Re: [Qemu-devel] Question about aio_poll and glib aio_ctx_dispatch

Re: [Qemu-devel] Question about aio_poll and glib aio_ctx_dispatch

[Li Qiang]

Sent it to qemu-devel.

Li Qiang <liq3ea@gmail.com> 于2018年12月19日周三 下午6:04写道：

> Hello Paolo, Stefan, Fam and all,
>
> Here I have a question about 'aio_poll'.
> IIUC the 'aio_poll' is (mostly) used for synchronous IO
> as I see a lot of code like this:
> while(condition)
>  aio_poll();
>
> However it seems the 'aio_poll' and 'aio_ctx_dispatch' both poll the fd.
> So what happened when the 'fd' has events, which function will be wakeup?
>
> Thanks,
> Li Qiang
>
>

Re: [Qemu-devel] Question about aio_poll and glib aio_ctx_dispatch

[Paolo Bonzini]

On 19/12/18 11:05, Li Qiang wrote:
> Sent it to qemu-devel.
> 
> Li Qiang <liq3ea@gmail.com <mailto:liq3ea@gmail.com>> 于2018年12月19日周
> 三 下午6:04写道：
> 
>     Hello Paolo, Stefan, Fam and all,
> 
>     Here I have a question about 'aio_poll'.
>     IIUC the 'aio_poll' is (mostly) used for synchronous IO
>     as I see a lot of code like this: 
>     while(condition)
>      aio_poll();
> 
>     However it seems the 'aio_poll' and 'aio_ctx_dispatch' both poll the fd.
>     So what happened when the 'fd' has events, which function will be
>     wakeup?

Roughly speaking, aio_poll is used for synchronous IO and within I/O
threads; aio_ctx_dispatch is used within the main thread.

Both end up calling aio_dispatch_handlers and timerlistgroup_run_timers.

Paolo

[Qemu-devel] 答复: Question about aio_poll and glib aio_ctx_dispatch

[Li Qiang]

Hello Paolo

Thanks for your kind reply.

Yes, aio_poll and aio_ctx_dispatch mostly run in different threads, though
Sometimes they can run in a thread nested from Fam’s slides:
→http://events17.linuxfoundation.org/sites/events/files/slides/Improving%20the%20QEMU%20Event%20Loop%20-%203.pdf

So you mean we can poll the same fd in two threads? If so , the ‘fd’ in ‘qemu_aio_context’ will be run twice, I think there’s 
something I understand wrong.

Let’s take an example. In the ‘v9fs_reset’.

void v9fs_reset(V9fsState *s)
{
    VirtfsCoResetData data = { .pdu = { .s = s }, .done = false };
    Coroutine *co;

    while (!QLIST_EMPTY(&s->active_list)) {
        aio_poll(qemu_get_aio_context(), true);
    }

    co = qemu_coroutine_create(virtfs_co_reset, &data);
    qemu_coroutine_enter(co);

    while (!data.done) {
        aio_poll(qemu_get_aio_context(), true);
    }
}

Here we use aio_poll to wait the pending action to complete.
Here aio_poll will poll the fds in ‘qemu_aio_context’ in vcpu thread,
However, the main loop is also poll the fds in ‘qemu_aio_context’.
If some ‘fd’ in ‘qemu_aio_context’ has events, the two thread will be wakeup?
You say both will call aio_dispatch_handlers and timerlistgroup_run_timers.
But the are the same fd, how can this happen?

Thanks,
Li Qiang


发件人: Paolo Bonzini
发送时间: 2018年12月20日 4:42
收件人: Li Qiang; stefanha@redhat.com; fam@euphon.net; Qemu Developers; 李强
主题: Re: Question about aio_poll and glib aio_ctx_dispatch

On 19/12/18 11:05, Li Qiang wrote:
> Sent it to qemu-devel.
> 
> Li Qiang <liq3ea@gmail.com <mailto:liq3ea@gmail.com>> 于2018年12月19日周
> 三 下午6:04写道：
> 
>     Hello Paolo, Stefan, Fam and all,
> 
>     Here I have a question about 'aio_poll'.
>     IIUC the 'aio_poll' is (mostly) used for synchronous IO
>     as I see a lot of code like this: 
>     while(condition)
>      aio_poll();
> 
>     However it seems the 'aio_poll' and 'aio_ctx_dispatch' both poll the fd.
>     So what happened when the 'fd' has events, which function will be
>     wakeup?

