Date: Fri, 11 Sep 2009 13:01:54 -0400 From: John Baldwin <jhb@freebsd.org> To: Luigi Rizzo <rizzo@iet.unipi.it> Cc: Juergen Lock <nox@jelal.kn-bremen.de>, Avi Kivity <avi@qumranet.com>, qemu-devel@nongnu.org, freebsd-current@freebsd.org, Jan Kiszka <jan.kiszka@web.de>, Mohammed Gamal <m.gamal005@gmail.com> Subject: Re: FreeBSD timing issues and qemu (was: Re: [Qemu-devel] Re: Breakage with local APIC routing) Message-ID: <200909111301.55692.jhb@freebsd.org> In-Reply-To: <20090911170317.GA33232@onelab2.iet.unipi.it> References: <4A93BF0C.8040601@web.de> <200909111123.00257.jhb@freebsd.org> <20090911170317.GA33232@onelab2.iet.unipi.it>
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On Friday 11 September 2009 1:03:17 pm Luigi Rizzo wrote:
> On Fri, Sep 11, 2009 at 11:22:59AM -0400, John Baldwin wrote:
> > On Thursday 10 September 2009 3:08:00 pm Luigi Rizzo wrote:
> ...
> > > > 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.
> > > >
> > >
> > > 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;
> >
> > I would fix the style in the first hunk (spaces around '-') but I think
you
> > should commit this and get it into 8.0. I think a per-CPU ticks might
prove
> > very problematic as 'ticks' is rather widely used (though I would find
that
> > cleaner perhaps).
>
> i will ask permission to re -- i was hoping to get some feedback
> on the thread on -current but no response so far :(
>
> Note that the per-cpu ticks i was proposing were only visible to the
> timing wheels, which don't use absolute timeouts anyways.
> So i think the mechanism would be quite safe: right now, when you
> request a callout after x ticks, the code first picks a CPU
> (with some criteria), then puts the request in the timer wheel for
> that CPU using (now) the global 'ticks'. Replacing ticks with cc->cc_ticks,
> would completely remove the races in insertion and removal.
>
> I actually find the per-cpu ticks even less intrusive than this change.
Well, it depends. If TCP ever started using per-CPU callouts (i.e.
callout_reset_on()) it would probably need to start using the per-CPU ticks
instead of the global ticks, etc. You could have 'ticks' just be == to CPU
0's ticks perhaps.
--
John Baldwin
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