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    Subject[tip:perf/core] perf_events: Fix default watermark calculation
    Commit-ID:  8904b18046c2f050107f6449e887e7c1142b9ab9
    Gitweb: http://git.kernel.org/tip/8904b18046c2f050107f6449e887e7c1142b9ab9
    Author: Stephane Eranian <eranian@gmail.com>
    AuthorDate: Fri, 20 Nov 2009 22:19:57 +0100
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Sat, 21 Nov 2009 14:11:41 +0100

    perf_events: Fix default watermark calculation

    This patch fixes the default watermark value for the sampling
    buffer. With the existing calculation (watermark =
    max(PAGE_SIZE, max_size / 2)), no notification was ever received
    when the buffer was exactly 1 page. This was because you would
    never cross the threshold (there is no partial samples).

    In certain configuration, there was no possibilty detecting the
    problem because there was not enough space left to store the
    LOST record.In fact, there may be a more generic problem here.
    The kernel should ensure that there is alaways enough space to
    store one LOST record.

    This patch sets the default watermark to half the buffer size.
    With such limit, we are guaranteed to get a notification even
    with a single page buffer assuming no sample is bigger than a
    page.

    Signed-off-by: Stephane Eranian <eranian@gmail.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Paul Mackerras <paulus@samba.org>
    LKML-Reference: <20091120212509.344964101@chello.nl>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>
    LKML-Reference: <1256302576-6169-1-git-send-email-eranian@gmail.com>
    ---
    kernel/perf_event.c | 2 +-
    1 files changed, 1 insertions(+), 1 deletions(-)

    diff --git a/kernel/perf_event.c b/kernel/perf_event.c
    index 3ede098..718fa93 100644
    --- a/kernel/perf_event.c
    +++ b/kernel/perf_event.c
    @@ -2340,7 +2340,7 @@ perf_mmap_data_init(struct perf_event *event, struct perf_mmap_data *data)
    }

    if (!data->watermark)
    - data->watermark = max_t(long, PAGE_SIZE, max_size / 2);
    + data->watermark = max_size / 2;


    rcu_assign_pointer(event->data, data);

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