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

On Thu, Sep 10, 2009 at 07:46:40PM +0200, Juergen Lock wrote:
> 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
> _at__at_ -323,7 +323,7 _at__at_ 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.
> 

as mentioned in the followup message in that thread,
you also need this change in callout_tick()

        mtx_lock_spin_flags(&cc->cc_lock, MTX_QUIET);
     -  for (; (cc->cc_softticks - ticks) < 0; cc->cc_softticks++) {
     +  for (; (cc->cc_softticks - ticks) <= 0; cc->cc_softticks++) {
                bucket = cc->cc_softticks & callwheelmask;

cheers
luigi