Re: [Qemu-devel] [RFC v4 04/71] cpu: make qemu_work_cond per-cpu

[Alex Bennée]

Emilio G. Cota <address@hidden> writes:

> This eliminates the need to use the BQL to queue CPU work.
>
> While at it, give the per-cpu field a generic name ("cond") since
> it will soon be used for more than just queueing CPU work.
>
> Signed-off-by: Emilio G. Cota <address@hidden>

Reviewed-by: Alex Bennée <address@hidden>

> ---
>  include/qom/cpu.h |  6 ++--
>  cpus-common.c     | 72 ++++++++++++++++++++++++++++++++++++++---------
>  cpus.c            |  2 +-
>  qom/cpu.c         |  1 +
>  4 files changed, 63 insertions(+), 18 deletions(-)
>
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index 7fdb5a2be0..2fad537a4f 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -316,6 +316,7 @@ struct qemu_work_item;
>   * @mem_io_vaddr: Target virtual address at which the memory was accessed.
>   * @kvm_fd: vCPU file descriptor for KVM.
>   * @lock: Lock to prevent multiple access to per-CPU fields.
> + * @cond: Condition variable for per-CPU events.
>   * @work_list: List of pending asynchronous work.
>   * @trace_dstate_delayed: Delayed changes to trace_dstate (includes all 
> changes
>   *                        to @trace_dstate).
> @@ -358,6 +359,7 @@ struct CPUState {
>
>      QemuMutex lock;
>      /* fields below protected by @lock */
> +    QemuCond cond;
>      QSIMPLEQ_HEAD(, qemu_work_item) work_list;
>
>      CPUAddressSpace *cpu_ases;
> @@ -769,12 +771,10 @@ bool cpu_is_stopped(CPUState *cpu);
>   * @cpu: The vCPU to run on.
>   * @func: The function to be executed.
>   * @data: Data to pass to the function.
> - * @mutex: Mutex to release while waiting for @func to run.
>   *
>   * Used internally in the implementation of run_on_cpu.
>   */
> -void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data,
> -                   QemuMutex *mutex);
> +void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data 
> data);
>
>  /**
>   * run_on_cpu:
> diff --git a/cpus-common.c b/cpus-common.c
> index 2913294cb7..71469c85ce 100644
> --- a/cpus-common.c
> +++ b/cpus-common.c
> @@ -26,7 +26,6 @@
>  static QemuMutex qemu_cpu_list_lock;
>  static QemuCond exclusive_cond;
>  static QemuCond exclusive_resume;
> -static QemuCond qemu_work_cond;
>
>  /* >= 1 if a thread is inside start_exclusive/end_exclusive.  Written
>   * under qemu_cpu_list_lock, read with atomic operations.
> @@ -42,7 +41,6 @@ void qemu_init_cpu_list(void)
>      qemu_mutex_init(&qemu_cpu_list_lock);
>      qemu_cond_init(&exclusive_cond);
>      qemu_cond_init(&exclusive_resume);
> -    qemu_cond_init(&qemu_work_cond);
>  }
>
>  void cpu_list_lock(void)
> @@ -113,23 +111,37 @@ struct qemu_work_item {
>      bool free, exclusive, done;
>  };
>
> -static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
> +/* Called with the CPU's lock held */
> +static void queue_work_on_cpu_locked(CPUState *cpu, struct qemu_work_item 
> *wi)
>  {
> -    qemu_mutex_lock(&cpu->lock);
>      QSIMPLEQ_INSERT_TAIL(&cpu->work_list, wi, node);
>      wi->done = false;
> -    qemu_mutex_unlock(&cpu->lock);
>
>      qemu_cpu_kick(cpu);
>  }
>
> -void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data,
> -                   QemuMutex *mutex)
> +static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
> +{
> +    cpu_mutex_lock(cpu);
> +    queue_work_on_cpu_locked(cpu, wi);
> +    cpu_mutex_unlock(cpu);
> +}
> +
> +void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data)
>  {
>      struct qemu_work_item wi;
> +    bool has_bql = qemu_mutex_iothread_locked();
> +
> +    g_assert(no_cpu_mutex_locked());
>
>      if (qemu_cpu_is_self(cpu)) {
> -        func(cpu, data);
> +        if (has_bql) {
> +            func(cpu, data);
> +        } else {
> +            qemu_mutex_lock_iothread();
