linux-kernel - RE: [EXTERNAL] Re: [PATCH v4] perf inject --jit: Remove //anon mmap events

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Date:   Wed, 27 May 2020 19:27:28 +0000
From:   Steve MacLean <Steve.MacLean@...rosoft.com>
To:     Ian Rogers <irogers@...gle.com>,
        Steve MacLean <steve.maclean@...ux.microsoft.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Stephane Eranian <eranian@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: RE: [EXTERNAL] Re: [PATCH v4] perf inject --jit: Remove //anon mmap
 events

>> ** Implemented solution
>>
>> This patch solves the issue by removing // anon mmap events for any 
>> process which has a valid jit-<pid>.dump file.
>>
>> It tracks on a per process basis to handle the case where some running 
>> apps support jit-<pid>.dump, but some only support perf-<pid>.map.
>>
>> It adds new assumptions:
>> * // anon mmap events are only required for perf-<pid>.map support.
>> * An app that uses jit-<pid>.dump, no longer needs perf-<pid>.map 
>> support. It assumes that any perf-<pid>.map info is inferior.
>>
>> *** Details
>>
>> Use thread->priv to store whether a jitdump file has been processed
>>
>> During "perf inject --jit", discard "//anon*" mmap events for any pid 
>> which has sucessfully processed a jitdump file.
>
>
> Thanks Steve this is an important fix! As //anon could be for malloc or other uses, should the stripping behavior be behind a flag? 
>
> Ian

I hadn't anticipated a need to preserve the //anon mmap events when profiling JIT generated code.

As far as I know mmap events are captured by perf only for mapping code to symbols.  File mappings are kept
by the change.  Only // anon mappings are stripped.  (Only for processes which emitted jitdump files.)
And these are stripped only during the `perf inject --jit` step. I believe the // Anon mapping are only 
generally useful for mapping JIT code.

I suppose if someone was trying to count mmap events it might be confusing, but `perf inject --jit` creates 
synthetic mmap file events which would also make this scenario confusing.

I personally don't see a good reason to add a flag.  I also don't see a simple way either.  Not running `perf inject --jit`
would preserve existing behavior w/o jitdump support.  Without stripping the anon events jitdump support is painfully
broken....