linux-kernel - Re: [PATCH V3 04/12] perf mmap: introduce perf_mmap__read

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Date:   Tue, 2 Jan 2018 16:33:43 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     kan.liang@...el.com
Cc:     acme@...nel.org, peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, wangnan0@...wei.com,
        jolsa@...nel.org, ak@...ux.intel.com, yao.jin@...ux.intel.com,
        kernel-team@....com
Subject: Re: [PATCH V3 04/12] perf mmap: introduce perf_mmap__read_done

Hello,

On Thu, Dec 21, 2017 at 10:08:46AM -0800, kan.liang@...el.com wrote:
> From: Kan Liang <kan.liang@...el.com>
> 
> The direction of overwrite mode is backward. The last mmap__read_event
> will set tail to map->prev. Need to correct the map->prev to head which
> is the end of next read.

Why do you update the map->prev needlessly then?  I think we don't
need it for overwrite/backward mode, right?

Also I guess the current code might miss some events since the head
can be different between _read_init() and _read_done(), no?

Thanks,
Namhyung


> 
> It will be used later.
> 
> Signed-off-by: Kan Liang <kan.liang@...el.com>
> ---
>  tools/perf/util/mmap.c | 11 +++++++++++
>  tools/perf/util/mmap.h |  1 +
>  2 files changed, 12 insertions(+)
> 
> diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
> index a844a2f..4aaeb64 100644
> --- a/tools/perf/util/mmap.c
> +++ b/tools/perf/util/mmap.c
> @@ -343,3 +343,14 @@ int perf_mmap__push(struct perf_mmap *md, bool overwrite,
>  out:
>  	return rc;
>  }
> +
> +/*
> + * Mandatory for overwrite mode
> + * The direction of overwrite mode is backward.
> + * The last mmap__read_event will set tail to map->prev.
> + * Need to correct the map->prev to head which is the end of next read.
> + */
> +void perf_mmap__read_done(struct perf_mmap *map)
> +{
> +	map->prev = perf_mmap__read_head(map);
> +}
> diff --git a/tools/perf/util/mmap.h b/tools/perf/util/mmap.h
> index abe9b9f..2df27c1 100644
> --- a/tools/perf/util/mmap.h
> +++ b/tools/perf/util/mmap.h
> @@ -96,4 +96,5 @@ size_t perf_mmap__mmap_len(struct perf_mmap *map);
>  
>  int perf_mmap__read_init(struct perf_mmap *map, bool overwrite,
>  			 u64 *start, u64 *end);
> +void perf_mmap__read_done(struct perf_mmap *map);
>  #endif /*__PERF_MMAP_H */
> -- 
> 2.5.5
>