> +            func(cpu, data);
> +            qemu_mutex_unlock_iothread();
> +        }
>          return;
>      }
>
> @@ -139,13 +151,34 @@ void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, 
> run_on_cpu_data data,
>      wi.free = false;
>      wi.exclusive = false;
>
> -    queue_work_on_cpu(cpu, &wi);
> +    cpu_mutex_lock(cpu);
> +    queue_work_on_cpu_locked(cpu, &wi);
> +
> +    /*
> +     * We are going to sleep on the CPU lock, so release the BQL.
> +     *
> +     * During the transition to per-CPU locks, we release the BQL _after_
> +     * having kicked the destination CPU (from queue_work_on_cpu_locked 
> above).
> +     * This makes sure that the enqueued work will be seen by the CPU
> +     * after being woken up from the kick, since the CPU sleeps on the BQL.
> +     * Once we complete the transition to per-CPU locks, we will release
> +     * the BQL earlier in this function.
> +     */
> +    if (has_bql) {
> +        qemu_mutex_unlock_iothread();
> +    }
> +
>      while (!atomic_mb_read(&wi.done)) {
>          CPUState *self_cpu = current_cpu;
>
> -        qemu_cond_wait(&qemu_work_cond, mutex);
> +        qemu_cond_wait(&cpu->cond, &cpu->lock);
>          current_cpu = self_cpu;
>      }
> +    cpu_mutex_unlock(cpu);
> +
> +    if (has_bql) {
> +        qemu_mutex_lock_iothread();
> +    }
>  }
>
>  void async_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data 
> data)
> @@ -307,6 +340,7 @@ void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func 
> func,
>  void process_queued_cpu_work(CPUState *cpu)
>  {
>      struct qemu_work_item *wi;
> +    bool has_bql = qemu_mutex_iothread_locked();
>
>      qemu_mutex_lock(&cpu->lock);
>      if (QSIMPLEQ_EMPTY(&cpu->work_list)) {
> @@ -324,13 +358,23 @@ void process_queued_cpu_work(CPUState *cpu)
>               * BQL, so it goes to sleep; start_exclusive() is sleeping too, 
> so
>               * neither CPU can proceed.
>               */
> -            qemu_mutex_unlock_iothread();
> +            if (has_bql) {
> +                qemu_mutex_unlock_iothread();
> +            }
>              start_exclusive();
>              wi->func(cpu, wi->data);
>              end_exclusive();
> -            qemu_mutex_lock_iothread();
> +            if (has_bql) {
> +                qemu_mutex_lock_iothread();
> +            }
>          } else {
> -            wi->func(cpu, wi->data);
> +            if (has_bql) {
> +                wi->func(cpu, wi->data);
> +            } else {
> +                qemu_mutex_lock_iothread();
> +                wi->func(cpu, wi->data);
> +                qemu_mutex_unlock_iothread();
> +            }
>          }
>          qemu_mutex_lock(&cpu->lock);
>          if (wi->free) {
> @@ -340,5 +384,5 @@ void process_queued_cpu_work(CPUState *cpu)
>          }
>      }
>      qemu_mutex_unlock(&cpu->lock);
> -    qemu_cond_broadcast(&qemu_work_cond);
> +    qemu_cond_broadcast(&cpu->cond);
>  }
> diff --git a/cpus.c b/cpus.c
> index 38cc9e1278..d0b7f8e02d 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -1236,7 +1236,7 @@ void qemu_init_cpu_loop(void)
>
>  void run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data)
>  {
> -    do_run_on_cpu(cpu, func, data, &qemu_global_mutex);
> +    do_run_on_cpu(cpu, func, data);
>  }
>
>  static void qemu_kvm_destroy_vcpu(CPUState *cpu)
> diff --git a/qom/cpu.c b/qom/cpu.c
> index d0758c907d..bb031a3a6a 100644
> --- a/qom/cpu.c
> +++ b/qom/cpu.c
> @@ -373,6 +373,7 @@ static void cpu_common_initfn(Object *obj)
>      cpu->nr_threads = 1;
>
>      qemu_mutex_init(&cpu->lock);
> +    qemu_cond_init(&cpu->cond);
>      QSIMPLEQ_INIT(&cpu->work_list);
>      QTAILQ_INIT(&cpu->breakpoints);
>      QTAILQ_INIT(&cpu->watchpoints);


--
Alex Bennée