From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Wed, 6 Jul 2016 19:11:57 +0000 (-0400)
Subject: Add generic fallback for perf counter read
X-Git-Tag: v2.9.0-rc1~42
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=a3a8d943e8ed60a6d60790d3b143ee6ac1b28ce3;p=lttng-ust.git

Add generic fallback for perf counter read

On x86, test the cap_user_rdpmc flag and fall-back on a system call if
unset.

On all other architectures, use the system call fallback.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/liblttng-ust/lttng-context-perf-counters.c b/liblttng-ust/lttng-context-perf-counters.c
index a7e1b63f..dbb1e6b1 100644
--- a/liblttng-ust/lttng-context-perf-counters.c
+++ b/liblttng-ust/lttng-context-perf-counters.c
@@ -80,6 +80,22 @@ size_t perf_counter_get_size(struct lttng_ctx_field *field, size_t offset)
 	return size;
 }
 
+static
+uint64_t read_perf_counter_syscall(
+		struct lttng_perf_counter_thread_field *thread_field)
+{
+	uint64_t count;
+
+	if (caa_unlikely(thread_field->fd < 0))
+		return 0;
+
+	if (caa_unlikely(read(thread_field->fd, &count, sizeof(count))
+				< sizeof(count)))
+		return 0;
+
+	return count;
+}
+
 #if defined(__x86_64__) || defined(__i386__)
 
 static
@@ -92,13 +108,8 @@ uint64_t rdpmc(unsigned int counter)
 	return low | ((uint64_t) high) << 32;
 }
 
-static bool arch_perf_use_read(void)
-{
-	return false;
-}
-
 static
-uint64_t read_perf_counter(
+uint64_t arch_read_perf_counter(
 		struct lttng_perf_counter_thread_field *thread_field)
 {
 	uint32_t seq, idx;
@@ -113,7 +124,7 @@ uint64_t read_perf_counter(
 		cmm_barrier();
 
 		idx = pc->index;
-		if (idx) {
+		if (caa_likely(pc->cap_user_rdpmc && idx)) {
 			int64_t pmcval;
 
 			pmcval = rdpmc(idx - 1);
@@ -122,7 +133,8 @@ uint64_t read_perf_counter(
 			pmcval >>= 64 - pc->pmc_width;
 			count = pc->offset + pmcval;
 		} else {
-			count = 0;
+			/* Fall-back on system call if rdpmc cannot be used. */
+			return read_perf_counter_syscall(thread_field);
 		}
 		cmm_barrier();
 	} while (CMM_LOAD_SHARED(pc->lock) != seq);
@@ -130,34 +142,33 @@ uint64_t read_perf_counter(
 	return count;
 }
 
-#elif defined (__ARM_ARCH_7A__)
-
-static bool arch_perf_use_read(void)
-{
-	return true;
-}
-
 static
-uint64_t read_perf_counter(
-		struct lttng_perf_counter_thread_field *thread_field)
+int arch_perf_keep_fd(struct lttng_perf_counter_thread_field *thread_field)
 {
-	uint64_t count;
+	struct perf_event_mmap_page *pc = thread_field->pc;
 
-	if (caa_unlikely(thread_field->fd < 0))
+	if (!pc)
 		return 0;
+	return !pc->cap_user_rdpmc;
+}
 
-	if (caa_unlikely(read(thread_field->fd, &count, sizeof(count))
-				< sizeof(count)))
-		return 0;
+#else
 
-	return count;
+/* Generic (slow) implementation using a read system call. */
+static
+uint64_t arch_read_perf_counter(
+		struct lttng_perf_counter_thread_field *thread_field)
+{
+	return read_perf_counter_syscall(thread_field);
 }
 
-#else /* defined(__x86_64__) || defined(__i386__) || defined(__ARM_ARCH_7A__) */
-
-#error "Perf event counters are only supported on x86 and ARMv7 so far."
+static
+int arch_perf_keep_fd(struct lttng_perf_counter_thread_field *thread_field)
+{
+	return 1;
+}
 
-#endif /* #else defined(__x86_64__) || defined(__i386__) || defined(__ARM_ARCH_7A__) */
+#endif
 
 static
 int sys_perf_event_open(struct perf_event_attr *attr,
@@ -205,7 +216,7 @@ struct perf_event_mmap_page *setup_perf(
 	if (perf_addr == MAP_FAILED)
 		perf_addr = NULL;
 
-	if (!arch_perf_use_read()) {
+	if (!arch_perf_keep_fd(thread_field)) {
 		close_perf_fd(thread_field->fd);
 		thread_field->fd = -1;
 	}
@@ -330,7 +341,7 @@ uint64_t wrapper_perf_counter_read(struct lttng_ctx_field *field)
 
 	perf_field = field->u.perf_counter;
 	perf_thread_field = get_thread_field(perf_field);
-	return read_perf_counter(perf_thread_field);
+	return arch_read_perf_counter(perf_thread_field);
 }
 
 static