illumos: manual page: cpc_bind

CPC_BIND_CURLWP(3CPC) CPU Performance Counters Library Functions

NAME

cpc_bind_curlwp, cpc_bind_pctx, cpc_bind_cpu, cpc_unbind,
cpc_request_preset, cpc_set_restart - bind request sets to hardware
counters

SYNOPSIS

cc [ flag... ] file... -lcpc [ library... ]
#include <libcpc.h>

int cpc_bind_curlwp(cpc_t *cpc, cpc_set_t *set, uint_t flags);

int cpc_bind_pctx(cpc_t *cpc, pctx_t *pctx, id_t id, cpc_set_t *set,
uint_t flags);

int cpc_bind_cpu(cpc_t *cpc, processorid_t id, cpc_set_t *set,
uint_t flags);

int cpc_unbind(cpc_t *cpc, cpc_set_t *set);

int cpc_request_preset(cpc_t *cpc, int index, uint64_t preset);

int cpc_set_restart(cpc_t *cpc, cpc_set_t *set);

DESCRIPTION

These functions program the processor's hardware counters according to
the requests contained in the set argument. If these functions are
successful, then upon return the physical counters will have been
assigned to count events on behalf of each request in the set, and each
counter will be enabled as configured.

The cpc_bind_curlwp() function binds the set to the calling LWP. If
successful, a performance counter context is associated with the LWP that
allows the system to virtualize the hardware counters to that specific
LWP.

By default, the system binds the set to the current LWP only. If the
CPC_BIND_LWP_INHERIT flag is present in the flags argument, however, any
subsequent LWPs created by the current LWP will inherit a copy of the
request set. The newly created LWP will have its virtualized 64-bit
counters initialized to the preset values specified in set, and the
counters will be enabled and begin counting events on behalf of the new
LWP. This automatic inheritance behavior can be useful when dealing with
multithreaded programs to determine aggregate statistics for the program
as a whole.

If the CPC_BIND_LWP_INHERIT flag is specified and any of the requests in
the set have the CPC_OVF_NOTIFY_EMT flag set, the process will
immediately dispatch a SIGEMT signal to the freshly created LWP so that
it can preset its counters appropriately on the new LWP. This
initialization condition can be detected using cpc_set_sample(3CPC) and
looking at the counter value for any requests with CPC_OVF_NOTIFY_EMT
set. The value of any such counters will be UINT64_MAX.

The cpc_bind_pctx() function binds the set to the LWP specified by the
pctx-id pair, where pctx refers to a handle returned from libpctx and id
is the ID of the desired LWP in the target process. If successful, a
performance counter context is associated with the specified LWP and the
system virtualizes the hardware counters to that specific LWP. The flags
argument is reserved for future use and must always be 0.

The cpc_bind_cpu() function binds the set to the specified CPU and
measures events occurring on that CPU regardless of which LWP is running.
Only one such binding can be active on the specified CPU at a time. As
long as any application has bound a set to a CPU, per-LWP counters are
unavailable and any attempt to use either cpc_bind_curlwp() or
cpc_bind_pctx() returns EAGAIN. The first invocation of cpc_bind_cpu()
invalidates all currently bound per-LWP counter sets, and any attempt to
sample an invalidated set returns EAGAIN. To bind to a CPU, the library
binds the calling LWP to the measured CPU with processor_bind(2). The
application must not change its processor binding until after it has
unbound the set with cpc_unbind(). The flags argument is reserved for
future use and must always be 0.

The cpc_request_preset() function updates the preset and current value
stored in the indexed request within the currently bound set, thereby
changing the starting value for the specified request for the calling LWP
only, which takes effect at the next call to cpc_set_restart().

When a performance counter counting on behalf of a request with the
CPC_OVF_NOTIFY_EMT flag set overflows, the performance counters are
frozen and the LWP to which the set is bound receives a SIGEMT signal.
The cpc_set_restart() function can be called from a SIGEMT signal handler
function to quickly restart the hardware counters. Counting begins from
each request's original preset (see cpc_set_add_request(3CPC)), or from
the preset specified in a prior call to cpc_request_preset().
Applications performing performance counter overflow profiling should use
the cpc_set_restart() function to quickly restart counting after
receiving a SIGEMT overflow signal and recording any relevant program
state.

