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From:   Steve MacLean <Steve.MacLean@microsoft.com>
To:     Jiri Olsa <jolsa@redhat.com>,
        Steve MacLean <steve.maclean@linux.microsoft.com>,
        Stephane Eranian <eranian@google.com>
CC:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v3] perf inject --jit: Remove //anon mmap events
Thread-Topic: [PATCH v3] perf inject --jit: Remove //anon mmap events
Thread-Index: AQHVkCoSdDKHEwaGv0G9a4q1EVvH9qd1/B4AgAnxV+A=
Date:   Sat, 9 Nov 2019 16:49:25 +0000
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> > While a JIT is jitting code it will eventually need to commit more=20
> > pages and change these pages to executable permissions.
> >=20
> > Typically the JIT will want these colocated to minimize branch displace=
ments.
> >=20
> > The kernel will coalesce these anonymous mapping with identical=20
> > permissions before sending an MMAP event for the new pages. This means=
=20
> > the mmap event for the new pages will include the older pages.
> >=20
> > These anonymous mmap events will obscure the jitdump injected pseudo ev=
ents.
> > This means that the jitdump generated symbols, machine code, debugging=
=20
> > info, and unwind info will no longer be used.
> >=20
> > Observations:
> >=20
> > When a process emits a jit dump marker and a jitdump file, the=20
> > perf-xxx.map file represents inferior information which has been=20
> > superceded by the jitdump jit-xxx.dump file.
> >=20
> > Further the '//anon*' mmap events are only required for the legacy=20
> > perf-xxx.map mapping.
> >=20
> > When attaching to an existing process, the synthetic anon map events=20
> > are given a time stamp of -1. These should not obscure the jitdump=20
> > events which have an actual time.
> >=20
> > Summary:
> >=20
> > Use thread->priv to store whether a jitdump file has been processed
>=20
> I'm ok wih the implementation but not sure about the described JIT/mmap l=
ogic, Stephane?
>=20
> jirka

The kernel only seems to coalesce the anonymous mappings when the allocatio=
ns grow beyond 64K. It may not affect JITs for smaller sets of JITted code.=
  I would guess a javascript JIT engine might not hit this type of problem =
often.

@Stephane Eranian could you comment.

@Jiri Olsa I am happy to expand the explanation if it would be helpful.