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perf_counter: Allow for a wakeup watermark 

Currently we wake the mmap() consumer once every PAGE_SIZE of data
and/or once event wakeup_events when specified.

For high speed sampling this results in too many wakeups wrt. the
buffer size, hence change this.

We move the default wakeup limit to 1/4-th the buffer size, and
provide for means to manually specify this limit.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra 2009-09-17 19:01:10 +02:00 committed by Ingo Molnar
parent 850bc73ffc
commit 2667de81f3
2 changed files with 27 additions and 15 deletions
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10
include/linux/perf_counter.h
View File

@ -199,10 +199,14 @@ struct perf_counter_attr {
inherit_stat : 1, /* per task counts */
enable_on_exec : 1, /* next exec enables */
task : 1, /* trace fork/exit */
watermark : 1, /* wakeup_watermark */
__reserved_1 : 50;
__reserved_1 : 49;
__u32 wakeup_events; /* wakeup every n events */
union {
__u32 wakeup_events; /* wakeup every n events */
__u32 wakeup_watermark; /* bytes before wakeup */
};
__u32 __reserved_2;
__u64 __reserved_3;
@ -521,6 +525,8 @@ struct perf_mmap_data {
atomic_t wakeup; /* needs a wakeup */
atomic_t lost; /* nr records lost */
long watermark; /* wakeup watermark */
struct perf_counter_mmap_page *user_page;
void *data_pages[0];
};

32
kernel/perf_counter.c
View File

@ -2176,6 +2176,13 @@ static int perf_mmap_data_alloc(struct perf_counter *counter, int nr_pages)
data->nr_pages = nr_pages;
atomic_set(&data->lock, -1);
if (counter->attr.watermark) {
data->watermark = min_t(long, PAGE_SIZE * nr_pages,
counter->attr.wakeup_watermark);
}
if (!data->watermark)
data->watermark = max(PAGE_SIZE, PAGE_SIZE * nr_pages / 4);
rcu_assign_pointer(counter->data, data);
return 0;
@ -2517,23 +2524,15 @@ struct perf_output_handle {
unsigned long flags;
};
static bool perf_output_space(struct perf_mmap_data *data,
unsigned int offset, unsigned int head)
static bool perf_output_space(struct perf_mmap_data *data, unsigned long tail,
unsigned long offset, unsigned long head)
{
unsigned long tail;
unsigned long mask;
if (!data->writable)
return true;
mask = (data->nr_pages << PAGE_SHIFT) - 1;
/*
* Userspace could choose to issue a mb() before updating the tail
* pointer. So that all reads will be completed before the write is
* issued.
*/
tail = ACCESS_ONCE(data->user_page->data_tail);
smp_rmb();
offset = (offset - tail) & mask;
head = (head - tail) & mask;
@ -2679,7 +2678,7 @@ static int perf_output_begin(struct perf_output_handle *handle,
{
struct perf_counter *output_counter;
struct perf_mmap_data *data;
unsigned int offset, head;
unsigned long tail, offset, head;
int have_lost;
struct {
struct perf_event_header header;
@ -2717,16 +2716,23 @@ static int perf_output_begin(struct perf_output_handle *handle,
perf_output_lock(handle);
do {
/*
* Userspace could choose to issue a mb() before updating the
* tail pointer. So that all reads will be completed before the
* write is issued.
*/
tail = ACCESS_ONCE(data->user_page->data_tail);
smp_rmb();
offset = head = atomic_long_read(&data->head);
head += size;
if (unlikely(!perf_output_space(data, offset, head)))
if (unlikely(!perf_output_space(data, tail, offset, head)))
goto fail;
} while (atomic_long_cmpxchg(&data->head, offset, head) != offset);
handle->offset = offset;
handle->head = head;
if ((offset >> PAGE_SHIFT) != (head >> PAGE_SHIFT))
if (head - tail > data->watermark)
atomic_set(&data->wakeup, 1);
if (have_lost) {