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

[Edgar E. Iglesias]

On Wed, Jan 19, 2011 at 03:02:26PM -0200, Marcelo Tosatti wrote:
> 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 <address@hidden>
> > > 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 <address@hidden>
> > > 
> > > 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.

OK, I suspected something like that. Thanks both of you for the info.
I'll give people a couple of days to complain at the patch, if noone
does I'll apply it.

Cheers