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

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Message-ID: <20171101123957.GA27215@krava>
Date:   Wed, 1 Nov 2017 13:39:57 +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 08:10:49PM +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) {
 		if (errno == EPERM) {