Thank you very much !
Thanks to your tools, I now understand how to trace some variable in the
source code.But I still don't know what is  HT siblings or  HT and core
siblings?Could you explain it a little more detail?
Thanks again!


On Thu, Mar 21, 2013 at 8:12 PM, George Dunlap
<George.Dunlap@eu.citrix.com>wrote:

> On Thu, Mar 21, 2013 at 9:53 AM, Demon King <kdmxen@gmail.com> wrote:
> > I am a new comer of xen-devel. When I read the source code of
> > sched_credit.c. I was confused by such codes below:
> >
> > if ( cpumask_test_cpu(cpu, per_cpu(cpu_core_mask, nxt)) )
> >         {
> >             /* We're on the same socket, so check the busy-ness of
> threads.
> >              * Migrate if # of idlers is less at all */
> >             ASSERT( cpumask_test_cpu(nxt, per_cpu(cpu_core_mask, cpu)) );
> >             migrate_factor = 1;
> >             cpumask_and(&cpu_idlers, &idlers, per_cpu(cpu_sibling_mask,
> > cpu));
> >             cpumask_and(&nxt_idlers, &idlers, per_cpu(cpu_sibling_mask,
> > nxt));
> >         }
> >
> > I don't know much about xen or linux. So I get confused in the per cpu
> > variable: cpu_core_mask and cpu_sibling_mask
> > Is anyone willing to tell me? Thank you! And may I ask you where to get
> > answers about this kind of question?
>
> A handy tool for looking through the hypervisor source code is as follows:
>
> $ find . -name "*.[cSh]" | xargs grep -H [rexexp]
>
> This will look for [regexp] in every .c, .h, and .S file.
>
> Doing this with "cpu_sibling_mask" turns up a couple of places, one of
> which is:
>
> xen/arch/x86/smpboot.c
>
> Which has the following comments:
>
> /* representing HT siblings of each logical CPU */
> DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_mask);
> /* representing HT and core siblings of each logical CPU */
> DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_core_mask);
>
> So if per_cpu(cpu_core_mask, cpu1) has bit cpu2 set, then cpu2 is a
> 'core sibling' of cpu1.  Same with cpu_sibling_mask, except then it's
> hyperthreads.
>
> To be honest, the code you're looking at now is very complicated and
> not very well commented, so it's not surprising you found it difficult
> to understand.  I had a plan at some point to update the comments to
> make it easier to follow.
>
>  -George
>