| EVCNT(9) | Kernel Developer's Manual | EVCNT(9) |
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);
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 counters 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:
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 commands 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 corresponding fields of the event counter structure it is initializing. The group and name arguments must be constant strings.
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.
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.)
Device drivers can use the evcnt_attach_dynamic() and evcnt_detach() functions to manage device-specific event counters. Statically configured 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);
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.
| January 14, 2011 | NetBSD 6.1 |