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From: Namhyung Kim <namhyung@kernel.org>
Date: Tue, 7 May 2024 14:55:53 -0700
Message-ID: <CAM9d7cj=zadZzQakNC7PeKWd5hfL83jvCRu-BuZ4EOzF2WPb-w@mail.gmail.com>
Subject: Re: [PATCH 2/5] fs/procfs: implement efficient VMA querying API for /proc/<pid>/maps
To: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>, Jiri Olsa <jolsa@kernel.org>, 
	Ian Rogers <irogers@google.com>, Greg KH <gregkh@linuxfoundation.org>, 
	Andrii Nakryiko <andrii@kernel.org>, linux-fsdevel@vger.kernel.org, brauner@kernel.org, 
	viro@zeniv.linux.org.uk, akpm@linux-foundation.org, 
	linux-kernel@vger.kernel.org, bpf@vger.kernel.org, linux-mm@kvack.org, 
	=?UTF-8?Q?Daniel_M=C3=BCller?= <deso@posteo.net>, 
	"linux-perf-use." <linux-perf-users@vger.kernel.org>
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On Mon, May 6, 2024 at 12:16=E2=80=AFPM Arnaldo Carvalho de Melo
<acme@kernel.org> wrote:
>
> On Mon, May 06, 2024 at 03:53:40PM -0300, Arnaldo Carvalho de Melo wrote:
> > On Mon, May 06, 2024 at 11:05:17AM -0700, Namhyung Kim wrote:
> > > On Mon, May 6, 2024 at 6:58=E2=80=AFAM Arnaldo Carvalho de Melo <acme=
@kernel.org> wrote:
> > > > On Sat, May 04, 2024 at 02:50:31PM -0700, Andrii Nakryiko wrote:
> > > > > On Sat, May 4, 2024 at 8:28=E2=80=AFAM Greg KH <gregkh@linuxfound=
ation.org> wrote:
> > > > > > On Fri, May 03, 2024 at 05:30:03PM -0700, Andrii Nakryiko wrote=
:
> > > > > > > Note also, that fetching VMA name (e.g., backing file path, o=
r special
> > > > > > > hard-coded or user-provided names) is optional just like buil=
d ID. If
> > > > > > > user sets vma_name_size to zero, kernel code won't attempt to=
 retrieve
> > > > > > > it, saving resources.
> >
> > > > > > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> >
> > > > > > Where is the userspace code that uses this new api you have cre=
ated?
> >
> > > > > So I added a faithful comparison of existing /proc/<pid>/maps vs =
new
> > > > > ioctl() API to solve a common problem (as described above) in pat=
ch
> > > > > #5. The plan is to put it in mentioned blazesym library at the ve=
ry
> > > > > least.
> > > > >
> > > > > I'm sure perf would benefit from this as well (cc'ed Arnaldo and
> > > > > linux-perf-user), as they need to do stack symbolization as well.
> >
> > > I think the general use case in perf is different.  This ioctl API is=
 great
> > > for live tracing of a single (or a small number of) process(es).  And
> > > yes, perf tools have those tracing use cases too.  But I think the
> > > major use case of perf tools is system-wide profiling.
> >
> > > For system-wide profiling, you need to process samples of many
> > > different processes at a high frequency.  Now perf record doesn't
> > > process them and just save it for offline processing (well, it does
> > > at the end to find out build-ID but it can be omitted).
> >
> > Since:
> >
> >   Author: Jiri Olsa <jolsa@kernel.org>
> >   Date:   Mon Dec 14 11:54:49 2020 +0100
> >   1ca6e80254141d26 ("perf tools: Store build id when available in PERF_=
RECORD_MMAP2 metadata events")
> >
> > We don't need to to process the events to find the build ids. I haven't
> > checked if we still do it to find out which DSOs had hits, but we
> > shouldn't need to do it for build-ids (unless they were not in memory
> > when the kernel tried to stash them in the PERF_RECORD_MMAP2, which I
> > haven't checked but IIRC is a possibility if that ELF part isn't in
> > memory at the time we want to copy it).
>
> > If we're still traversing it like that I guess we can have a knob and
> > make it the default to not do that and instead create the perf.data
> > build ID header table with all the build-ids we got from
> > PERF_RECORD_MMAP2, a (slightly) bigger perf.data file but no event
> > processing at the end of a 'perf record' session.
>
> But then we don't process the PERF_RECORD_MMAP2 in 'perf record', it
> just goes on directly to the perf.data file :-\

Yep, we don't process build-IDs at the end if --buildid-mmap
option is given.  It won't have build-ID header table but it's
not needed anymore and perf report can know build-ID from
MMAP2 directly.

Thanks,
Namhyung