[PATCH] perf: reorder perf_event_context to remove 8 bytes ofpadding on 64 bit builds

From: Richard Kennedy
Date: Tue May 15 2012 - 07:52:40 EST

Reorder structure perf_event_context to remove 8 bytes of padding on 64
bit builds, which also shrinks structure perf_cpu_context by 8 bytes.
Reducing its size to 256 bytes so allowing it to fit into one fewer
cache lines.

Signed-off-by: Richard Kennedy <richard@xxxxxxxxxxxxxxx>

patch against v3.4-rc7
compiled and tested on x86_64

I have given this patch some light testing and perf still seems to work

perf_pin_task_context() increments both the pin_count and the refcount
so it seems a sensible place to put pin_count so there isn't any padding
after refcount. Obviously we can reorder this structure in other ways if
you prefer.


diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index ddbb6a9..964c0cf 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1027,6 +1027,7 @@ struct perf_event_context {
int nr_freq;
int rotate_disable;
atomic_t refcount;
+ int pin_count;
struct task_struct *task;

@@ -1036,13 +1037,12 @@ struct perf_event_context {
u64 timestamp;

- * These fields let us detect when two contexts have both
- * been cloned (inherited) from a common ancestor.
+ * These fields (with pin_count) let us detect when two contexts
+ * have both been cloned (inherited) from a common ancestor.
struct perf_event_context *parent_ctx;
u64 parent_gen;
u64 generation;
- int pin_count;
int nr_cgroups; /* cgroup evts */
int nr_branch_stack; /* branch_stack evt */
struct rcu_head rcu_head;

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