[PATCH 3.16 053/294] perf/core: Invert perf_read_group() loops

From: Ben Hutchings
Date: Mon Nov 06 2017 - 19:29:13 EST


3.16.50-rc1 review patch. If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

commit fa8c269353d560b7c28119ad7617029f92e40b15 upstream.

In order to enable the use of perf_event_read(.group = true), we need
to invert the sibling-child loop nesting of perf_read_group().

Currently we iterate the child list for each sibling, this precludes
using group reads. Flip things around so we iterate each group for
each child.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
[ Made the patch compile and things. ]
Signed-off-by: Sukadev Bhattiprolu <sukadev@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Stephane Eranian <eranian@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Vince Weaver <vincent.weaver@xxxxxxxxx>
Link: http://lkml.kernel.org/r/1441336073-22750-7-git-send-email-sukadev@xxxxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
[bwh: Backported to 3.16 as a dependency of commit 2aeb18835476 ("perf/core: Fix
locking for children siblings group read"):
- Keep the function name perf_event_read_group()
- Keep using perf_event_read_value()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3565,50 +3565,71 @@ u64 perf_event_read_value(struct perf_ev
}
EXPORT_SYMBOL_GPL(perf_event_read_value);

-static int perf_event_read_group(struct perf_event *event,
- u64 read_format, char __user *buf)
+static void __perf_read_group_add(struct perf_event *leader,
+ u64 read_format, u64 *values)
{
- struct perf_event *leader = event->group_leader, *sub;
- struct perf_event_context *ctx = leader->ctx;
- int n = 0, size = 0, ret;
+ struct perf_event *sub;
+ int n = 1; /* skip @nr */
u64 count, enabled, running;
- u64 values[5];
-
- lockdep_assert_held(&ctx->mutex);

count = perf_event_read_value(leader, &enabled, &running);

- values[n++] = 1 + leader->nr_siblings;
+ /*
+ * Since we co-schedule groups, {enabled,running} times of siblings
+ * will be identical to those of the leader, so we only publish one
+ * set.
+ */
if (read_format & PERF_FORMAT_TOTAL_TIME_ENABLED)
values[n++] = enabled;
if (read_format & PERF_FORMAT_TOTAL_TIME_RUNNING)
values[n++] = running;
- values[n++] = count;
+
+ /*
+ * Write {count,id} tuples for every sibling.
+ */
+ values[n++] += count;
if (read_format & PERF_FORMAT_ID)
values[n++] = primary_event_id(leader);

- size = n * sizeof(u64);
-
- if (copy_to_user(buf, values, size))
- return -EFAULT;
-
- ret = size;
-
list_for_each_entry(sub, &leader->sibling_list, group_entry) {
- n = 0;
-
values[n++] = perf_event_read_value(sub, &enabled, &running);
if (read_format & PERF_FORMAT_ID)
values[n++] = primary_event_id(sub);
+ }
+}

- size = n * sizeof(u64);
+static int perf_event_read_group(struct perf_event *event,
+ u64 read_format, char __user *buf)
+{
+ struct perf_event *leader = event->group_leader, *child;
+ struct perf_event_context *ctx = leader->ctx;
+ int ret = event->read_size;
+ u64 *values;

- if (copy_to_user(buf + ret, values, size)) {
- return -EFAULT;
- }
+ lockdep_assert_held(&ctx->mutex);

- ret += size;
- }
+ values = kzalloc(event->read_size, GFP_KERNEL);
+ if (!values)
+ return -ENOMEM;
+
+ values[0] = 1 + leader->nr_siblings;
+
+ /*
+ * By locking the child_mutex of the leader we effectively
+ * lock the child list of all siblings.. XXX explain how.
+ */
+ mutex_lock(&leader->child_mutex);
+
+ __perf_read_group_add(leader, read_format, values);
+ list_for_each_entry(child, &leader->child_list, child_list)
+ __perf_read_group_add(child, read_format, values);
+
+ mutex_unlock(&leader->child_mutex);
+
+ if (copy_to_user(buf, values, event->read_size))
+ ret = -EFAULT;
+
+ kfree(values);

return ret;
}