Roughly speaking, aio_poll is used for synchronous IO and within I/O
threads; aio_ctx_dispatch is used within the main thread.

Both end up calling aio_dispatch_handlers and timerlistgroup_run_timers.

Paolo

Re: [Qemu-devel] Question about aio_poll and glib aio_ctx_dispatch

[Fam Zheng]

> On Dec 20, 2018, at 06:58, Li Qiang <liq3ea@163.com> wrote:
> 
> Hello Paolo
> 
> Thanks for your kind reply.
> 
> Yes, aio_poll and aio_ctx_dispatch mostly run in different threads, though
> Sometimes they can run in a thread nested from Fam’s slides:
> →http://events17.linuxfoundation.org/sites/events/files/slides/Improving%20the%20QEMU%20Event%20Loop%20-%203.pdf
> 
> So you mean we can poll the same fd in two threads? If so , the ‘fd’ in ‘qemu_aio_context’ will be run twice, I think there’s 
> something I understand wrong.
> 
> Let’s take an example. In the ‘v9fs_reset’.
> 
> void v9fs_reset(V9fsState *s)
> {
>    VirtfsCoResetData data = { .pdu = { .s = s }, .done = false };
>    Coroutine *co;
> 
>    while (!QLIST_EMPTY(&s->active_list)) {
>        aio_poll(qemu_get_aio_context(), true);
>    }
> 
>    co = qemu_coroutine_create(virtfs_co_reset, &data);
>    qemu_coroutine_enter(co);
> 
>    while (!data.done) {
>        aio_poll(qemu_get_aio_context(), true);
>    }
> }
> 
> Here we use aio_poll to wait the pending action to complete.
> Here aio_poll will poll the fds in ‘qemu_aio_context’ in vcpu thread,
> However, the main loop is also poll the fds in ‘qemu_aio_context’.
> If some ‘fd’ in ‘qemu_aio_context’ has events, the two thread will be wakeup?
> You say both will call aio_dispatch_handlers and timerlistgroup_run_timers.
> But the are the same fd, how can this happen?


I think in this case BQL is used to synchronize them so when v9fs_reset runs, the main loop doesn't.

Thanks,
Fam

> 
> Thanks,
> Li Qiang
> 
> 
> 发件人: Paolo Bonzini
> 发送时间: 2018年12月20日 4:42
> 收件人: Li Qiang; stefanha@redhat.com; fam@euphon.net; Qemu Developers; 李强
> 主题: Re: Question about aio_poll and glib aio_ctx_dispatch
> 
> On 19/12/18 11:05, Li Qiang wrote:
>> Sent it to qemu-devel.
>> 
>> Li Qiang <liq3ea@gmail.com <mailto:liq3ea@gmail.com>> 于2018年12月19日周
>> 三 下午6:04写道：
>> 
>>    Hello Paolo, Stefan, Fam and all,
>> 
>>    Here I have a question about 'aio_poll'.
>>    IIUC the 'aio_poll' is (mostly) used for synchronous IO
>>    as I see a lot of code like this: 
>>    while(condition)
>>     aio_poll();
>> 
>>    However it seems the 'aio_poll' and 'aio_ctx_dispatch' both poll the fd.
>>    So what happened when the 'fd' has events, which function will be
>>    wakeup?
> 
> Roughly speaking, aio_poll is used for synchronous IO and within I/O
> threads; aio_ctx_dispatch is used within the main thread.
> 
> Both end up calling aio_dispatch_handlers and timerlistgroup_run_timers.
> 
> Paolo
> 

