Re: what the meaning of the per cpu variable cpu_core_mask and cpu_sibling_mask?

[George Dunlap <george.dunlap@eu.citrix.com>]

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On 21/03/13 13:49, Demon King wrote:
> 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!

Please don't top-post.

Hyperthreads (HT) in the same core are siblings, and cores in the same 
socket / numa node are siblings.

  -George

>
>
> On Thu, Mar 21, 2013 at 8:12 PM, George Dunlap 
> <George.Dunlap@eu.citrix.com <mailto:George.Dunlap@eu.citrix.com>> wrote:
>
>     On Thu, Mar 21, 2013 at 9:53 AM, Demon King <kdmxen@gmail.com
>     <mailto: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
>
>


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