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Subject[PATCH 3/5] perf: Convert the perf output buffer to local_t
Since there is now only a single writer, we can use local_t instead
and avoid all these pesky LOCK insn.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/perf_event.h | 15 +++++++--------
kernel/perf_event.c | 30 +++++++++++++++---------------
2 files changed, 22 insertions(+), 23 deletions(-)

Index: linux-2.6/include/linux/perf_event.h
===================================================================
--- linux-2.6.orig/include/linux/perf_event.h
+++ linux-2.6/include/linux/perf_event.h
@@ -485,6 +485,7 @@ struct perf_guest_info_callbacks {
#include <linux/ftrace.h>
#include <linux/cpu.h>
#include <asm/atomic.h>
+#include <asm/local.h>

#define PERF_MAX_STACK_DEPTH 255

@@ -588,20 +589,18 @@ struct perf_mmap_data {
#ifdef CONFIG_PERF_USE_VMALLOC
struct work_struct work;
#endif
- int data_order;
+ int data_order; /* allocation order */
int nr_pages; /* nr of data pages */
int writable; /* are we writable */
int nr_locked; /* nr pages mlocked */

atomic_t poll; /* POLL_ for wakeups */
- atomic_t events; /* event_id limit */

- atomic_long_t head; /* write position */
-
- atomic_t wakeup; /* needs a wakeup */
- atomic_t lost; /* nr records lost */
-
- atomic_t nest; /* nested writers */
+ local_t head; /* write position */
+ local_t nest; /* nested writers */
+ local_t events; /* event limit */
+ local_t wakeup; /* needs a wakeup */
+ local_t lost; /* nr records lost */

long watermark; /* wakeup watermark */

Index: linux-2.6/kernel/perf_event.c
===================================================================
--- linux-2.6.orig/kernel/perf_event.c
+++ linux-2.6/kernel/perf_event.c
@@ -2916,7 +2916,7 @@ static void perf_output_get_handle(struc
struct perf_mmap_data *data = handle->data;

preempt_disable();
- atomic_inc(&data->nest);
+ local_inc(&data->nest);
}

static void perf_output_put_handle(struct perf_output_handle *handle)
@@ -2925,13 +2925,13 @@ static void perf_output_put_handle(struc
unsigned long head;

again:
- head = atomic_long_read(&data->head);
+ head = local_read(&data->head);

/*
* IRQ/NMI can happen here, which means we can miss a head update.
*/

- if (!atomic_dec_and_test(&data->nest))
+ if (!local_dec_and_test(&data->nest))
return;

/*
@@ -2945,12 +2945,12 @@ again:
* Now check if we missed an update, rely on the (compiler)
* barrier in atomic_dec_and_test() to re-read data->head.
*/
- if (unlikely(head != atomic_long_read(&data->head))) {
- atomic_inc(&data->nest);
+ if (unlikely(head != local_read(&data->head))) {
+ local_inc(&data->nest);
goto again;
}

- if (atomic_xchg(&data->wakeup, 0))
+ if (local_xchg(&data->wakeup, 0))
perf_output_wakeup(handle);

preempt_enable();
@@ -3031,7 +3031,7 @@ int perf_output_begin(struct perf_output
if (!data->nr_pages)
goto out;

- have_lost = atomic_read(&data->lost);
+ have_lost = local_read(&data->lost);
if (have_lost)
size += sizeof(lost_event);

@@ -3045,24 +3045,24 @@ int perf_output_begin(struct perf_output
*/
tail = ACCESS_ONCE(data->user_page->data_tail);
smp_rmb();
- offset = head = atomic_long_read(&data->head);
+ offset = head = local_read(&data->head);
head += size;
if (unlikely(!perf_output_space(data, tail, offset, head)))
goto fail;
- } while (atomic_long_cmpxchg(&data->head, offset, head) != offset);
+ } while (local_cmpxchg(&data->head, offset, head) != offset);

handle->offset = offset;
handle->head = head;

if (head - tail > data->watermark)
- atomic_inc(&data->wakeup);
+ local_inc(&data->wakeup);

if (have_lost) {
lost_event.header.type = PERF_RECORD_LOST;
lost_event.header.misc = 0;
lost_event.header.size = sizeof(lost_event);
lost_event.id = event->id;
- lost_event.lost = atomic_xchg(&data->lost, 0);
+ lost_event.lost = local_xchg(&data->lost, 0);

perf_output_put(handle, lost_event);
}
@@ -3070,7 +3070,7 @@ int perf_output_begin(struct perf_output
return 0;

fail:
- atomic_inc(&data->lost);
+ local_inc(&data->lost);
perf_output_put_handle(handle);
out:
rcu_read_unlock();
@@ -3086,10 +3086,10 @@ void perf_output_end(struct perf_output_
int wakeup_events = event->attr.wakeup_events;

if (handle->sample && wakeup_events) {
- int events = atomic_inc_return(&data->events);
+ int events = local_inc_return(&data->events);
if (events >= wakeup_events) {
- atomic_sub(wakeup_events, &data->events);
- atomic_inc(&data->wakeup);
+ local_sub(wakeup_events, &data->events);
+ local_inc(&data->wakeup);
}
}




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