Messages in this thread | | | From | Steve MacLean <> | Subject | RE: [PATCH v3] perf inject --jit: Remove //anon mmap events | 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 > > pages and change these pages to executable permissions. > > > > Typically the JIT will want these colocated to minimize branch displacements. > > > > The kernel will coalesce these anonymous mapping with identical > > permissions before sending an MMAP event for the new pages. This means > > the mmap event for the new pages will include the older pages. > > > > These anonymous mmap events will obscure the jitdump injected pseudo events. > > This means that the jitdump generated symbols, machine code, debugging > > info, and unwind info will no longer be used. > > > > Observations: > > > > When a process emits a jit dump marker and a jitdump file, the > > perf-xxx.map file represents inferior information which has been > > superceded by the jitdump jit-xxx.dump file. > > > > Further the '//anon*' mmap events are only required for the legacy > > perf-xxx.map mapping. > > > > When attaching to an existing process, the synthetic anon map events > > are given a time stamp of -1. These should not obscure the jitdump > > events which have an actual time. > > > > Summary: > > > > Use thread->priv to store whether a jitdump file has been processed > > I'm ok wih the implementation but not sure about the described JIT/mmap logic, Stephane? > > jirka
The kernel only seems to coalesce the anonymous mappings when the allocations 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.
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