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

[Paolo Bonzini]

On 01/04/2010 09:24 PM, Anthony Liguori wrote:
> I'm not a huge fan of this for a couple reasons.  The first is that
> it introduces a subtle semantic change.  Previously, timers always
> ran before bottom halves whereas after this change, timers may run
> after some bottoms halves but before others.

I see what you mean, and you are right: qemu_bh_new adds a
bottom half at the beginning of the queue, so it's pretty much
guaranteed that a ptimer's bottom half will run _before_ the alarm timer's.

There are three possible fixes:

1) make async.c use a tail queue.  Fixes the bug, but it is too clever IMHO.

2) in tcg_exec, where there is

        if (timer_alarm_pending) {
            timer_alarm_pending = 0;
            break;
        }

instead check if any bottom half is scheduled.  With this change, after 
the timers run, if the ptimer's bottom half hadn't run TCG would not 
execute code, qemu_bh_calculate_timeout would make main_loop_wait 
nonblocking, and the ptimer's bottom half would execute right away.

BTW after my series the above check will test whether the timer bottom 
half is scheduled, so in some sense this could be considered a bugfix 
that would be placed _very early_ in the series or could even go in 
independently.

3) Both of the above.  2 would provide the fix and 1 would provide a 
performance improvement by avoiding the useless looping.

> But more importantly, I think timer dispatch needs to be part of the
> select loop.  malc has a git tree that replaces host alarm timers
> with select() timeouts.  This has a lot of really nice properties
> like it eliminates the need for signals and EINTR handling.  A move
> like this would likely make this more difficult.

Not necessarily, or at least, splitting qemu-timer.c may make it 
marginally more difficult but not having a bottom half for timers.

With qemu-timer.c split you'd need something like

   if (rc == 0) host_alarm_handler ();

after the select loop.  I suppose you could basically revert this patch 
and move timer handling into host_alarm_handler, but the code should 
work independent of this patch.  This patch (modulo your other 
objection) just adds a level of indirection but doesn't change the 
overall structure of the main loop.

Paolo