Re: [PATCH v15 03/12] perf tools: Update perf evlist mmap related APIs and helpers

[Wangnan (F)]

On 2016/7/13 22:06, Jiri Olsa wrote:
> On Tue, Jul 12, 2016 at 10:00:03AM +0000, Wang Nan wrote:
>
> SNIP
>
> > -union perf_event *perf_evlist__mmap_read_forward(struct perf_evlist *evlist, int idx)
> > +union perf_event *perf_mmap__read_forward(struct perf_mmap *md, bool overwrite)
> >   {
> > -	struct perf_mmap *md = &evlist->mmap[idx];
> >   	u64 head;
> >   	u64 old = md->prev;
> >   
> > @@ -795,13 +795,12 @@ union perf_event *perf_evlist__mmap_read_forward(struct perf_evlist *evlist, int
> >   
> >   	head = perf_mmap__read_head(md);
> >   
> > -	return perf_mmap__read(md, evlist->overwrite, old, head, &md->prev);
> > +	return perf_mmap__read(md, overwrite, old, head, &md->prev);
> >   }
> >   
> >   union perf_event *
> > -perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)
> > +perf_mmap__read_backward(struct perf_mmap *md, bool overwrite __maybe_unused)
> >   {
> > -	struct perf_mmap *md = &evlist->mmap[idx];
> >   	u64 head, end;
> >   	u64 start = md->prev;
> >   
> > @@ -836,6 +835,20 @@ perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx)
> >   	return perf_mmap__read(md, false, start, end, &md->prev);
> should you pas evlist->overwrite in here?

No need. The second arg of perf_mmap__read() is not 'overwrite', but 
'check_messup'.
When set to 'true', stop reading if caller require a buffer larger than 
half of the
size of the ring buffer and report a warning. Only forward, read-only 
ring buffer
requires this check. We can always read the whole backward ring buffer 
because we
pause it before reading.

I'll add more comments here.

Thank you.

> thanks,
> jirka