perf_counter: add an mmap method to allow userspace to read hardware counters
Impact: new feature giving performance improvement
This adds the ability for userspace to do an mmap on a hardware counter
fd and get access to a read-only page that contains the information
needed to translate a hardware counter value to the full 64-bit
counter value that would be returned by a read on the fd. This is
useful on architectures that allow user programs to read the hardware
counters, such as PowerPC.
The mmap will only succeed if the counter is a hardware counter
monitoring the current process.
On my quad 2.5GHz PowerPC 970MP machine, userspace can read a counter
and translate it to the full 64-bit value in about 30ns using the
mmapped page, compared to about 830ns for the read syscall on the
counter, so this does give a significant performance improvement.
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Orig-LKML-Reference: <20090323172417.297057964@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 18dc17d..40b324e 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -143,6 +143,17 @@
#define PERF_COUNTER_IOC_ENABLE _IO('$', 0)
#define PERF_COUNTER_IOC_DISABLE _IO('$', 1)
+/*
+ * Structure of the page that can be mapped via mmap
+ */
+struct perf_counter_mmap_page {
+ __u32 version; /* version number of this structure */
+ __u32 compat_version; /* lowest version this is compat with */
+ __u32 lock; /* seqlock for synchronization */
+ __u32 index; /* hardware counter identifier */
+ __s64 offset; /* add to hardware counter value */
+};
+
#ifdef __KERNEL__
/*
* Kernel-internal data types and definitions:
@@ -278,6 +289,9 @@
int oncpu;
int cpu;
+ /* pointer to page shared with userspace via mmap */
+ unsigned long user_page;
+
/* read() / irq related data */
wait_queue_head_t waitq;
/* optional: for NMIs */
@@ -361,6 +375,7 @@
extern int hw_perf_group_sched_in(struct perf_counter *group_leader,
struct perf_cpu_context *cpuctx,
struct perf_counter_context *ctx, int cpu);
+extern void perf_counter_update_userpage(struct perf_counter *counter);
extern void perf_counter_output(struct perf_counter *counter,
int nmi, struct pt_regs *regs);