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

From: Steve MacLean
Date: Wed May 27 2020 - 15:27:42 EST

>> ** 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