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SubjectRe: perf: [patch] regression with PERF_EVENT_IOC_REFRESH
On Sun, 29 May 2011, Vince Weaver wrote:

> On Sat, 28 May 2011, Peter Zijlstra wrote:
>
> > On Fri, 2011-05-27 at 23:38 -0400, Vince Weaver wrote:
> > > on that note (and while trying to document exactly what the ioctls do) it
> > > seems that a PERF_EVENT_IOC_REFRESH with an argument of anything higher
> > > than one does not work on kernels 2.6.36 and newer. The behavior acts
> > > as if 1 was passed, even if you pass in, say, 3.
> >
> > Urgh, no that should definitely work. Thanks for the test-case, I'll
> > work on that (probably not until Monday though, but who knows).
> >
>
> after a painfully long bisection, it turns out that this problem was in
> theory introduced by the following commit:
>
> d57e34fdd60be7ffd0b1d86bfa1a553df86b7172
>
> perf: Simplify the ring-buffer logic: make perf_buffer_alloc() do everything needed
>
> I'll see if I can come up with a patch, but it's a bit non-obvious why
> this commit is affecting the REFRESH value at all.

the problem was the mentioned commit tried to optimize the use of
watermark and wakeup_watermark without taking into account that
wakeup_watermark is a union with wakeup_events.

The patch below *should* fix it, but something unrelated has broken
overflow support between 2.6.39 and 3.0-rc1 which I haven't had time to
investigate. The overflow count is suddenly about 10x what it should be
though. So the below is semi-untested and I possibly need to do another
bisect. *sigh*

Vince

Signed-off-by: Vince Weaver <vweaver1@cl320.eecs.utk.edu>


diff --git a/kernel/events/core.c b/kernel/events/core.c
index d863b3c..e3ff283 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3403,12 +3403,14 @@ unlock:
static unsigned long perf_data_size(struct perf_buffer *buffer);

static void
-perf_buffer_init(struct perf_buffer *buffer, long watermark, int flags)
+perf_buffer_init(struct perf_buffer *buffer,
+ long watermark,
+ long wakeup_watermark, int flags)
{
long max_size = perf_data_size(buffer);

if (watermark)
- buffer->watermark = min(max_size, watermark);
+ buffer->watermark = min(max_size, wakeup_watermark);

if (!buffer->watermark)
buffer->watermark = max_size / 2;
@@ -3451,7 +3453,8 @@ static void *perf_mmap_alloc_page(int cpu)
}

static struct perf_buffer *
-perf_buffer_alloc(int nr_pages, long watermark, int cpu, int flags)
+perf_buffer_alloc(int nr_pages, long watermark,
+ long wakeup_watermark, int cpu, int flags)
{
struct perf_buffer *buffer;
unsigned long size;
@@ -3476,7 +3479,7 @@ perf_buffer_alloc(int nr_pages, long watermark, int cpu, int flags)

buffer->nr_pages = nr_pages;

- perf_buffer_init(buffer, watermark, flags);
+ perf_buffer_init(buffer, watermark, wakeup_watermark, flags);

return buffer;

@@ -3568,7 +3571,8 @@ static void perf_buffer_free(struct perf_buffer *buffer)
}

static struct perf_buffer *
-perf_buffer_alloc(int nr_pages, long watermark, int cpu, int flags)
+perf_buffer_alloc(int nr_pages, long watermark,
+ long wakeup_watermark, int cpu, int flags)
{
struct perf_buffer *buffer;
unsigned long size;
@@ -3592,7 +3596,7 @@ perf_buffer_alloc(int nr_pages, long watermark, int cpu, int flags)
buffer->page_order = ilog2(nr_pages);
buffer->nr_pages = 1;

- perf_buffer_init(buffer, watermark, flags);
+ perf_buffer_init(buffer, watermark, wakeup_watermark, flags);

return buffer;

@@ -3787,7 +3791,9 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
if (vma->vm_flags & VM_WRITE)
flags |= PERF_BUFFER_WRITABLE;

- buffer = perf_buffer_alloc(nr_pages, event->attr.wakeup_watermark,
+ buffer = perf_buffer_alloc(nr_pages,
+ event->attr.watermark,
+ event->attr.wakeup_watermark,
event->cpu, flags);
if (!buffer) {
ret = -ENOMEM;

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