Messages in this thread | | | Date | Mon, 11 Nov 2019 12:33:11 +0100 | From | Jiri Olsa <> | Subject | Re: [PATCH v3] perf inject --jit: Remove //anon mmap events |
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On Sat, Nov 09, 2019 at 04:49:25PM +0000, Steve MacLean wrote: > > > 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 superceded by the jitdump jit-xxx.dump file.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.
that'd be great, thanks
jirka
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