Re: [PATCH] perf tools: fix: Force backward ring buffer mapped readonly

[Wangnan (F)]

On 2017/10/11 21:16, Liang, Kan wrote:
perf record's --overwrite option doesn't work as we expect.
For example:

[SNIP]


In the above example we get same records from the backward ring buffer
all the time. Overwriting is not triggered.

This commit maps backward ring buffers readonly, make it overwritable.
It is safe because we assume backward ring buffer always overwritable
in other part of code.

After applying this patch:

     $ ~/linux/tools/perf$ sudo ./perf record -m 4 -e raw_syscalls:* -g --
overwrite \
                                   --switch-output=1s --tail-synthesize   dd
if=/dev/zero of=/dev/null

[SNIP]



Signed-off-by: Wang Nan <wangn...@huawei.com>
Cc: Liang Kan <kan.li...@intel.com>
Cc: Arnaldo Carvalho de Melo <a...@kernel.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Jiri Olsa <jo...@kernel.org>
Cc: Namhyung Kim <namhy...@kernel.org>
Cc: Alexander Shishkin <alexander.shish...@linux.intel.com>
Cc: Adrian Hunter <adrian.hun...@intel.com>
Cc: Andi Kleen <a...@linux.intel.com>
Cc: Li Zefan <lize...@huawei.com>
---
  tools/perf/util/evlist.c | 7 ++++++-
  1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index c6c891e..a86b0d2 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -799,12 +799,14 @@ 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 = _mp;
+       struct mmap_params backward_mp;

        evlist__for_each_entry(evlist, evsel) {
                struct perf_mmap *maps = evlist->mmap;
@@ -815,6 +817,9 @@ static int perf_evlist__mmap_per_evsel(struct
perf_evlist *evlist, int idx,
                if (evsel->attr.write_backward) {
                        output = _output_backward;
                        maps = evlist->backward_mmap;
+                       backward_mp = *mp;
+                       backward_mp.prot &= ~PROT_WRITE;
+                       mp = &backward_mp;

                        if (!maps) {
                                maps = perf_evlist__alloc_mmap(evlist);
So it's trying to support per-event overwrite.
How about the global --overwrite option?

Not only the per-event overwrite. See the example above. The overall 
--overwrite
option is also respected. In perf_evsel__config, per-event evsel 
'backward' setting
is set based on overall '--overwrite' and per-event '/overwrite/' setting.

I think we should use opts->overwrite to replace the hard code 'false' for
perf_evlist__mmap_ex as well.

Thanks,
Kan