FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing)

[Juergen Lock]

On Wed, Sep 09, 2009 at 10:46:16PM +0200, Luigi Rizzo wrote:
> On Mon, Sep 07, 2009 at 10:59:55PM +0200, Juergen Lock wrote:
> > [I'm copying freebsd-current at FreeBSD.org because ppl there might know
> > more about this...]
> > 
> >  qemu on FreeBSD hosts used to be able to run a (FreeBSD at least) guest
> > with the same HZ as the host (like, 1000) with (mostly) proper timing
> > once, but no longer. :(  It seems there are two problems involved:
> > 
> >  a) use of apic seems to cause the clock irq rate to be doubled to 2 * HZ
> > (can anyone explain why?), i.e. a FreeBSD 7 guest on a FreeBSD 7 host
> > only gets proper timing after setting hint.apic.0.disabled=1 via the
> > loader.  (as can be verified by `vmstat -i' and `time sleep 2' in an
> > installed guest or via the fixit->cdrom/dvd shell on a FreeBSD livefs
> > or dvd1 iso.)
> > 
> >  b) qemu running on FreeBSD 8 hosts (and most likely head) has the
> > additional problem of running its timers only at HZ/2 when using
> > setitimer(2) (called `-clock unix' in qemu), as seen below.  (as also
> 
> this problem in 8.x is caused by the bug i described here yesterday:
> 
> http://lists.freebsd.org/pipermail/freebsd-current/2009-September/011393.html
> 
> In qeumu, the setitimer call (in file vl.c) has a timeout of 1 tick
> which maps to callout_reset(..., 1, ...) and because (due to the bug)
> 8.x processes callouts 1 tick late, this effectively halves the clock rate.
> 
Thanx for the pointer!

 The proposed patch in that post didn't make a different here tho,
guest still sees only half host HZ clock irq rate. (i.e. ~500 Hz.)

 Here is the patch I used, to make sure I patched what you meant...

Index: sys/kern/kern_timeout.c
@@ -323,7 +323,7 @@ softclock(void *arg)
 	steps = 0;
 	cc = (struct callout_cpu *)arg;
 	CC_LOCK(cc);
-	while (cc->cc_softticks != ticks) {
+	while (cc->cc_softticks-1 != ticks) {
 		/*
 		 * cc_softticks may be modified by hard clock, so cache
 		 * it while we work on a given bucket.

> > seen below, timer_settime(2) aka `-clock dynticks' in qemu behaves
> > even worse, but that is similarly true on FreeBSD 7 which is why
> > I removed the patch that enabled that from our qemu port(s) a few
> > days ago.)  And the only reason FreeBSD 8 guests are usually less
> > affected by these problems is they now reduce their HZ to 100 when
> > they detect being run in a VM.  (which makes sense for other reasons
> > as well, don't get me wrong...  but of course doesn't help when the
> > host is running with HZ=100 too.)
> 
> cheers
> luigi

 Cheers,
	Juergen