Re: [Xen-devel] A clocksource question

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 03/10/2010 05:19 PM, Joanna Rutkowska wrote:
> On 11/03/2010 02:06, Jeremy Fitzhardinge wrote:
>    
> > On 03/10/2010 04:52 PM, Joanna Rutkowska wrote:
> >      
> > > BTW, how does the clocksource=jiffies work on a pvops kernel in Dom0?
> > >
> > >        
> > Not very well.  clocksource=jiffies just sets up timer interrupts at
> > approx 100ms intervals and assumes that's 100ms.  You get very low res
> > timers and timekeeping.
> >
> >      
> So, how to explain that there is no wallclock drift *at all*, even in a
> long run -- uptime of a few days, in Dom0, when it uses the jiffies source?
>    
You said earlier that you were seeing clock drift with 
clocksource=jiffies in dom0.
> Anyway, I assume that the "xen" clock source is much more fine grained
> (1ms?)
The xen clocksource has nanosecond resolution.  But clocksources are 
different from event sources, and so the ns resolution of time 
measurement doesn't have much relationship to the timer precision (which 
is always going to use the xen event source, which is also ns 
resolution, but it will tend to fold together timer events which are 
closer than 50us).
>   and so, maybe my kbd hiccups are caused by some code executed by
> the timer interrupt too frequently (maybe too much code executes per
> each timer interrupt, because of some other bug)? Just a though...
>    
Possibly, but doesn't seem too likely.  Can you tell if your hiccups are 
accompanied by bursts of timer interrupts in /proc/interrupts?
There is another possibility, which is that the scheduler is getting 
confused by xen's scheduler clock.  Rather than just scheduling based on 
real time, we try to take into account time stolen from a vcpu so that 
it isn't credited against the process (which may have had all its time 
stolen by another domain).  But that could just be confusing things.
Does this help?

diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
index 0d3f07c..9029885 100644
--- a/arch/x86/xen/time.c
+++ b/arch/x86/xen/time.c
@@ -161,6 +161,9 @@ static void do_stolen_accounting(void)
  */
 unsigned long long xen_sched_clock(void)
 {
+#if 1
+       return xen_clocksource_read();
+#else
        struct vcpu_runstate_info state;
        cycle_t now;
        u64 ret;
@@ -190,6 +193,7 @@ unsigned long long xen_sched_clock(void)
        preempt_enable();

        return ret;
+#endif
 }





    J

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