Re: [PATCH 1/2] perf mmap: Fix perf backward recording

[Wangnan (F)]

On 2017/11/1 20:39, Jiri Olsa wrote:
> On Wed, Nov 01, 2017 at 08:10:49PM +0800, Wangnan (F) wrote:
>
> SNIP
>
> > > > > diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> > > > > index c6c891e154a6..27ebe355e794 100644
> > > > > --- a/tools/perf/util/evlist.c
> > > > > +++ b/tools/perf/util/evlist.c
> > > > > @@ -838,6 +838,11 @@ static int perf_evlist__mmap_per_evsel(struct perf_evlist *evlist, int idx,
> > > > >                    if (*output == -1) {
> > > > >                            *output = fd;
> > > > > +                       if (evsel->attr.write_backward)
> > > > > +                               mp->prot = PROT_READ;
> > > > > +                       else
> > > > > +                               mp->prot = PROT_READ | PROT_WRITE;
> > > > > +
> > > > If evlist->overwrite is true, PROT_WRITE should be unset even if
> > > > write_backward is
> > > > not set. If you want to delay the setting of mp->prot, you need to consider
> > > > both evlist->overwrite and evsel->attr.write_backward.
> > > I thought evsel->attr.write_backward should be set when
> > > evlist->overwrite is set.  Do you mean following case?
> > >
> > >     perf record --overwrite -e 'cycles/no-overwrite/'
> > No. evlist->overwrite is unrelated to '--overwrite'. This is why I
> > said the concept of 'overwrite' and 'backward' is ambiguous.
> >
> > perf record never sets 'evlist->overwrite'. What '--overwrite' actually
> > does is setting write_backward. Some testcases needs overwrite evlist.
> did not check deeply, but so why can't we do the below?
>
> jirka
>
>
> ---
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index f4d9fc54b382..4643c487bd29 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -300,7 +300,7 @@ static int record__mmap_evlist(struct record *rec,
>   	struct record_opts *opts = &rec->opts;
>   	char msg[512];
>   
> -	if (perf_evlist__mmap_ex(evlist, opts->mmap_pages, false,
> +	if (perf_evlist__mmap_ex(evlist, opts->mmap_pages, opts->overwrite,
>   				 opts->auxtrace_mmap_pages,
>   				 opts->auxtrace_snapshot_mode) < 0) {

perf_evlist__mmap_ex's overwrite argument is used to control evlist->mmap.

Consider Namhyung's example:

  perf record --overwrite -e 'cycles/no-overwrite/'

In this case both evlist->mmap and evlist->backward_mmap would be set to overwrite.
'cycles' will be put into evlist->mmap, so it will be set to overwrite incorrectly.

Patch 2/2 clarifies these concepts. What we want is the flight recorder mode,
not only a read only ring buffer. Even flight recorder mode is selected, we can
still have a normal ring buffer which keep output data.

Thank you.