The cpc_unbind() function unbinds the set from the resource to which it
is bound. All hardware resources associated with the bound set are freed
and if the set was bound to a CPU, the calling LWP is unbound from the
corresponding CPU. See processor_bind(2).

RETURN VALUES

Upon successful completion these functions return 0. Otherwise, -1 is
returned and errno is set to indicate the error.

ERRORS

Applications wanting to get detailed error values should register an
error handler with cpc_seterrhndlr(3CPC). Otherwise, the library will
output a specific error description to stderr.

These functions will fail if:

EACCES
For cpc_bind_curlwp(), the system has Pentium 4 processors
with HyperThreading and at least one physical processor has
more than one hardware thread online. See NOTES.

For cpc_bind_cpu(), the process does not have the cpc_cpu
privilege to access the CPU's counters.

For cpc_bind_curlwp(), cpc_bind_cpc(), and cpc_bind_pctx(),
access to the requested hypervisor event was denied.

EAGAIN
For cpc_bind_curlwp() and cpc_bind_pctx(), the performance
counters are not available for use by the application.

For cpc_bind_cpu(), another process has already bound to this
CPU. Only one process is allowed to bind to a CPU at a time
and only one set can be bound to a CPU at a time.

EINVAL
The set does not contain any requests or cpc_set_add_request()
was not called.

The value given for an attribute of a request is out of range.

The system could not assign a physical counter to each request
in the system. See NOTES.

One or more requests in the set conflict and might not be
programmed simultaneously.

The set was not created with the same cpc handle.

For cpc_bind_cpu(), the specified processor does not exist.

For cpc_unbind(), the set is not bound.

For cpc_request_preset() and cpc_set_restart(), the calling
LWP does not have a bound set.

ENOSYS
For cpc_bind_cpu(), the specified processor is not online.

ENOTSUP
The cpc_bind_curlwp() function was called with the
CPC_OVF_NOTIFY_EMT flag, but the underlying processor is not
capable of detecting counter overflow.

ESRCH
For cpc_bind_pctx(), the specified LWP in the target process
does not exist.

EXAMPLES

Example 1: Use hardware performance counters to measure events in a

process.

The following example demonstrates how a standalone application can be
instrumented with the libcpc(3LIB) functions to use hardware performance
counters to measure events in a process. The application performs 20
iterations of a computation, measuring the counter values for each
iteration. By default, the example makes use of two counters to measure
external cache references and external cache hits. These options are only
appropriate for UltraSPARC processors. By setting the EVENT0 and EVENT1
environment variables to other strings (a list of which can be obtained
from the -h option of the cpustat(8) or cputrack(1) utilities), other
events can be counted. The error() routine is assumed to be a user-
provided routine analogous to the familiar printf(3C) function from the C
library that also performs an exit(2) after printing the message.

#include <inttypes.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <libcpc.h>
#include <errno.h>

int
main(int argc, char *argv[])
{
int iter;
char *event0 = NULL, *event1 = NULL;
cpc_t *cpc;
cpc_set_t *set;
cpc_buf_t *diff, *after, *before;
int ind0, ind1;
uint64_t val0, val1;

if ((cpc = cpc_open(CPC_VER_CURRENT)) == NULL)
error("perf counters unavailable: %s", strerror(errno));

if ((event0 = getenv("EVENT0")) == NULL)
event0 = "EC_ref";
if ((event1 = getenv("EVENT1")) == NULL)
event1 = "EC_hit";

if ((set = cpc_set_create(cpc)) == NULL)
error("could not create set: %s", strerror(errno));

if ((ind0 = cpc_set_add_request(cpc, set, event0, 0, CPC_COUNT_USER, 0,
NULL)) == -1)
error("could not add first request: %s", strerror(errno));

if ((ind1 = cpc_set_add_request(cpc, set, event1, 0, CPC_COUNT_USER, 0,
NULL)) == -1)
error("could not add first request: %s", strerror(errno));

if ((diff = cpc_buf_create(cpc, set)) == NULL)
error("could not create buffer: %s", strerror(errno));
if ((after = cpc_buf_create(cpc, set)) == NULL)
error("could not create buffer: %s", strerror(errno));
if ((before = cpc_buf_create(cpc, set)) == NULL)
error("could not create buffer: %s", strerror(errno));

if (cpc_bind_curlwp(cpc, set, 0) == -1)
error("cannot bind lwp%d: %s", _lwp_self(), strerror(errno));

for (iter = 1; iter <= 20; iter++) {

if (cpc_set_sample(cpc, set, before) == -1)
break;

