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    Subject[PATCH 02/10] perf tool: fix: Don't discard prev in backward mode
    Date
    From: Kan Liang <kan.liang@intel.com>

    Perf record can switch output. The new output should only store the data
    after switching. However, in overwrite backward mode, the new output
    still have the data from old output.

    At the end of mmap_read, the position of processed ring buffer is saved
    in md->prev. Next mmap_read should be end in md->prev.
    However, the md->prev is discarded. So next mmap_read has to process
    whole valid ring buffer, which definitely include the old processed
    data.

    Set the prev as the end of the range in backward mode.

    Signed-off-by: Kan Liang <kan.liang@intel.com>
    ---
    tools/perf/util/evlist.c | 14 +++++++++++++-
    1 file changed, 13 insertions(+), 1 deletion(-)

    diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
    index 33b8837..7d23cf5 100644
    --- a/tools/perf/util/evlist.c
    +++ b/tools/perf/util/evlist.c
    @@ -742,13 +742,25 @@ static int
    rb_find_range(void *data, int mask, u64 head, u64 old,
    u64 *start, u64 *end, bool backward)
    {
    + int ret;
    +
    if (!backward) {
    *start = old;
    *end = head;
    return 0;
    }

    - return backward_rb_find_range(data, mask, head, start, end);
    + ret = backward_rb_find_range(data, mask, head, start, end);
    +
    + /*
    + * The start and end from backward_rb_find_range is the range for all
    + * valid data in ring buffer.
    + * However, part of the data is processed previously.
    + * Reset the end to drop the processed data
    + */
    + *end = old;
    +
    + return ret;
    }

    /*
    --
    2.5.5
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