linux-kernel - Re: [RFC 3/4] perf tool: Introduce PERF_RECORD_KMOD_SEC

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Message-ID: <437ccc68-e07f-de9a-4a0d-602b0d8189dd@amd.com>
Date:   Mon, 16 Jan 2023 19:04:12 +0530
From:   Ravi Bangoria <ravi.bangoria@....com>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     acme@...nel.org, jolsa@...nel.org, namhyung@...nel.org,
        irogers@...gle.com, kan.liang@...ux.intel.com,
        peterz@...radead.org, mark.rutland@....com, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, james.clark@....com,
        german.gomez@....com, leo.yan@...aro.org,
        alexey.v.bayduraev@...ux.intel.com,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        sandipan.das@....com, ananth.narayan@....com,
        santosh.shukla@....com, Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [RFC 3/4] perf tool: Introduce PERF_RECORD_KMOD_SEC_MAP

Hi Adrian,

On 16-Jan-23 11:44 AM, Adrian Hunter wrote:
> On 10/01/23 07:58, Ravi Bangoria wrote:
>> Introduce, perf tool only, synthetic event type PERF_RECORD_KMOD_SEC_MAP.
>> Also add stub code for it. This event will be used to save/restore kernel
>> module section maps to/from perf.data file. This is needed because kernel
>> module elfs does not contain program header table and thus there is no
>> easy way to find out how kernel would have loaded module sections in the
>> memory.
> 
> Currently machine__addnew_module_map() adds a map for a
> module, and then perf_event__synthesize_modules() creates
> an MMAP/MMAP2 event for it.  Why can't we do that for the
> sections?

Do you mean why not use MMAP/MMAP2 for module sections as well?
I thought about that but:
- PERF_RECORD_MMAP has no field which can help segregate normal vs section
  specific maps.
- I can probably introduce special MAP_ flag and use PERF_RECORD_MMAP2. But
  that flag will be non-standard (perf tool only), which is ugly.
- I also need additional hacks while synthesizing section specific MMAP/
  MMAP2 (e.g.: store section name and file path in filename[]) and similar
  hacks while parsing them at perf report time.

So I felt introducing PERF_RECORD_KMOD_SEC_MAP is relatively cleaner approach.

Thanks,
Ravi