/* ==> Computation to be measured goes here <== */

if (cpc_set_sample(cpc, set, after) == -1)
break;

cpc_buf_sub(cpc, diff, after, before);
cpc_buf_get(cpc, diff, ind0, &val0);
cpc_buf_get(cpc, diff, ind1, &val1);

(void) printf("%3d: %" PRId64 " %" PRId64 "\n", iter,
val0, val1);
}

if (iter != 21)
error("cannot sample set: %s", strerror(errno));

cpc_close(cpc);

return (0);
}

Example 2: Write a signal handler to catch overflow signals.

The following example builds on Example 1 and demonstrates how to write
the signal handler to catch overflow signals. A counter is preset so that
it is 1000 counts short of overflowing. After 1000 counts the signal
handler is invoked.

The signal handler:

cpc_t *cpc;
cpc_set_t *set;
cpc_buf_t *buf;
int index;

void
emt_handler(int sig, siginfo_t *sip, void *arg)
{
ucontext_t *uap = arg;
uint64_t val;

if (sig != SIGEMT || sip->si_code != EMT_CPCOVF) {
psignal(sig, "example");
psiginfo(sip, "example");
return;
}

(void) printf("lwp%d - si_addr %p ucontext: %%pc %p %%sp %p\n",
_lwp_self(), (void *)sip->si_addr,
(void *)uap->uc_mcontext.gregs[PC],
(void *)uap->uc_mcontext.gregs[SP]);

if (cpc_set_sample(cpc, set, buf) != 0)
error("cannot sample: %s", strerror(errno));

cpc_buf_get(cpc, buf, index, &val);

(void) printf("0x%" PRIx64"\n", val);
(void) fflush(stdout);

/*
* Update a request's preset and restart the counters. Counters which
* have not been preset with cpc_request_preset() will resume counting
* from their current value.
*/
(cpc_request_preset(cpc, ind1, val1) != 0)
error("cannot set preset for request %d: %s", ind1,
strerror(errno));
if (cpc_set_restart(cpc, set) != 0)
error("cannot restart lwp%d: %s", _lwp_self(), strerror(errno));
}

The setup code, which can be positioned after the code that opens the CPC
library and creates a set:

#define PRESET (UINT64_MAX - 999ull)

struct sigaction act;
...
act.sa_sigaction = emt_handler;
bzero(&act.sa_mask, sizeof (act.sa_mask));
act.sa_flags = SA_RESTART|SA_SIGINFO;
if (sigaction(SIGEMT, &act, NULL) == -1)
error("sigaction: %s", strerror(errno));

if ((index = cpc_set_add_request(cpc, set, event, PRESET,
CPC_COUNT_USER | CPC_OVF_NOTIFY_EMT, 0, NULL)) != 0)
error("cannot add request to set: %s", strerror(errno));

if ((buf = cpc_buf_create(cpc, set)) == NULL)
error("cannot create buffer: %s", strerror(errno));

if (cpc_bind_curlwp(cpc, set, 0) == -1)
error("cannot bind lwp%d: %s", _lwp_self(), strerror(errno));

for (iter = 1; iter <= 20; iter++) {
/* ==> Computation to be measured goes here <== */
}

cpc_unbind(cpc, set); /* done */

ATTRIBUTES

NOTES

When a set is bound, the system assigns a physical hardware counter to
count on behalf of each request in the set. If such an assignment is not
possible for all requests in the set, the bind function returns -1 and
sets errno to EINVAL. The assignment of requests to counters depends on
the capabilities of the available counters. Some processors (such as
Pentium 4) have a complicated counter control mechanism that requires the
reservation of limited hardware resources beyond the actual counters. It
could occur that two requests for different events might be impossible to
count at the same time due to these limited hardware resources. See the
processor manual as referenced by cpc_cpuref(3CPC) for details about the
underlying processor's capabilities and limitations.

