On 04/08/20 12:29, Stefan Hajnoczi wrote:
> On Tue, Aug 04, 2020 at 06:28:04AM +0100, Stefan Hajnoczi wrote:
>> @@ -597,15 +574,38 @@ bool aio_poll(AioContext *ctx, bool blocking)
>> * system call---a single round of run_poll_handlers_once suffices.
>> */
>> if (timeout || ctx->fdmon_ops->need_wait(ctx)) {
>> + /*
>> + * aio_notify can avoid the expensive event_notifier_set if
>> + * everything (file descriptors, bottom halves, timers) will
>> + * be re-evaluated before the next blocking poll(). This is
>> + * already true when aio_poll is called with blocking == false;
>> + * if blocking == true, it is only true after poll() returns,
>> + * so disable the optimization now.
>> + */
>> + if (timeout) {
>> + atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
>> + /*
>> + * Write ctx->notify_me before computing the timeout
>> + * (reading bottom half flags, etc.). Pairs with
>> + * smp_mb in aio_notify().
>> + */
>> + smp_mb();
>> +
>> + /* Check again in case a shorter timer was added */
>> + timeout = qemu_soonest_timeout(timeout,
>> aio_compute_timeout(ctx));
>> + }
>> +
>> ret = ctx->fdmon_ops->wait(ctx, &ready_list, timeout);
>> - }
>>
>> - if (blocking) {
>> - /* Finish the poll before clearing the flag. */
>> - atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me)
>> - 2);
>> - aio_notify_accept(ctx);
>> + if (timeout) {
>> + /* Finish the poll before clearing the flag. */
>> + atomic_store_release(&ctx->notify_me,
>> + atomic_read(&ctx->notify_me) - 2);
>> + }
>> }
>
> Hi Paolo,
> We can avoid calling aio_compute_timeout() like this, what do you think?
I don't understand :) except I guess you mean we can avoid the second
call. Can you post either a complete patch with this squashed, or a 4th
patch (whatever you think is best)?
Paolo
> bool use_notify_me = timeout != 0;
>
> if (use_notify_me) {
> atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
> /*
> * Write ctx->notify_me before computing the timeout
> * (reading bottom half flags, etc.). Pairs with
> * smp_mb in aio_notify().
> */
> smp_mb();
>
> /* Don't block if aio_notify() was called */
> if (atomic_read(ctx->notified)) {
> timeout = 0;
> }
> }
>
> ret = ctx->fdmon_ops->wait(ctx, &ready_list, timeout);
>
> if (use_notify_me) {
> /* Finish the poll before clearing the flag. */
> atomic_store_release(&ctx->notify_me,
> atomic_read(&ctx->notify_me) - 2);
> }
>
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