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RE: [PATCH] perf tool: Don't discard prev in backward mode




> 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@xxxxxxxxxx>
> Cc: Liang Kan <kan.liang@xxxxxxxxx>
> ---

The patch looks good to me.

Tested-by: Kan Liang <kan.liang@xxxxxxxxx>


>  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)) {
> --
> 2.9.3