Re: [Qemu-devel] Question about aio_poll and glib aio_ctx_dispatch

[Li Qiang]

Fam Zheng <fam@euphon.net> 于2018年12月20日周四 上午9:26写道：

>
>
> > On Dec 20, 2018, at 06:58, Li Qiang <liq3ea@163.com> wrote:
> >
> > Hello Paolo
> >
> > Thanks for your kind reply.
> >
> > Yes, aio_poll and aio_ctx_dispatch mostly run in different threads,
> though
> > Sometimes they can run in a thread nested from Fam’s slides:
> > →
> http://events17.linuxfoundation.org/sites/events/files/slides/Improving%20the%20QEMU%20Event%20Loop%20-%203.pdf
> >
> > So you mean we can poll the same fd in two threads? If so , the ‘fd’ in
> ‘qemu_aio_context’ will be run twice, I think there’s
> > something I understand wrong.
> >
> > Let’s take an example. In the ‘v9fs_reset’.
> >
> > void v9fs_reset(V9fsState *s)
> > {
> >    VirtfsCoResetData data = { .pdu = { .s = s }, .done = false };
> >    Coroutine *co;
> >
> >    while (!QLIST_EMPTY(&s->active_list)) {
> >        aio_poll(qemu_get_aio_context(), true);
> >    }
> >
> >    co = qemu_coroutine_create(virtfs_co_reset, &data);
> >    qemu_coroutine_enter(co);
> >
> >    while (!data.done) {
> >        aio_poll(qemu_get_aio_context(), true);
> >    }
> > }
> >
> > Here we use aio_poll to wait the pending action to complete.
> > Here aio_poll will poll the fds in ‘qemu_aio_context’ in vcpu thread,
> > However, the main loop is also poll the fds in ‘qemu_aio_context’.
> > If some ‘fd’ in ‘qemu_aio_context’ has events, the two thread will be
> wakeup?
> > You say both will call aio_dispatch_handlers and
> timerlistgroup_run_timers.
> > But the are the same fd, how can this happen?
>
>
>
Thanks Fam,


> I think in this case BQL is used to synchronize them so when v9fs_reset
> runs, the main loop doesn't.
>

I think you're right. The main loop will not hold BQL when polling. But it
will acquire the BQL before it can
do the dispatch. So even if an 'fd' events occurs in this case, only the
aio_poll's dispatch can run.

Thanks,
Li Qiang



>
> Thanks,
> Fam
>
> >
> > Thanks,
> > Li Qiang
> >
> >
> > 发件人: Paolo Bonzini
> > 发送时间: 2018年12月20日 4:42
> > 收件人: Li Qiang; stefanha@redhat.com; fam@euphon.net; Qemu Developers; 李强
> > 主题: Re: Question about aio_poll and glib aio_ctx_dispatch
> >
> > On 19/12/18 11:05, Li Qiang wrote:
> >> Sent it to qemu-devel.
> >>
> >> Li Qiang <liq3ea@gmail.com <mailto:liq3ea@gmail.com>> 于2018年12月19日周
> >> 三 下午6:04写道：
> >>
> >>    Hello Paolo, Stefan, Fam and all,
> >>
> >>    Here I have a question about 'aio_poll'.
> >>    IIUC the 'aio_poll' is (mostly) used for synchronous IO
> >>    as I see a lot of code like this:
> >>    while(condition)
> >>     aio_poll();
> >>
> >>    However it seems the 'aio_poll' and 'aio_ctx_dispatch' both poll the
> fd.
> >>    So what happened when the 'fd' has events, which function will be
> >>    wakeup?
> >
> > Roughly speaking, aio_poll is used for synchronous IO and within I/O
> > threads; aio_ctx_dispatch is used within the main thread.
> >
> > Both end up calling aio_dispatch_handlers and timerlistgroup_run_timers.
> >
> > Paolo
> >
>
>
>