Messages in this thread | | | Date | Wed, 22 Aug 2012 09:09:10 -0700 | Subject | Re: [PATCH] perf: do not flush maps on COMM for perf report | From | Luigi Semenzato <> |
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On Wed, Aug 22, 2012 at 12:28 AM, Ingo Molnar <mingo@kernel.org> wrote: > > * Luigi Semenzato <semenzato@chromium.org> wrote: > >> This fixes a long-standing bug caused by the lack of separate >> COMM and EXEC record types, which makes "perf report" lose >> track of symbols when a process renames itself. >> >> With this fix (suggested by Stephane Eranian), a COMM (rename) >> no longer flushes the maps, which is the correct behavior. >> An EXEC also no longer flushes the maps, but this doesn't >> matter because as new mappings are created (for the executable >> and the libraries) the old mappings are automatically removed. >> This is not by accident: the functionality is necessary because >> DLLs can be explicitly loaded at any time with dlopen(), >> possibly on top of existing text, so "perf report" handles >> correctly the clobbering of new mappings on top of old ones. >> >> An alternative patch (which I proposed earlier) would be to >> introduce a separate PERF_RECORD_EXEC type, but it is a much >> larger change (about 300 lines) and is not necessary. > > It would be nice to add that too - we already have FORK/EXIT, > this seems like a natural extension.
Yes. Adding PERF_RECORD_EXEC is/would be the right long-term solution. But there are two issues.
1. One nice aspect of perf is that perf.data files and "perf report" are compatible across a large number of versions. Adding PERF_RECORD_EXEC breaks compatibility in a somewhat unpleasant manner. New perf.data files won't work with old versions of perf and *might* fail poorly (segv) although this situation is difficult to analyze.
2. Adding a new record type is messy. It replicates a lot of boilerplate code, much of it in the kernel, and affects many parts of the system. It adds to size, complexity, and likelihood of new bugs.
I would prioritize the "would be nice" category as follows.
1. Improve the handling of unknown record types for future better backward compatibility. (Small change.)
2. Refactor/cleanup code to improve readability and robustness. (Big change, but can be broken into many smaller pieces.)
3. Add PERF_RECORD_EXEC.
If there is consensus, I might be able to give a shot to 1 and 2 (courtesy of Google).
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