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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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