[Qemu-devel] Re: MIPS, io-thread, icount and wfi

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jan 18, 2011 at 11:00:57AM +0100, Jan Kiszka wrote:
> On 2011-01-18 01:19, Edgar E. Iglesias wrote:
> > On Mon, Jan 17, 2011 at 11:03:08AM +0100, Edgar E. Iglesias wrote:
> >> Hi,
> >>
> >> I'm running an io-thread enabled qemu-system-mipsel with icount.
> >> When the guest (linux) goes to sleep through the wait insn (waiting
> >> to be woken up by future timer interrupts), the thing deadlocks.
> >>
> >> IIUC, this is because vm timers are driven by icount, but the CPU is
> >> halted so icount makes no progress and time stands still.
> >>
> >> I've locally disabled vcpu halting when icount is enabled, that
> >> works around my problem but of course makes qemu consume 100% host cpu.
> >>
> >> I don't know why I only see this problem with io-thread builds?
> >> Could be related timing and luck.
> >>
> >> Would be interesting to know if someone has any info on how this was
> >> intended to work (if it was)? And if there are ideas for better
> >> workarounds or fixes that don't disable vcpu halting entirely.
> > 
> > Hi,
> > 
> > I've found the problem. For some reason io-thread builds use a
> > static timeout for wait loops. The entire chunk of code that
> > makes sure qemu_icount makes forward progress when the CPU's
> > are idle has been ifdef'ed away...
> > 
> > This fixes the problem for me, hopefully without affecting
> > io-thread runs without icount.
> > 
> > commit 0f4f3a919952500b487b438c5520f07a1c6be35b
> > Author: Edgar E. Iglesias <edgar@axis.com>
> > Date:   Tue Jan 18 01:01:57 2011 +0100
> > 
> >     qemu-timer: Fix timeout calc for io-thread with icount
> >     
> >     Make sure we always make forward progress with qemu_icount to
> >     avoid deadlocks. For io-thread, use the static 1000 timeout
> >     only if icount is disabled.
> >     
> >     Signed-off-by: Edgar E. Iglesias <edgar@axis.com>
> > 
> > diff --git a/qemu-timer.c b/qemu-timer.c
> > index 95814af..db1ec49 100644
> > --- a/qemu-timer.c
> > +++ b/qemu-timer.c
> > @@ -110,7 +110,6 @@ static int64_t cpu_get_clock(void)
> >      }
> >  }
> >  
> > -#ifndef CONFIG_IOTHREAD
> >  static int64_t qemu_icount_delta(void)
> >  {
> >      if (!use_icount) {
> > @@ -124,7 +123,6 @@ static int64_t qemu_icount_delta(void)
> >          return cpu_get_icount() - cpu_get_clock();
> >      }
> >  }
> > -#endif
> >  
> >  /* enable cpu_get_ticks() */
> >  void cpu_enable_ticks(void)
> > @@ -1077,9 +1075,17 @@ void quit_timers(void)
> >  
> >  int qemu_calculate_timeout(void)
> >  {
> > -#ifndef CONFIG_IOTHREAD
> >      int timeout;
> >  
> > +#ifdef CONFIG_IOTHREAD
> > +    /* When using icount, making forward progress with qemu_icount when the
> > +       guest CPU is idle is critical. We only use the static io-thread timeout
> > +       for non icount runs.  */
> > +    if (!use_icount) {
> > +        return 1000;
> > +    }
> > +#endif
> > +
> >      if (!vm_running)
> >          timeout = 5000;
> >      else {
> > @@ -1110,8 +1116,5 @@ int qemu_calculate_timeout(void)
> >      }
> >  
> >      return timeout;
> > -#else /* CONFIG_IOTHREAD */
> > -    return 1000;
> > -#endif
> >  }
> >  
> > 
> > 
> 
> This logic and timeout values were imported on iothread merge. And I bet
> at least the timeout value of 1s (vs. 5s) can still be found in
> qemu-kvm. Maybe someone over there can remember the rationales behind
> choosing this value.
> 
> Jan

This timeout is for the main select() call. So there is not a lot
of reasoning, how long to wait when there's no activity on the file
descriptors.