[Qemu-devel] Re: [PATCH 11/19] use a bottom half to run timers

[Paolo Bonzini]

On 01/05/2010 12:59 AM, Anthony Liguori wrote:
> When I think of a main loop, I think of something that provides the
> following three services
>
> 1) fd based events
> 2) time based events
> 3) an idle callback
>
> The problem we have is that bottom halves aren't quite idle callbacks.
> They have some really weird properties about the guarantees of when
> they're executed.  At any rate, you really cannot express a time based
> event as an idle callback because you need to setup the select() timeout
> based on the next deadline and then dispatch timer event based on
> selects return.  idle functions have none of this ability.

I see bottom halves as a fourth service, providing the ability to 
synchronize stuff that needs to execute in a particular thread 
(guaranteeing thread-safety and especially signal-safety).

In some sense, the problem is that bottom halves conflate this service 
_and_ idle callbacks, and that is the weird property.  Idle callbacks 
are used basically only for DMA, and real-world DMA does not have at all 
the semantics that qemu_bh_schedule_idle provides (which in turn is very 
tricky and not exactly what comments in qemu_bh_update_timeout promise).

Compared to qemu_bh_schedule_idle, bottom halves have sane semantics. 
It would be nicer IMO to remove qemu_bh_schedule_idle, have first-class 
idle callbacks, and leave bottom halves as they are now.  I'll put that 
on the todo list. :-)

That said, I'll try to replace this patch with one that doesn't use a 
bottom half.

Paolo