[tip:perfcounters/core] perf_counter: fix perf_poll()

Peter Zijlstra
2009-03-24T13:07:37Z

Commit-ID: 47a6aae483f6b98509a4565ad2d8916de77ee078
Gitweb: http://git.kernel.org/tip/47a6aae483f6b98509a4565ad2d8916de77ee078
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 24 Mar 2009 13:18:16 +0100
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Tue, 24 Mar 2009 13:28:56 +0100

perf_counter: fix perf_poll()

Impact: fix kerneltop 100% CPU usage

Only return a poll event when there's actually been one, poll_wait()
doesn't actually wait for the waitq you pass it, it only enqueues
you on it.

Only once all FDs have been iterated and none of thm returned a
poll-event will it schedule().

Also make it return POLL_HUP when there's not mmap() area to read from.

Further, fix a silly bug in the write code.

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arjan van de Ven <arjan@infradead.org>
LKML-Reference: <1237897096.24918.181.camel@twins>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

---
include/linux/perf_counter.h | 1 +
kernel/perf_counter.c | 14 ++++++++++++--
2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 2b5e66d..48212c1 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -246,6 +246,7 @@ struct file;
struct perf_mmap_data {
struct rcu_head rcu_head;
int nr_pages;
+ atomic_t wakeup;
atomic_t head;
struct perf_counter_mmap_page *user_page;
void *data_pages[0];
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 0dfe910..affe227 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1161,7 +1161,16 @@ perf_read(struct file *file, char __user *buf, size_t count, loff_t *ppos)
static unsigned int perf_poll(struct file *file, poll_table *wait)
{
struct perf_counter *counter = file->private_data;
- unsigned int events = POLLIN;
+ struct perf_mmap_data *data;
+ unsigned int events;
+
+ rcu_read_lock();
+ data = rcu_dereference(counter->data);
+ if (data)
+ events = atomic_xchg(&data->wakeup, 0);
+ else
+ events = POLL_HUP;
+ rcu_read_unlock();

poll_wait(file, &counter->waitq, wait);

@@ -1425,7 +1434,7 @@ static int perf_output_write(struct perf_counter *counter, int nmi,

do {
offset = head = atomic_read(&data->head);
- head += sizeof(u64);
+ head += size;
} while (atomic_cmpxchg(&data->head, offset, head) != offset);

wakeup = (offset >> PAGE_SHIFT) != (head >> PAGE_SHIFT);
@@ -1446,6 +1455,7 @@ static int perf_output_write(struct perf_counter *counter, int nmi,
* generate a poll() wakeup for every page boundary crossed
*/
if (wakeup) {
+ atomic_xchg(&data->wakeup, POLL_IN);
__perf_counter_update_userpage(counter, data);
if (nmi) {
counter->wakeup_pending = 1;