EVCNT(9)               NetBSD Kernel Developer's Manual               EVCNT(9)

NAME
     evcnt, evcnt_attach_dynamic, evcnt_attach_static, evcnt_detach -- generic
     event counter framework

SYNOPSIS
     #include <sys/evcnt.h>

     void
     evcnt_attach_dynamic(struct evcnt *ev, int type,
         const struct evcnt *parent, const char *group, const char *name);

     void
     evcnt_attach_static(struct evcnt *ev);

     void
     evcnt_detach(struct evcnt *ev);

DESCRIPTION
     The NetBSD generic event counter framework is designed to provide a flex-
     ible and hierarchical event counting facility, which is useful for track-
     ing system events (including device interrupts).

     The fundamental component of this framework is the evcnt structure.  Its
     user-accessible fields are:

     struct evcnt {
             uint64_t        ev_count;       /* how many have occurred */
             TAILQ_ENTRY(evcnt) ev_list;     /* entry on list of all counters */
             unsigned char   ev_type;        /* counter type; see below */
             unsigned char   ev_grouplen;    /* 'group' len, excluding NUL */
             unsigned char   ev_namelen;     /* 'name' len, excluding NUL */
             const struct evcnt *ev_parent;  /* parent, for hierarchical ctrs */
             const char      *ev_group;      /* name of group */
             const char      *ev_name;       /* name of specific event */
     };

     The system maintains a global linked list of all active event counters.
     This list, called allevents, may grow or shrink over time as event coun-
     ters are dynamically added to and removed from the system.

     Each event counter is marked (in the ev_type field) with the type of
     event being counted.  The following types are currently defined:

           EVCNT_TYPE_MISC  Miscellaneous; doesn't fit into one of the other
                            types.

           EVCNT_TYPE_INTR  Interrupt counter, reported by vmstat -i.

           EVCNT_TYPE_TRAP  Processor trap style events.

     Each event counter also has a group name (ev_group) and an event name
     (ev_name) which are used to identify the counter.  The group name may be
     shared by a set of counters.  For example, device interrupt counters
     would use the name of the device whose interrupts are being counted as
     the group name.  The counter name is meant to distinguish the counter
     from others in its group (and need not be unique across groups).  Both
     names should be understandable by users, since they are printed by com-
     mands like vmstat(1).  The constant EVCNT_STRING_MAX is defined to be the
     maximum group or event name length in bytes (including the trailing NUL).
     In the current implementation it is 256.

     To support hierarchical tracking of events, each event counter can name a
     ``parent'' event counter.  For instance, interrupt dispatch code could
     have an event counter per interrupt line, and devices could each have
     counters for the number of interrupts that they were responsible for
     causing.  In that case, the counter for a device on a given interrupt
     line would have the line's counter as its parent.  The value NULL is used
     to indicate that a counter has no parent.  A counter's parent must be
     attached before the counter is attached, and detached after the counter
     is detached.

     The EVCNT_INITIALIZER() macro can be used to provide a static initializer
     for an event counter structure.  It is invoked as EVCNT_INITIALIZER(type,
     parent, group, name), and its arguments will be placed into the corre-
     sponding fields of the event counter structure it is initializing.  The
     group and name arguments must be constant strings.

FUNCTIONS
     The following is a brief description of each function in the framework:

     evcnt_attach_dynamic(ev, type, parent, group, name)
             Attach the event counter structure pointed to by ev to the system
             event list.  The event counter is cleared and its fields initial-
             ized using the arguments to the function call.  The contents of
             the remaining elements in the structure (e.g., the name lengths)
             are calculated, and the counter is added to the system event
             list.

             The strings specified as the group and counter names must persist
             (with the same value) throughout the life of the event counter;
             they are referenced by, not copied into, the counter.

     evcnt_attach_static(ev)
             Attach the statically-initialized event counter structure pointed
             to by ev to the system event list.  The event counter is assumed
             to be statically initialized using the EVCNT_INITIALIZER() macro.
             This function simply calculates structure elements' values as
             appropriate (e.g., the string lengths), and adds the counter to
             the system event list.

     evcnt_detach(ev)
             Detach the event counter structure pointed to by ev from the sys-
             tem event list.

