Re: [PATCH] perf: skip unnecessary userspace map synthesis

[Max Kellermann]

On Tue, Oct 13, 2026 at 3:29 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> Thanks for the contribution! I can see if there's a user event then
> evlist__needs_user_mmaps will return true, otherwise we'll return
> false if there's a kernel event but true if the list is empty or just
> contains dummy events. Why do the empty list and dummy case need the
> user mmap events?

I checked it again (and got confused a bit more than I should). Is the
empty list really possible? If no event is specified, "cycles" is the
implicit default in both tools.

With "perf -e dummy", the kernel emits user mmap records even with
"exclude_user", so this would be a possibly user-visible change.  Do
you want me to remove the "dummy" check, or do you want me to add a
comment explaining this?  Or is it possible to check whether a dummy
event is guaranteed to never require user mmaps?

--
Max Kellermann
Principal Architect
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