[libvirt] RFC: APIs to manage KSM

[Osier Yang]

Triggered by the requirement to control the new sys knob
/sys/kernel/mm/ksm/merge_nodes for a NUMA aware host by
patch:

http://permalink.gmane.org/gmane.linux.kernel.mm/81497

It shows there is no methods to manage the KSM in
libvirt. This proposes the new APIs like following, thanks
for suggestions/idea in advance.

== The macros and enums for the params' fields

    /* virNodeKSMRunMode:
     *
     * Represents the modes of /sys/kernel/mm/ksm/run.
     */
    typedef enum {
        /* Stop ksmd from running but keep merged pages */
        VIR_NODE_STOP_KSM_KEEP_MERGED_PAGES = 0,

        /* Start ksmd */
        VIR_NODE_START_KSM                  = 1,

        /* Stop ksmd and and unmerge all pages currently merged */
        VIR_NODE_STOP_KSM_UNMERGE_PAGES     = 2,
    } virNodeKSMRunMode;

   /*
    * VIR_NODE_KSM_PAGES_TO_SCAN:
    *
    * Macro for typed parameter that represents how many present pages
    * to scan before ksmd goes to sleep.
    */
   # define VIR_NODE_KSM_PAGES_TO_SCAN     "pages_to_scan"

   /*
    * VIR_NODE_KSM_SLEEP_MILLISECS:
    *
    * Macro for typed parameter that represents how many milliseconds
    * ksmd should sleep before next scan.
    */
   # define VIR_NODE_KSM_SLEEP_MILLISECS   "sleep_millisecs"

   /*
    * VIR_NODE_KSM_RUN:
    *
    * Macro for typed parameter that is used to control ksmd's state, as
    * an int containing a virNodeKSMRunMode value.
    */
   # define VIR_NODE_KSM_RUN                "run"

   /*
    * VIR_NODE_KSM_PAGES_SHARED:
    *
    * Macro for typed parameter that represents how many ksm shared pages
    * are being used.
    */
   # define VIR_NODE_KSM_PAGES_SHARED       "pages_shared"

   /*
    * VIR_NODE_KSM_PAGES_SHARING:
    *
    * Macro for typed parameter that represents how many sites are
    * sharing the pages i.e. how much saved.
    */
   # define VIR_NODE_KSM_PAGES_SHARING      "pages_sharing"

   /* VIR_NODE_KSM_PAGES_UNSHARED:
    *
    * Macro for typed parameter that represents how many pages unique
    * but repeatedly checked for merging.
    */
   # define VIR_NODE_KSM_PAGES_UNSHARED     "pages_unshared"

   /* VIR_NODE_KSM_PAGES_VOLATILE:
    *
    * Macro for typed parameter that represents how many pages changing
    * too fast to be placed in a tree.
    */
   # define VIR_NODE_KSM_PAGES_VOLATILE     "pages_volatile"

   /* VIR_NODE_KSM_FULL_SCAN:
    *
    * Macro for typed parameter that represents how many times all
    * mergeable areas have been scanned.
    */
   # define VIR_NODE_KSM_FULL_SCAN          "full_scan"

== API to Get KSM parameters.

   /*
    * virNodeGetKSMParameters:
    * @conn: pointer to the hypervisor connection
    * @params: pointer to memory parameter object
    *          (return value, allocated by the caller)
    * @nparams: pointer to number of memory parameters; input and output
    * @flags: extra flags; not used yet, so callers should always pass 0
    *
    * ... Detailed comments ommitted ...
    *
    * Returns 0 in case of success, and -1 in case of failure.
    */
   int
   virNodeGetKSMParameters(virConnectPtr conn,
                           virTypedParameterPtr params,
                           int *nparams,
                           unsigned int flags);

== API to set KSM parameters.

    Currently only "run", "pages_to_scan", and "sleep_millisecs"
    are writable. The new boolearn sys knob "merge_nodes" could
    be added once it's acceptted by kernel upstream.

   /*
    * virNodeSetKSMParameters:
    * @conn: pointer to the hypervisor connection
    * @params: pointer to scheduler parameter objects
    * @naprams: number of scheduler parameter objects
    *          (this value can be the same or less than the returned
    *           value nparams of virDomainGetSchedulerType)
    * @flags: extra flags; not used yet, so callers should always pass 0
    *
    * ... Detailed comments ommitted ...
    *
    * Returns 0 in case of success, -1 in case of failure.
    */
   int
   virNodeSetKSMParameters(virConnectPtr conn,
                           virTypedParameterPtr params,
                           int nparams,
                           unsigned int flags);

BTW, the other thing libvirt misses is to tell ksmtuned to retune
when a domain is started or destroyed. Which is outlined in
the Red Hat Virtualization Administration Guide doc, but it's
not true. So either we have a hole in libvirt, or lacks a fix
for the document.

<...>
The KSM tuning service
The ksmtuned service does not have any options. The ksmtuned service 
loops and adjusts ksm. The ksmtuned service is notified by libvirt when 
a guest is created or destroyed.
</...>

https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Administration_Guide/chap-KSM.html

Regards,
Osier