From: "Liang, Kan" <kan.liang@intel.com>
To: "Wangnan (F)" <wangnan0@huawei.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"acme@kernel.org" <acme@kernel.org>,
"jolsa@redhat.com" <jolsa@redhat.com>,
"namhyung@kernel.org" <namhyung@kernel.org>
Subject: RE: [PATCH 2/2] perf record: Replace 'overwrite' by 'flightrecorder' for better naming
Date: Wed, 1 Nov 2017 16:13:43 +0000 [thread overview]
Message-ID: <37D7C6CF3E00A74B8858931C1DB2F077537DC3B1@SHSMSX103.ccr.corp.intel.com> (raw)
In-Reply-To: <01f61c50-c176-f9fc-8711-b1308b455bd0@huawei.com>
> On 2017/11/1 23:04, Liang, Kan wrote:
> >> On 2017/11/1 22:22, Liang, Kan wrote:
> >>>> On 2017/11/1 21:26, Liang, Kan wrote:
> >>>>>> The meaning of perf record's "overwrite" option and many
> "overwrite"
> >>>>>> in source code are not clear. In perf's code, the 'overwrite' has
> >>>>>> 2
> >> meanings:
> >>>>>> 1. Make ringbuffer readonly (perf_evlist__mmap_ex's argument).
> >>>>>> 2. Set evsel's "backward" attribute (in apply_config_terms).
> >>>>>>
> >>>>>> perf record doesn't use meaning 1 at all, but have a overwrite
> >>>>>> option, its real meaning is setting backward.
> >>>>>>
> >>>>> I don't understand here.
> >>>>> 'overwrite' has 2 meanings. perf record only support 1.
> >>>>> It should be a bug, and need to be fixed.
> >>>> Not a bug, but ambiguous.
> >>>>
> >>>> Perf record doesn't need overwrite main channel (we have two
> channels:
> >>>> evlist->mmap is main channel and evlist->backward_mmap is backward
> >>>> evlist->channel),
> >>>> but some testcases require it, and your new patchset may require it.
> >>>> 'perf record --overwrite' doesn't set main channel overwrite. What
> >>>> it does
> >> is
> >>>> moving all evsels to backward channel, and we can move some evsels
> >> back to
> >>>> the main channel by /no-overwrite/ setting. This behavior is hard
> >>>> to understand.
> >>>>
> >>> As my understanding, the 'main channel' should depends on what user
> sets.
> >>> If --overwrite is applied, then evlist->backward_mmap should be the
> >>> 'main channel'. evlist->overwrite should be set to true as well.
> >> Then it introduces a main channel switching mechanism, and we need
> >> checking evlist->overwrite and another factor to determine which one
> >> is the main channel. Make things more complex.
> > We should check the evlist->overwrite.
> > Now, all perf tools force evlist->overwrite = false. I think it doesn’t make
> sense.
> >
> > What is another factor?
>
> If we support mixed channel as well as forward overwrite ring buffer,
> evlist->overwrite is not enough.
I think you have a detailed analysis regarding to the weakness of 'forward overwrite'.
commit ID 9ecda41acb97 ("perf/core: Add ::write_backward attribute to perf event").
There is no perf tools use 'forward overwrite' mode.
The only users are three perf test cases. We can change them to 'backward overwrite'
I think it's OK to discard the 'forward overwrite' mode
>
> > I don't think it will be too complex.
> >
> > In perf_evlist__mmap_ex, we just need to add a check.
> > If (!overwrite)
> > evlist->mmap = perf_evlist__alloc_mmap(evlist); else
> > evlist->backward_mmap = perf_evlist__alloc_mmap(evlist);
> >
> > In perf_evlist__mmap_per_evsel, we already handle per-event overwrite.
> > It just need to add some similar codes to handler per-event nonoverwrite.
>
> Then the logic becomes:
>
> if (check write_backward) {
> maps = evlist->backward_mmap;
> if (!maps) {
> maps = perf_evlist__alloc_mmap(...);
> if (!maps) {
> /* error processing */
> }
> evlist->backward_mmap = maps;
> if (evlist->bkw_mmap_state == BKW_MMAP_NOTREADY)
> perf_evlist__toggle_bkw_mmap(evlist, BKW_MMAP_RUNNING);
> }
> } else {
> maps = evlist->mmap;
> if (!maps) {
> maps = perf_evlist__alloc_mmap(...);
> if (!maps) {
> /* error processing */
> }
> evlist->mmap = maps;
> ....
> }
> }
>
Thanks.
It looks good to me.
Kan
> > For other codes, they should already check evlist->mmap and evlist-
> >backward_mmap.
> > So they probably don't need to change.
> >
> >
> > Thanks,
> > Kan
> >
> >
>
prev parent reply other threads:[~2017-11-01 16:13 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-01 5:53 [PATCH 0/2] perf record: Fix --overwrite and clarify concepts Wang Nan
2017-11-01 5:53 ` [PATCH 1/2] perf mmap: Fix perf backward recording Wang Nan
2017-11-01 9:49 ` Namhyung Kim
2017-11-01 10:32 ` Wangnan (F)
2017-11-01 12:00 ` Namhyung Kim
2017-11-01 12:10 ` Wangnan (F)
2017-11-01 12:39 ` Jiri Olsa
2017-11-01 12:56 ` Wangnan (F)
2017-11-02 15:12 ` Jiri Olsa
2017-11-01 13:57 ` Liang, Kan
2017-11-01 16:12 ` Wangnan (F)
2017-11-01 16:22 ` Liang, Kan
2017-11-02 5:34 ` Namhyung Kim
2017-11-02 13:25 ` Liang, Kan
2017-11-02 14:59 ` Jiri Olsa
2017-11-01 5:53 ` [PATCH 2/2] perf record: Replace 'overwrite' by 'flightrecorder' for better naming Wang Nan
2017-11-01 10:03 ` Namhyung Kim
2017-11-01 10:17 ` Wangnan (F)
2017-11-01 12:03 ` Namhyung Kim
2017-11-01 13:26 ` Liang, Kan
2017-11-01 14:05 ` Wangnan (F)
2017-11-01 14:22 ` Liang, Kan
2017-11-01 14:44 ` Wangnan (F)
2017-11-01 15:04 ` Liang, Kan
2017-11-01 16:00 ` Wangnan (F)
2017-11-01 16:13 ` Liang, Kan [this message]
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