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Detect when the kernel multiplexed hw counters. #2421

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Dec 14, 2019
Merged

Detect when the kernel multiplexed hw counters. #2421

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28 changes: 23 additions & 5 deletions src/PerfCounters.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -260,10 +260,25 @@ static const uint64_t IN_TX = 1ULL << 32;
static const uint64_t IN_TXCP = 1ULL << 33;

static int64_t read_counter(ScopedFd& fd) {
int64_t val;
ssize_t nread = read(fd, &val, sizeof(val));
DEBUG_ASSERT(nread == sizeof(val));
return val;
struct read_format {
int64_t value;
int64_t time_enabled;
int64_t time_running;
} data;
errno = 0;
ssize_t nread = read(fd, &data, sizeof(data));
if (nread < 0) {
FATAL() << "Failed to read counter";
}
DEBUG_ASSERT(nread == sizeof(data));
if (data.time_enabled != data.time_running) {
FATAL()
<< "\nCounter was multiplexed to a ratio of "
<< (data.time_running * 1.0 / data.time_enabled)
<< ".\nCheck that other software is not using performance counters on\n"
"this CPU.";
}
return data.value;
}

static ScopedFd start_counter(pid_t tid, int group_fd,
Expand All @@ -272,11 +287,14 @@ static ScopedFd start_counter(pid_t tid, int group_fd,
if (disabled_txcp) {
*disabled_txcp = false;
}
struct perf_event_attr tmp_attr = *attr;
// Needed to detect when the kernel is multiplexing counters.
tmp_attr.read_format =
PERF_FORMAT_TOTAL_TIME_ENABLED | PERF_FORMAT_TOTAL_TIME_RUNNING;
int fd = syscall(__NR_perf_event_open, attr, tid, -1, group_fd, 0);
if (0 > fd && errno == EINVAL && attr->type == PERF_TYPE_RAW &&
(attr->config & IN_TXCP)) {
// The kernel might not support IN_TXCP, so try again without it.
struct perf_event_attr tmp_attr = *attr;
tmp_attr.config &= ~IN_TXCP;
fd = syscall(__NR_perf_event_open, &tmp_attr, tid, -1, group_fd, 0);
if (fd >= 0) {
Expand Down