RE: [PATCH 2/2] perf record: Replace 'overwrite' by 'flightrecorder' for better naming

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 15:04:40 +0000	[thread overview]
Message-ID: <37D7C6CF3E00A74B8858931C1DB2F077537DC2F2@SHSMSX103.ccr.corp.intel.com> (raw)
In-Reply-To: <24733178-7c63-2841-13e7-d4c68ae7efe2@huawei.com>

> 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?

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.  

For other codes, they should already check evlist->mmap and evlist->backward_mmap.
So they probably don't need to change.

Thanks,
Kan