[libvirt] [PATCH 1/2] cgroup: Add accessors for cpuset.memory_migrate

[Martin Kletzander]

On Thu, Mar 19, 2015 at 05:25:28PM -0600, Eric Blake wrote:
>On 03/19/2015 05:11 PM, John Ferlan wrote:
>
>>> +int
>>> +virCgroupSetCpusetMemoryMigrate(virCgroupPtr group, bool migrate)
>>> +{
>>> +    return virCgroupSetValueStr(group,
>>> +                                VIR_CGROUP_CONTROLLER_CPUSET,
>>> +                                "cpuset.memory_migrate",
>>> +                                migrate ? "1" : "0");
>>> +}
>>
>> are my eyes deceiving me?...  boolean here (1 or 0)
>
>No, the kernel really works this way - it takes the human-readable
>string "1" or "0" rather than the C byte '\1' or '\0', so this is the
>lowest level in the stack where we convert our convenient bool for
>everyone above us into the string literal the kernel cgroup interface
>expects.
>

Yes, exactly.  I could've also used SetValueU64(), but that formats
that value which is unnecessary overhead.

>>> +int
>>> +virCgroupGetCpusetMemoryMigrate(virCgroupPtr group, bool *migrate)
>>> +{
>>> +    unsigned long long value = 0;
>>> +    int ret = virCgroupGetValueU64(group,
>>
>> But we're getting a U64 here? for what's documented as "contains a flag
>> (0 or 1)
>>
>>> +                                   VIR_CGROUP_CONTROLLER_CPUSET,
>>> +                                   "cpuset.memory_migrate",
>>> +                                   &value);
>>> +    *migrate = !!value;
>
>And here, this is one easy way to turn the kernel's string back into a
>bool (it might also be possible to read as a string, and then do
>*str=='1', rather than going through an intermediate integer conversion,
>but I'm not sure it is worth the micro-optimization).
>

With one exception.  If kernel changes the possible values to
something else (let's say they'll add "2" for some special new
meaning like "move aggressively"), this code will still work.

>--
>Eric Blake   eblake redhat com    +1-919-301-3266
>Libvirt virtualization library http://libvirt.org
>
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