linux-kernel - Re: [RFC/PATCH 00/14] perf record: Add support to store data in directory

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Message-ID: <CABPqkBTOM2gLvv89ywkpWDKniS2L37XhKPjNLtR8WCY3VuYXzw@mail.gmail.com>
Date:   Mon, 11 Feb 2019 11:30:07 -0800
From:   Stephane Eranian <eranian@...gle.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Jiri Olsa <jolsa@...hat.com>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Song Liu <songliubraving@...com>
Subject: Re: [RFC/PATCH 00/14] perf record: Add support to store data in directory

Arnaldo,

On Mon, Feb 11, 2019 at 10:55 AM Arnaldo Carvalho de Melo
<acme@...nel.org> wrote:
>
> Em Mon, Feb 11, 2019 at 10:34:16AM -0800, Stephane Eranian escreveu:
> > Jiri,
> >
> > On Mon, Feb 11, 2019 at 2:20 AM Jiri Olsa <jolsa@...hat.com> wrote:
> > >
> > > On Tue, Feb 05, 2019 at 02:37:27PM +0100, Jiri Olsa wrote:
> > > > On Mon, Feb 04, 2019 at 02:44:37PM -0800, Stephane Eranian wrote:
> > > > > Jiri,
> > > > >
> > > > > While you're looking at the output format, I think it would be good
> > > > > time to simplify the code handling perf.data file.
> > > > > Today, perf record can emit in two formats: file mode or pipe mode.
> > > > > This adds complexity in the code and
> > > > > is error prone as the file mode path is tested more than the pipe mode
> > > > > path. We have run into multiple issues with
> > > > > the pipe mode in recent years. There is no real reason why we need to
> > > > > maintain two formats. If I recall, the pipe format
> > > > > was introduced because on pipes you cannot lseek to update the headers
> > > > > and therefore some of the information present as tables
> > > > > updated on the fly needed to be generated as pseudo records by the
> > > > > tool. I believe that the pipe format covers all the needs and could
> > > > > supersede the file mode format. That would simplify code in perf
> > > > > record and eliminate the risk of errors when new headers
> > > > > are introduced.
> > > >
> > > > yep, I think we have almost all the features covered for pipe mode,
> > > > and we have all necessary events to describe events features
> > > >
> > > > so with some effort we could switch off the superfluos file header
> > > > and use only events to describe events ;-) make sense, I'll check
> > > > on it
> > >
> > > so following features are not synthesized:
> > >
> > >         FEAT_OPN(TRACING_DATA,  tracing_data,   false),
> > >         FEAT_OPN(BUILD_ID,      build_id,       false),
> > >         FEAT_OPN(BRANCH_STACK,  branch_stack,   false),
> > >         FEAT_OPN(AUXTRACE,      auxtrace,       false),
> > >         FEAT_OPN(STAT,          stat,           false),
> > >         FEAT_OPN(CACHE,         cache,          true),
> > >
> > What do you need for BRANCH_STACK?
> >
> > > I think all could be added and worked around with exception
> > > of BUILD_ID, which we store at the end (after processing
> > > all data) and we need it early in the report phase
> > >
> > Buildids are injected after the fact via perf inject when in pipe mode.
> >
> > > maybe it's time to re-think that buildid -> mmap event
> > > association again, because it's pain in current implementation
> > > as well
> > >
> > Sure, but what do you propose?
>
> this keeps resurfacing, the idea is to have the building go together
> with the PERF_RECORD_MMAP3 event, i.e. as part of setting up an
> executable mapping the loader would get the buildid and ask the kernel
> to keep it aroung, then when a PERF_RECORD_MMAP needs to be issued, it
> can include the build id, so tooling will not need to get it.
>
And how would the dynamic loader (ld.so) communicate the buildid to the kernel?
How would that work for statically linked binaries.
I think you're say the kernel would parse the ELF header looking for
that note section
and extract the buildid from there. Is that what you are proposing?

> Alternatively, we would have a separate thread to process
> PERF_RECORD_MMAP events, and as soon as it gets one from the kernel,
> augment it straight away with the build-id it reads from the ELF file,
> i.e. no need to have the kernel provide it, do it just like we do with
> PERF_RECORD_BPF_EVENT, which reminds me Song probably already posted
> thise bits...
>
But that would not work in pipe mode, wouldn't it?
Unless that thread intercepts everything pushed to the pipe looking
for MMAP records.

> > > looks like bpf code is actualy getting build ids and storing
> > > it for the callchains in kernel.. we can check if we can do
> > > something similar for mmap events
> > >
> > > jirka
>
> --
>
> - Arnaldo