[RFC v2] sketch of an unwinder cache interface for elfutils libdwfl

[Serhei Makarov]

On Thu, Mar 20, 2025, at 10:28 AM, Serhei Makarov wrote:
> On Tue, Dec 10, 2024, at 4:42 PM, Serhei Makarov wrote:
>> This email sketches an 'unwinder cache' interface for libdwfl, derived 
>> from recent eu-stacktrace code [1] and based on Christian Hergert's 
>> adaptation of eu-stacktrace as sysprof-live-unwinder [2]. The intent is 
>> to remove the need for a lot of boilerplate code that would be 
>> identical among profiling tools that use libdwfl to unwind perf_events 
>> stack samples. But since this becomes a library design, it's in need of 
>> feedback and bikeshedding.
> In advance of finishing up the Dwfl_Process_Tracker patchset
> (initial version currently under review),
> wanted to post an update summarizing the current api design.

Redid performance analysis based on the released code. I'm seeing a reduction of sysprof-live-unwinder overhead from 7~8% to 3~6% (with the framepointer version of sysprof providing a baseline of about 1.5%). So there is some variance (the lower the overhead gets, the harder it is to keep conditions identical, apparently), but the performance is moving along; the next bottleneck to look at is dwfl_linux_proc_report. I may want to automate the performance testing procedure to get fully exact numbers I'm comfortable with reporting.

There is an issue with spontaneous exit of sysprofd (cleanly, with a "Stopping RAPL monitor" message) that I'm trying to understand. Hypotheses -- may be a polkit issue, or may be sysprof-live-unwinder not handling some error result gracefully. (The eu-stacktrace tool + prototype sysprof patches don't exhibit this behaviour.) I expect I'll need to make another revision of the sysprof-live-unwinder patches at https://git.sr.ht/~serhei/sysprof-experiments/log/serhei/live-unwinder

With the Elf caching in Dwfl_Process_Tracker, I counted (via attached simple patch) how many times an Elf struct was retrieved from cache vs how many were newly created. On a quick test with a swaywm system the 'created' number stabilized at ~186 created Elf structs with the 'cached' number ~400 and rising as I keep running the profiler. On gnome3, stabilizes at ~282 structs with the 'cached' number ~500 and rising. Obviously, this is not super meaningful, as the number can be made arbitrarily good by running the profiler for longer periods of time :p but it's worth verifying that the caching works.

All the best,
     Serhei
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