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

[Wangnan (F)]

On 2017/11/1 20:00, Namhyung Kim wrote:
> On Wed, Nov 01, 2017 at 06:32:50PM +0800, Wangnan (F) wrote:
> > On 2017/11/1 17:49, Namhyung Kim wrote:
> > > Hi,
> > >
> > > On Wed, Nov 01, 2017 at 05:53:26AM +0000, Wang Nan wrote:
> > > > perf record backward recording doesn't work as we expected: it never
> > > > overwrite when ring buffer full.
> > > >
> > > > Test:
> > > >
> > > > (Run a busy printing python task background like this:
> > > >
> > > >    while True:
> > > >        print 123
> > > >
> > > > send SIGUSR2 to perf to capture snapshot.)
> > [SNIP]
> >
> > > > Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> > > > ---
> > > >    tools/perf/util/evlist.c | 8 +++++++-
> > > >    1 file changed, 7 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> > > > index c6c891e..4c5daba 100644
> > > > --- a/tools/perf/util/evlist.c
> > > > +++ b/tools/perf/util/evlist.c
> > > > @@ -799,22 +799,28 @@ perf_evlist__should_poll(struct perf_evlist *evlist __maybe_unused,
> > > >    }
> > > >    static int perf_evlist__mmap_per_evsel(struct perf_evlist *evlist, int idx,
> > > > -				       struct mmap_params *mp, int cpu_idx,
> > > > +				       struct mmap_params *_mp, int cpu_idx,
> > > >    				       int thread, int *_output, int *_output_backward)
> > > >    {
> > > >    	struct perf_evsel *evsel;
> > > >    	int revent;
> > > >    	int evlist_cpu = cpu_map__cpu(evlist->cpus, cpu_idx);
> > > > +	struct mmap_params *mp;
> > > >    	evlist__for_each_entry(evlist, evsel) {
> > > >    		struct perf_mmap *maps = evlist->mmap;
> > > > +		struct mmap_params rdonly_mp;
> > > >    		int *output = _output;
> > > >    		int fd;
> > > >    		int cpu;
> > > > +		mp = _mp;
> > > >    		if (evsel->attr.write_backward) {
> > > >    			output = _output_backward;
> > > >    			maps = evlist->backward_mmap;
> > > > +			rdonly_mp = *_mp;
> > > > +			rdonly_mp.prot &= ~PROT_WRITE;
> > > > +			mp = &rdonly_mp;
> > > >    			if (!maps) {
> > > >    				maps = perf_evlist__alloc_mmap(evlist);
> > > > --
> > > What about this instead (not tested)?
> > >
> > >
> > > 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.

Thank you.