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SubjectRe: [PATCH] perf tool: Don't discard prev in backward mode
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Hi Kan, Please see if this patch works for you.


On 2017/10/14 7:16, Wang Nan wrote:
> 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. That also brings extra overhead.
>
> 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 if it is
> not overwriten. That avoids to process duplicate data.
> However, the md->prev is discarded. So next mmap_read has to process
> whole valid ring buffer, which probably include the old processed
> data.
>
> Avoid calling backward_rb_find_range() when md->prev is still
> available.
>
> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> Cc: Liang Kan <kan.liang@intel.com>
> ---
> tools/perf/util/mmap.c | 33 +++++++++++++++------------------
> 1 file changed, 15 insertions(+), 18 deletions(-)
>
> diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
> index 9fe5f9c..df1de55 100644
> --- a/tools/perf/util/mmap.c
> +++ b/tools/perf/util/mmap.c
> @@ -287,18 +287,6 @@ static int backward_rb_find_range(void *buf, int mask, u64 head, u64 *start, u64
> return -1;
> }
>
> -static int rb_find_range(void *data, int mask, u64 head, u64 old,
> - u64 *start, u64 *end, bool backward)
> -{
> - if (!backward) {
> - *start = old;
> - *end = head;
> - return 0;
> - }
> -
> - return backward_rb_find_range(data, mask, head, start, end);
> -}
> -
> int perf_mmap__push(struct perf_mmap *md, bool overwrite, bool backward,
> void *to, int push(void *to, void *buf, size_t size))
> {
> @@ -310,19 +298,28 @@ int perf_mmap__push(struct perf_mmap *md, bool overwrite, bool backward,
> void *buf;
> int rc = 0;
>
> - if (rb_find_range(data, md->mask, head, old, &start, &end, backward))
> - return -1;
> + start = backward ? head : old;
> + end = backward ? old : head;
>
> if (start == end)
> return 0;
>
> size = end - start;
> if (size > (unsigned long)(md->mask) + 1) {
> - WARN_ONCE(1, "failed to keep up with mmap data. (warn only once)\n");
> + if (!backward) {
> + WARN_ONCE(1, "failed to keep up with mmap data. (warn only once)\n");
>
> - md->prev = head;
> - perf_mmap__consume(md, overwrite || backward);
> - return 0;
> + md->prev = head;
> + perf_mmap__consume(md, overwrite || backward);
> + return 0;
> + }
> +
> + /*
> + * Backward ring buffer is full. We still have a chance to read
> + * most of data from it.
> + */
> + if (backward_rb_find_range(data, md->mask, head, &start, &end))
> + return -1;
> }
>
> if ((start & md->mask) + size != (end & md->mask)) {


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