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Subject[tip:perfcounters/core] perf_counter: add an mmap method to allow userspace to read hardware counters
Commit-ID:  36e6cd42be5579128495e7d9e678638f4945de6e
Gitweb: http://git.kernel.org/tip/36e6cd42be5579128495e7d9e678638f4945de6e
Author: Paul Mackerras <paulus@samba.org>
AuthorDate: Mon, 23 Mar 2009 18:22:08 +0100
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Mon, 23 Mar 2009 21:45:09 +0100

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>
LKML-Reference: <20090323172417.297057964@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
arch/powerpc/kernel/perf_counter.c | 6 +++
include/linux/perf_counter.h | 15 +++++++
kernel/perf_counter.c | 76 ++++++++++++++++++++++++++++++++++++
3 files changed, 97 insertions(+), 0 deletions(-)
diff --git a/arch/powerpc/kernel/perf_counter.c b/arch/powerpc/kernel/perf_counter.c
index d056515..e434928 100644
--- a/arch/powerpc/kernel/perf_counter.c
+++ b/arch/powerpc/kernel/perf_counter.c
@@ -417,6 +417,8 @@ void hw_perf_restore(u64 disable)
atomic64_set(&counter->hw.prev_count, val);
counter->hw.idx = hwc_index[i] + 1;
write_pmc(counter->hw.idx, val);
+ if (counter->user_page)
+ perf_counter_update_userpage(counter);
}
mb();
cpuhw->mmcr[0] |= MMCR0_PMXE | MMCR0_FCECE;
@@ -572,6 +574,8 @@ static void power_perf_disable(struct perf_counter *counter)
ppmu->disable_pmc(counter->hw.idx - 1, cpuhw->mmcr);
write_pmc(counter->hw.idx, 0);
counter->hw.idx = 0;
+ if (counter->user_page)
+ perf_counter_update_userpage(counter);
break;
}
}
@@ -698,6 +702,8 @@ static void record_and_restart(struct perf_counter *counter, long val,
write_pmc(counter->hw.idx, val);
atomic64_set(&counter->hw.prev_count, val);
atomic64_set(&counter->hw.period_left, left);
+ if (counter->user_page)
+ perf_counter_update_userpage(counter);

/*
* Finally record data if requested.
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 @@ struct perf_counter_hw_event {
#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 @@ struct perf_counter {
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 perf_counter_task_enable(void);
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);
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index ce34bff..d9cfd90 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1177,6 +1177,7 @@ static int perf_release(struct inode *inode, struct file *file)
mutex_unlock(&counter->mutex);
mutex_unlock(&ctx->mutex);

+ free_page(counter->user_page);
free_counter(counter);
put_context(ctx);

@@ -1346,12 +1347,87 @@ static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
return err;
}

+void perf_counter_update_userpage(struct perf_counter *counter)
+{
+ struct perf_counter_mmap_page *userpg;
+
+ if (!counter->user_page)
+ return;
+ userpg = (struct perf_counter_mmap_page *) counter->user_page;
+
+ ++userpg->lock;
+ smp_wmb();
+ userpg->index = counter->hw.idx;
+ userpg->offset = atomic64_read(&counter->count);
+ if (counter->state == PERF_COUNTER_STATE_ACTIVE)
+ userpg->offset -= atomic64_read(&counter->hw.prev_count);
+ smp_wmb();
+ ++userpg->lock;
+}
+
+static int perf_mmap_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
+{
+ struct perf_counter *counter = vma->vm_file->private_data;
+
+ if (!counter->user_page)
+ return VM_FAULT_SIGBUS;
+
+ vmf->page = virt_to_page(counter->user_page);
+ get_page(vmf->page);
+ return 0;
+}
+
+static struct vm_operations_struct perf_mmap_vmops = {
+ .fault = perf_mmap_fault,
+};
+
+static int perf_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ struct perf_counter *counter = file->private_data;
+ unsigned long userpg;
+
+ if (!(vma->vm_flags & VM_SHARED) || (vma->vm_flags & VM_WRITE))
+ return -EINVAL;
+ if (vma->vm_end - vma->vm_start != PAGE_SIZE)
+ return -EINVAL;
+
+ /*
+ * For now, restrict to the case of a hardware counter
+ * on the current task.
+ */
+ if (is_software_counter(counter) || counter->task != current)
+ return -EINVAL;
+
+ userpg = counter->user_page;
+ if (!userpg) {
+ userpg = get_zeroed_page(GFP_KERNEL);
+ mutex_lock(&counter->mutex);
+ if (counter->user_page) {
+ free_page(userpg);
+ userpg = counter->user_page;
+ } else {
+ counter->user_page = userpg;
+ }
+ mutex_unlock(&counter->mutex);
+ if (!userpg)
+ return -ENOMEM;
+ }
+
+ perf_counter_update_userpage(counter);
+
+ vma->vm_flags &= ~VM_MAYWRITE;
+ vma->vm_flags |= VM_RESERVED;
+ vma->vm_ops = &perf_mmap_vmops;
+ return 0;
+}
+
static const struct file_operations perf_fops = {
.release = perf_release,
.read = perf_read,
.poll = perf_poll,
.unlocked_ioctl = perf_ioctl,
.compat_ioctl = perf_ioctl,
+ .mmap = perf_mmap,
};

/*

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