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Message-ID: <Zjksc3yqvkocS18M@x1>
Date: Mon, 6 May 2024 16:16:03 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
Greg KH <gregkh@...uxfoundation.org>,
Andrii Nakryiko <andrii@...nel.org>, linux-fsdevel@...r.kernel.org,
brauner@...nel.org, viro@...iv.linux.org.uk,
akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
bpf@...r.kernel.org, linux-mm@...ck.org,
Daniel Müller <deso@...teo.net>,
"linux-perf-use." <linux-perf-users@...r.kernel.org>
Subject: Re: [PATCH 2/5] fs/procfs: implement efficient VMA querying API for
/proc/<pid>/maps
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 AM Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> > > On Sat, May 04, 2024 at 02:50:31PM -0700, Andrii Nakryiko wrote:
> > > > On Sat, May 4, 2024 at 8:28 AM Greg KH <gregkh@...uxfoundation.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, or special
> > > > > > hard-coded or user-provided names) is optional just like build 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@...nel.org>
>
> > > > > Where is the userspace code that uses this new api you have created?
>
> > > > So I added a faithful comparison of existing /proc/<pid>/maps vs new
> > > > ioctl() API to solve a common problem (as described above) in patch
> > > > #5. The plan is to put it in mentioned blazesym library at the very
> > > > 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@...nel.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 :-\
Humm, perhaps the sideband thread...
- Arnaldo
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