Re: [PATCH v3] perf inject --jit: Remove //anon mmap events

[Jiri Olsa]

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 superceded by the jitdump jit-xxx.dump file.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