Re: [PATCH v1 03/11] perf: Allow for multiple ring buffers per event

[Peter Zijlstra]

How about something like this for the itrace thing?

You would mmap() the regular buffer; when write ->aux_{offset,size} in
the control page. After which you can do a second mmap() with the .pgoff
matching the aux_offset you gave and .length matching the aux_size you
gave.

This way the mmap() content still looks like a single linear file (could
be sparse if you leave a hole, although we could require the aux_offset
to match the end of the data section).

And there is still the single event->rb, not more.

Then, when data inside that aux data store changes they should inject an
PERF_RECORD_AUX to indicate this did happen, which ties it back into the
normal event flow.

With this there should be no difficult page table tricks or anything.

The patch is way incomplete but should sketch enough of the idea..

So the aux_head/tail values should also be in the file space and not
start at 0 again, similar for the offsets in the AUX record.

---
 include/uapi/linux/perf_event.h | 19 +++++++++++++++
 kernel/events/core.c            | 51 +++++++++++++++++++++++++++++++++++++----
 kernel/events/internal.h        |  6 +++++
 kernel/events/ring_buffer.c     |  8 +------
 4 files changed, 72 insertions(+), 12 deletions(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 853bc1ccb395..adef7c0f1e7c 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -491,6 +491,13 @@ struct perf_event_mmap_page {
 	 */
 	__u64   data_head;		/* head in the data section */
 	__u64	data_tail;		/* user-space written tail */
+	__u64	data_offset;
+	__u64	data_size;
+
+	__u64	aux_head;
+	__u64	aux_tail;
+	__u64	aux_offset;
+	__u64	aux_size;
 };
 
 #define PERF_RECORD_MISC_CPUMODE_MASK		(7 << 0)
@@ -705,6 +712,18 @@ enum perf_event_type {
 	 */
 	PERF_RECORD_MMAP2			= 10,
 
+	/*
+	 * Records that new data landed in the AUX buffer part.
+	 *
+	 * struct {
+	 * 	struct perf_event_header	header;
+	 *
+	 * 	u64				aux_offset;
+	 * 	u64				aux_size;
+	 * };
+	 */
+	PERF_RECORD_AUX				= 11,
+
 	PERF_RECORD_MAX,			/* non-ABI */
 };
 
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5129b1201050..993995a23b73 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4016,7 +4016,7 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 
 static const struct vm_operations_struct perf_mmap_vmops = {
 	.open		= perf_mmap_open,
-	.close		= perf_mmap_close,
+	.close		= perf_mmap_close, /* non mergable */
 	.fault		= perf_mmap_fault,
 	.page_mkwrite	= perf_mmap_fault,
 };
@@ -4030,6 +4030,7 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	struct ring_buffer *rb;
 	unsigned long vma_size;
 	unsigned long nr_pages;
+	unsigned long pgoff;
 	long user_extra, extra;
 	int ret = 0, flags = 0;
 
@@ -4045,7 +4046,50 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 		return -EINVAL;
 
 	vma_size = vma->vm_end - vma->vm_start;
-	nr_pages = (vma_size / PAGE_SIZE) - 1;
+
+	if (vma->vm_pgoff == 0) {
+		nr_pages = (vma_size / PAGE_SIZE) - 1;
+	} else {
+		if (!event->rb)
+			return -EINVAL;
+
+		nr_pages = vma_size / PAGE_SIZE;
+
+		mutex_lock(&event->mmap_mutex);
+		ret = -EINVAL;
+		if (!event->rb)
+			goto err_aux_unlock;
+
+		if (!atomic_inc_not_zero(&event->rb->mmap_count))
+			goto err_aux_unlock;
+
+		if (userpg->aux_offset < userpg->data_offset + userpg->data_size)
+			goto err_aux_unlock;
+
+		pgoff = userpg->aux_offset;
+		if (pgoff & ~PAGE_MASK)
+			goto err_aux_unlock;
+
+		pgoff >>= PAGE_SHIFT;
+		if (pgoff != vma->vm_pgoff)
+			goto err_aux_unlock;
+
+		/* XXX do we want to allow !power_of_2 sizes, for AUX?  */
+		if (nr_pages == 0 || !is_power_of_2(nr_pages))
+			goto err_aux_unlock;
+
+		if (vma_size != PAGE_SIZE * nr_pages)
+			goto err_aux_unlock;
+
+		if (userpg->aux_size != vma_size)
+			goto err_aux_unlock;
+			
+		ret = rb_alloc_aux(event->rb, userpg->aux_offset >> PAGE_SHIFT, nr_pages);
+
+err_aux_unlock:
+		mutex_unlock(&event->mmap_mutex);
+		return ret;
+	}
 
 	/*
 	 * If we have rb pages ensure they're a power-of-two number, so we
@@ -4057,9 +4101,6 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 	if (vma_size != PAGE_SIZE * (1 + nr_pages))
 		return -EINVAL;
 
-	if (vma->vm_pgoff != 0)
-		return -EINVAL;
-
 	WARN_ON_ONCE(event->ctx->parent_ctx);
 again:
 	mutex_lock(&event->mmap_mutex);
diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 569b218782ad..6258aaa36097 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -36,6 +36,7 @@ struct ring_buffer {
 	struct user_struct		*mmap_user;
 
 	struct perf_event_mmap_page	*user_page;
+	struct radix_tree_root		page_tree;
 	void				*data_pages[0];
 };
 
diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 146a5792b1d2..b82505325df0 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -251,13 +251,7 @@ ring_buffer_init(struct ring_buffer *rb, long watermark, int flags)
 struct page *
 perf_mmap_to_page(struct ring_buffer *rb, unsigned long pgoff)
 {
-	if (pgoff > rb->nr_pages)
-		return NULL;
-
-	if (pgoff == 0)
-		return virt_to_page(rb->user_page);
-
-	return virt_to_page(rb->data_pages[pgoff - 1]);
+	return radix_tree_lookup(&rb->page_tree, pgoff);
 }
 
 static void *perf_mmap_alloc_page(int cpu)

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