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

[Ian Rogers]

On Wed, May 27, 2020 at 12:27 PM Steve MacLean
<Steve.MacLean@xxxxxxxxxxxxx> wrote:
>
> >> ** 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....

Agreed that things are broken. In general only executable mappings are
held onto by perf, so it could be I'm over worrying about //anon
stripping breaking around memory allocations. We have some other use
cases for //anon at Google but they aren't impacted by jitdump. We
have also been trying to migrate jit caches to using memfd_create,
which has the same problem that this patch fixes for //anon. Fixing
memfd_create is a separate issue to //anon. I'll try to get a repro
for Java that demonstrates the problem and then add a Tested-by.

Thanks,
Ian