly call into
> qemu_wait_io_event. Also breakpoints don't work, not only in SMP mode.
> 
> The former issue is mostly fine with this patch:
> 
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 026980a..9f4191d 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -236,9 +236,8 @@ int cpu_exec(CPUState *env1)
>      asm("");
>      env = env1;
>  
> -    if (exit_request) {
> +    if (unlikely(exit_request)) {
>          env->exit_request = 1;
> -        exit_request = 0;
>      }
>  
>  #if defined(TARGET_I386)
> diff --git a/cpus.c b/cpus.c
> index fcd0f09..2ce839d 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -39,7 +39,6 @@
>  #define SIG_IPI SIGUSR1
>  #endif
>  
> -static CPUState *cur_cpu;
>  static CPUState *next_cpu;
>  
>  /***********************************************************/
> @@ -331,6 +330,7 @@ int qemu_init_main_loop(void)
>          return ret;
>  
>      qemu_cond_init(&qemu_pause_cond);
> +    qemu_cond_init(&qemu_system_cond);
>      qemu_mutex_init(&qemu_fair_mutex);
>      qemu_mutex_init(&qemu_global_mutex);
>      qemu_mutex_lock(&qemu_global_mutex);
> @@ -401,10 +401,12 @@ static void qemu_wait_io_event_common(CPUState *env)
>      flush_queued_work(env);
>  }
>  
> -static void qemu_wait_io_event(CPUState *env)
> +static void qemu_tcg_wait_io_event(void)
>  {
> +    CPUState *env;
> +
>      while (!tcg_has_work())
> -        qemu_cond_timedwait(env->halt_cond, &qemu_global_mutex, 1000);
> +        qemu_cond_timedwait(tcg_halt_cond, &qemu_global_mutex, 1000);
>  
>      qemu_mutex_unlock(&qemu_global_mutex);
>  
> @@ -417,7 +419,10 @@ static void qemu_wait_io_event(CPUState *env)
>      qemu_mutex_unlock(&qemu_fair_mutex);
>  
>      qemu_mutex_lock(&qemu_global_mutex);
> -    qemu_wait_io_event_common(env);
> +
> +    for (env = first_cpu; env != NULL; env = env->next_cpu) {
> +        qemu_wait_io_event_common(env);
> +    }
>  }
>  
>  static void qemu_kvm_eat_signal(CPUState *env, int timeout)
> @@ -502,7 +507,7 @@ static void *tcg_cpu_thread_fn(void *arg)
>  
>      while (1) {
>          tcg_cpu_exec();
> -        qemu_wait_io_event(cur_cpu);
> +        qemu_tcg_wait_io_event();
>      }
>  
>      return NULL;
> @@ -768,11 +773,11 @@ bool tcg_cpu_exec(void)
>  
>      if (next_cpu == NULL)
>          next_cpu = first_cpu;
> -    for (; next_cpu != NULL; next_cpu = next_cpu->next_cpu) {
> -        CPUState *env = cur_cpu = next_cpu;
> +    for (; next_cpu != NULL && !exit_request; next_cpu = next_cpu->next_cpu) {
> +        CPUState *env = next_cpu;
>  
>          qemu_clock_enable(vm_clock,
> -                          (cur_cpu->singlestep_enabled & SSTEP_NOTIMER) == 0);
> +                          (env->singlestep_enabled & SSTEP_NOTIMER) == 0);
>  
>          if (qemu_alarm_pending())
>              break;
> @@ -787,6 +792,7 @@ bool tcg_cpu_exec(void)
>              break;
>          }
>      }
> +    exit_request = 0;
>      return tcg_has_work();
>  }
>  
> 
> I haven't looked into the breakpoint bug yet as performance (likely
> VCPU scheduling) in iothread mode is still suffering compared to classic
> mode, specifically when you go up with the number of VCPUs (try -smp 8).

Hmm, retesting this, I cannot reproduce the performance regression
anymore. Likely I confused something or had instrumentations applied.

Locking up still works "reliably". :(

Jan

> And there is some race that cause a lock up in qemu_mutex_lock_iothread
> after a while (the cpu_unlink_tb seems to race with the linking - just a
> guess so far).
> 
> Anyway, all this is getting terribly complex, hard to grasp, and maybe
> therefore fragile as well. Specifically the SMP round-robin scheduling
> looks like it requires a redesign for iothread mode (my feeling is it
> only works by chance ATM). Part of the issue may not be new, but could
> have been shadowed by the frequently interrupting I/O load when using a
> single thread. The rest seems to lack a bit a user base...
> 
> Jan
> 



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