Re: [PATCH v2 3/4] perf tools: Support reading from backward ring buffer

[Arnaldo Carvalho de Melo]

Em Wed, Apr 27, 2016 at 02:19:22AM +0000, Wang Nan escreveu:
> perf_evlist__mmap_read_backward() is introduced for reading backward
> ring buffer. Different from reading forward, before reading, caller
> needs to call perf_evlist__mmap_read_catchup() first.
> 
> Backward ring buffer should be read from 'head' pointer, not '0'.
> perf_evlist__mmap_read_catchup() saves 'head' to 'md->prev', then
> make it remember the start position after each reading.
> 
> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Zefan Li <lizefan@xxxxxxxxxx>
> Cc: pi3orama@xxxxxxx
> ---
>  tools/perf/util/evlist.c | 39 +++++++++++++++++++++++++++++++++++++++
>  tools/perf/util/evlist.h |  4 ++++
>  2 files changed, 43 insertions(+)
> 
> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> index 17cd014..2e0b7b0 100644
> --- a/tools/perf/util/evlist.c
> +++ b/tools/perf/util/evlist.c
> @@ -766,6 +766,45 @@ union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx)
>  	return perf_mmap__read(md, evlist->overwrite, old, head, &md->prev);
>  }
>  
> +union perf_event *
> +perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)
> +{
> +	struct perf_mmap *md = &evlist->mmap[idx];
> +	u64 head, end;
> +	u64 start = md->prev;
> +
> +	/*
> +	 * Check if event was unmapped due to a POLLHUP/POLLERR.
> +	 */
> +	if (!atomic_read(&md->refcnt))
> +		return NULL;
> +
> +	/* NOTE: head is negative in this case */

What is this comment for, can you ellaborate? Are you double sure this
arithmetic with u64, negative values, that -head below are all ok?

I've applied the first two patches in this series.

I also need to check why we now need that catchup thing :-\

> +	head = perf_mmap__read_head(md);
> +
> +	if (!head)
> +		return NULL;
> +
> +	end = head + md->mask + 1;
> +
> +	if ((end - head) > -head)
> +		end = 0;
> +
> +	return perf_mmap__read(md, false, start, end, &md->prev);
> +}
> +
> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx)
> +{
> +	struct perf_mmap *md = &evlist->mmap[idx];
> +	u64 head;
> +
> +	if (!atomic_read(&md->refcnt))
> +		return;
> +
> +	head = perf_mmap__read_head(md);
> +	md->prev = head;
> +}
> +
>  static bool perf_mmap__empty(struct perf_mmap *md)
>  {
>  	return perf_mmap__read_head(md) == md->prev && !md->auxtrace_mmap.base;
> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
> index 208897a..85d1b59 100644
> --- a/tools/perf/util/evlist.h
> +++ b/tools/perf/util/evlist.h
> @@ -129,6 +129,10 @@ struct perf_sample_id *perf_evlist__id2sid(struct perf_evlist *evlist, u64 id);
>  
>  union perf_event *perf_evlist__mmap_read(struct perf_evlist *evlist, int idx);
>  
> +union perf_event *perf_evlist__mmap_read_backward(struct perf_evlist *evlist,
> +						  int idx);
> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int idx);
> +
>  void perf_evlist__mmap_consume(struct perf_evlist *evlist, int idx);
>  
>  int perf_evlist__open(struct perf_evlist *evlist);
> -- 
> 1.8.3.4