     Note that no method is provided to increment the value of an event
     counter.  Code incrementing an event counter should do so by directly
     accessing its ev_count field in a manner that is known to be safe.  For
     instance, additions to a device's event counters in the interrupt handler
     for that device will often be safe without additional protection (because
     interrupt handler entries for a given device have to be serialized).
     However, for other uses of event counters, additional locking or use of
     machine-dependent atomic operation may be appropriate.  (The overhead of
     using a mechanism that is guaranteed to be safe to increment every
     counter, regardless of actual need for such a mechanism where the counter
     is being incremented, would be too great.  On some systems, it might
     involve a global lock and several function calls.)

EXAMPLES
     This section includes a description on basic use of the framework and
     example usage of its functions.

     Device drivers can use the evcnt_attach_dynamic() and evcnt_detach()
     functions to manage device-specific event counters.  Statically config-
     ured system modules can use evcnt_attach_static() to configure global
     event counters.  Similarly, loadable modules can use
     evcnt_attach_static() to configure their global event counters,
     evcnt_attach_dynamic() to attach device-specific event counters, and
     evcnt_detach() to detach all counters when being unloaded.

     Device drivers that wish to use the generic event counter framework
     should place event counter structures in their ``softc'' structures.  For
     example, to keep track of the number of interrupts for a given device
     (broken down further into ``device readable'' and ``device writable''
     interrupts) a device driver might use:

     struct foo_softc {
             [ . . . ]
             struct evcnt sc_ev_intr;        /* interrupt count */
             struct evcnt sc_ev_intr_rd;     /* 'readable' interrupt count */
             struct evcnt sc_ev_intr_wr;     /* 'writable' interrupt count */
             [ . . . ]
     };

     In the device attach function, those counters would be registered with
     the system using the evcnt_attach_dynamic() function, using code like:

     void
     fooattach(device_t parent, device_t self, void *aux)
     {
             struct foo_softc *sc = device_private(self);

             [ . . . ]

             /* Initialize and attach event counters. */
             evcnt_attach_dynamic(&sc->sc_ev, EVCNT_TYPE_INTR,
                 NULL, device_xname(self), "intr");
             evcnt_attach_dynamic(&sc->sc_ev_rd, EVCNT_TYPE_INTR,
                 &sc->sc_ev, device_xname(self), "intr rd");
             evcnt_attach_dynamic(&sc->sc_ev_wr, EVCNT_TYPE_INTR,
                 &sc->sc_ev, device_xname(self), "intr wr");

             [ . . . ]
     }

     If the device can be detached from the system, its detach function should
     invoke evcnt_detach() on each attached counter (making sure to detach any
     ``parent'' counters only after detaching all children).

     Code like the following might be used to initialize a static event
     counter (in this example, one used to track CPU alignment traps):

             struct evcnt aligntrap_ev = EVCNT_INITIALIZER(EVCNT_TYPE_MISC,
                 NULL, "cpu", "aligntrap")

     To attach this event counter, code like the following could be used:

             evcnt_attach_static(&aligntrap_ev);

CODE REFERENCES
     The event counter framework itself is implemented within the file
     sys/kern/subr_evcnt.c.  Data structures and function prototypes for the
     framework are located in sys/sys/device.h.

     Event counters are used throughout the system.

     The vmstat(1) source file usr.bin/vmstat/vmstat.c shows an example of how
     to access event counters from user programs.

SEE ALSO
     vmstat(1)

HISTORY
     A set of interrupt counter interfaces with similar names to the inter-
     faces in the NetBSD generic event counter framework appeared as part of
     the new autoconfiguration system in 4.4BSD.  Those interfaces were never
     widely adopted in NetBSD because of limitations in their applicability.
     (Their use was limited to non-hierarchical, dynamically attached device
     interrupt counters.)  The NetBSD generic event counter framework first
     appeared in NetBSD 1.5.

AUTHORS
     The NetBSD generic event counter framework was designed and implemented
     by Chris Demetriou <cgd@NetBSD.org>.

NetBSD 6.1.5                   January 14, 2011                   NetBSD 6.1.5

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