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

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Date:   Thu, 2 Nov 2017 16:12:08 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     "Wangnan (F)" <wangnan0@...wei.com>
Cc:     Namhyung Kim <namhyung@...nel.org>, linux-kernel@...r.kernel.org,
        kan.liang@...el.com, acme@...nel.org, kernel-team@....com
Subject: Re: [PATCH 1/2] perf mmap: Fix perf backward recording

On Wed, Nov 01, 2017 at 08:56:32PM +0800, Wangnan (F) wrote:
> 
> 
> 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.

right, missed the separate mmaps..

so we have some code that uses evlist->overwrite, which is always
set to 'false' in perf record.. but in the crucial checks like
for perf_mmap__consume we use the 'backward' bool to save the day

that might need some consolidation as well.. we could keep the
overwrite flag in the struct perf_mmap.. that could simplify the code

jirka