Re: [Qemu-devel] [PATCH v5 13/13] cpu-exec: replace cpu->queued_work with GArray

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

Alex Bennée <alex.bennee@linaro.org> writes:

> Under times of high memory stress the additional small mallocs by a
> linked list are source of potential memory fragmentation. As we have
> worked hard to avoid mallocs elsewhere when queuing work we might as
> well do the same for the list. We convert the lists to a auto-resizeing
> GArray which will re-size in steps of powers of 2.
<snip>
> diff --git a/cpu-exec-common.c b/cpu-exec-common.c
> index 6d5da15..745d973 100644
> --- a/cpu-exec-common.c
> +++ b/cpu-exec-common.c
> @@ -113,17 +113,18 @@ void wait_safe_cpu_work(void)
>  static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
>  {
>      qemu_mutex_lock(&cpu->work_mutex);
> -    if (cpu->queued_work_first == NULL) {
> -        cpu->queued_work_first = wi;
> -    } else {
> -        cpu->queued_work_last->next = wi;
> +
> +    if (!cpu->queued_work) {
> +        cpu->queued_work = g_array_sized_new(true, true,
> +                             sizeof(struct qemu_work_item), 16);
>      }
> -    cpu->queued_work_last = wi;
> -    wi->next = NULL;
> -    wi->done = false;
> +    trace_queue_work_on_cpu(cpu->cpu_index, wi->safe,
> cpu->queued_work->len);

Oops, this was left over from testing, I've posted an updated version of
the patch.

> +
> +    g_array_append_val(cpu->queued_work, *wi);
>      if (wi->safe) {
>          atomic_inc(&safe_work_pending);
>      }
> +
>      qemu_mutex_unlock(&cpu->work_mutex);
>
>      if (!wi->safe) {
> @@ -138,6 +139,7 @@ static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
>  void run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data)
>  {
>      struct qemu_work_item wi;
> +    bool done = false;
>
>      if (qemu_cpu_is_self(cpu)) {
>          func(cpu, data);
> @@ -146,11 +148,11 @@ void run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data)
>
>      wi.func = func;
>      wi.data = data;
> -    wi.free = false;
>      wi.safe = false;
> +    wi.done = &done;
>
>      queue_work_on_cpu(cpu, &wi);
> -    while (!atomic_mb_read(&wi.done)) {
> +    while (!atomic_mb_read(&done)) {
>          CPUState *self_cpu = current_cpu;
>
>          qemu_cond_wait(&qemu_work_cond, qemu_get_cpu_work_mutex());
> @@ -160,70 +162,75 @@ void run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data)
>
>  void async_run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data)
>  {
> -    struct qemu_work_item *wi;
> +    struct qemu_work_item wi;
>
>      if (qemu_cpu_is_self(cpu)) {
>          func(cpu, data);
>          return;
>      }
>
> -    wi = g_malloc0(sizeof(struct qemu_work_item));
> -    wi->func = func;
> -    wi->data = data;
> -    wi->free = true;
> -    wi->safe = false;
> +    wi.func = func;
> +    wi.data = data;
> +    wi.safe = false;
> +    wi.done = NULL;
>
> -    queue_work_on_cpu(cpu, wi);
> +    queue_work_on_cpu(cpu, &wi);
>  }
>
>  void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func, void *data)
>  {
> -    struct qemu_work_item *wi;
> +    struct qemu_work_item wi;
>
> -    wi = g_malloc0(sizeof(struct qemu_work_item));
> -    wi->func = func;
> -    wi->data = data;
> -    wi->free = true;
> -    wi->safe = true;
> +    wi.func = func;
> +    wi.data = data;
> +    wi.safe = true;
> +    wi.done = NULL;
>
> -    queue_work_on_cpu(cpu, wi);
> +    queue_work_on_cpu(cpu, &wi);
>  }
>
>  void process_queued_cpu_work(CPUState *cpu)
>  {
>      struct qemu_work_item *wi;
> -
> -    if (cpu->queued_work_first == NULL) {
> -        return;
> -    }
> +    GArray *work_list = NULL;
> +    int i;
>
>      qemu_mutex_lock(&cpu->work_mutex);
> -    while (cpu->queued_work_first != NULL) {
> -        wi = cpu->queued_work_first;
> -        cpu->queued_work_first = wi->next;
> -        if (!cpu->queued_work_first) {
> -            cpu->queued_work_last = NULL;
> -        }
> -        if (wi->safe) {
> -            while (tcg_pending_threads) {
> -                qemu_cond_wait(&qemu_exclusive_cond,
> -                               qemu_get_cpu_work_mutex());
> +
> +    work_list = cpu->queued_work;
> +    cpu->queued_work = NULL;
> +
> +    qemu_mutex_unlock(&cpu->work_mutex);
> +
> +    if (work_list) {
> +
> +        g_assert(work_list->len > 0);
> +
> +        for (i = 0; i < work_list->len; i++) {
> +            wi = &g_array_index(work_list, struct qemu_work_item, i);
> +
> +            if (wi->safe) {
> +                while (tcg_pending_threads) {
> +                    qemu_cond_wait(&qemu_exclusive_cond,
> +                                   qemu_get_cpu_work_mutex());
> +                }
>              }
> -        }
> -        qemu_mutex_unlock(&cpu->work_mutex);
> -        wi->func(cpu, wi->data);
> -        qemu_mutex_lock(&cpu->work_mutex);
> -        if (wi->safe) {
> -            if (!atomic_dec_fetch(&safe_work_pending)) {
> -                qemu_cond_broadcast(&qemu_safe_work_cond);
> +
> +            wi->func(cpu, wi->data);
> +
> +            if (wi->safe) {
> +                if (!atomic_dec_fetch(&safe_work_pending)) {
> +                    qemu_cond_broadcast(&qemu_safe_work_cond);
> +                }
> +            }
> +
> +            if (wi->done) {
> +                atomic_mb_set(wi->done, true);
>              }
>          }
> -        if (wi->free) {
> -            g_free(wi);
> -        } else {
> -            atomic_mb_set(&wi->done, true);
> -        }
> +
> +        trace_process_queued_cpu_work(cpu->cpu_index,
> work_list->len);

And so was this.


--
Alex Bennée