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>

1 files changed, 76 insertions, 0 deletions

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index ce34bff07bda..d9cfd902140e 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,
 };
 
 /*