Re: [Xen-devel] Question about xentrace to trace s_time_t type of data

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 08/25/2014 02:31 PM, Konrad Rzeszutek Wilk wrote:
> On Sat, Aug 23, 2014 at 11:24:21PM -0400, Meng Xu wrote:
> > Hi,
> >
> > I'm trying to trace the scheduler-specific events for debug purpose by
> > using xentrace (instead of using printk). I read the trace code in credit
> > and credit2 scheduler (sched_credit.c and sched_credit2.c) and "followed"
> > the way credit2 wrote.
> >
> > I added the following code into the burn_budget() in my scheduler file,
> > sched_rt.c:
> >
> >          /* TRACE */
> >
> >          {
> >
> >              struct {
> >
> >                  unsigned dom:16,vcpu:16;
> >
> >                  s_time_t cur_budget;
> >
> >              } d;
> >
> >              d.dom = svc->vcpu->domain->domain_id;
> >
> >              d.vcpu = svc->vcpu->vcpu_id;
> >
> >              d.cur_budget = svc->cur_budget;
> >
> >              trace_var(TRC_RT_BUDGET_REPLENISH, 1,
> >
> >                        sizeof(d),
> >
> >                        (unsigned char *) &d);
> You put the virtual address addresss of your 'd' structure
> that is on the stack in the trace file. That does not contain any
> data except an address.
No; trace_var() takes a pointer to a struct and a size.  Meng is 
following the example he's seen in sched_credit2.c. :-)
The size of the structure is larger because the structure isn't 
"packed": by default it will try to make 64-bit elements 64-bit aligned 
by adding in "padding".  You can get around this in one of two ways:
1. Re-arranging the elements so they're naturally aligned.  In this 
case, you could do
struct {
    s_time_t cur_budget;
    unsigned dom:16, vcpu:16;
} d;

2. Adding "__packed" after "struct".  This tells the compiler not to try 
to align the bytes according to their size.
That explains why it's 4 words instead of 3, but I'm still a bit 
mistified why it's not something like
 [ 00010000 <garbage> 40000000 00000000 ]

I guess try with a packed structure and see what you get. :-)

 -George


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