Some processors can be configured to dispatch an interrupt when a
physical counter overflows. The most obvious use for this facility is to
ensure that the full 64-bit counter values are maintained without
repeated sampling. Certain hardware, such as the UltraSPARC processor,
does not record which counter overflowed. A more subtle use for this
facility is to preset the counter to a value slightly less than the
maximum value, then use the resulting interrupt to catch the counter
overflow associated with that event. The overflow can then be used as an
indication of the frequency of the occurrence of that event.

The interrupt generated by the processor might not be particularly
precise. That is, the particular instruction that caused the counter
overflow might be earlier in the instruction stream than is indicated by
the program counter value in the ucontext.

When a request is added to a set with the CPC_OVF_NOTIFY_EMT flag set,
then as before, the control registers and counter are preset from the
64-bit preset value given. When the flag is set, however, the kernel
arranges to send the calling process a SIGEMT signal when the overflow
occurs. The si_code member of the corresponding siginfo structure is set
to EMT_CPCOVF and the si_addr member takes the program counter value at
the time the overflow interrupt was delivered. Counting is disabled until
the set is bound again.

If the CPC_CAP_OVERFLOW_PRECISE bit is set in the value returned by
cpc_caps(3CPC), the processor is able to determine precisely which
counter has overflowed after receiving the overflow interrupt. On such
processors, the SIGEMT signal is sent only if a counter overflows and the
request that the counter is counting has the CPC_OVF_NOTIFY_EMT flag set.
If the capability is not present on the processor, the system sends a
SIGEMT signal to the process if any of its requests have the
CPC_OVF_NOTIFY_EMT flag set and any counter in its set overflows.

Different processors have different counter ranges available, though all
processors supported by Solaris allow at least 31 bits to be specified as
a counter preset value. Portable preset values lie in the range
UINT64_MAX to UINT64_MAX-INT32_MAX.

The appropriate preset value will often need to be determined
experimentally. Typically, this value will depend on the event being
measured as well as the desire to minimize the impact of the act of
measurement on the event being measured. Less frequent interrupts and
samples lead to less perturbation of the system.

If the processor cannot detect counter overflow, bind will fail and
return ENOTSUP. Only user events can be measured using this technique.
See Example 2.

Pentium 4
Most Pentium 4 events require the specification of an event mask for
counting. The event mask is specified with the emask attribute.

Pentium 4 processors with HyperThreading Technology have only one set of
hardware counters per physical processor. To use cpc_bind_curlwp() or
cpc_bind_pctx() to measure per-LWP events on a system with Pentium 4 HT
processors, a system administrator must first take processors in the
system offline until each physical processor has only one hardware thread
online (See the -p option to psrinfo(8)). If a second hardware thread is
brought online, all per-LWP bound contexts will be invalidated and any
attempt to sample or bind a CPC set will return EAGAIN.

Only one CPC set at a time can be bound to a physical processor with
cpc_bind_cpu(). Any call to cpc_bind_cpu() that attempts to bind a set to
a processor that shares a physical processor with a processor that
already has a CPU-bound set returns an error.

To measure the shared state on a Pentium 4 processor with HyperThreading,
the count_sibling_usr and count_sibling_sys attributes are provided for
use with cpc_bind_cpu(). These attributes behave exactly as the
CPC_COUNT_USER and CPC_COUNT_SYSTEM request flags, except that they act
on the sibling hardware thread sharing the physical processor with the
CPU measured by cpc_bind_cpu(). Some CPC sets will fail to bind due to
resource constraints. The most common type of resource constraint is an
ESCR conflict among one or more requests in the set. For example, the
branch_retired event cannot be measured on counters 12 and 13
simultaneously because both counters require the CRU_ESCR2 ESCR to
measure this event. To measure branch_retired events simultaneously on
more than one counter, use counters such that one counter uses CRU_ESCR2
and the other counter uses CRU_ESCR3. See the processor documentation for
details.

March 5, 2007 CPC_BIND_CURLWP(3CPC)