Re: [PATCH] profiling: remove prof_cpu_mask

[Linus Torvalds]

On Sat, 27 Jul 2024 at 14:20, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> Only people who indulge in nostalgia will notice :)

So let's see. Any bets on whether anybody actually notices?

I took Tetsuo's fix for the syzbot issue, and then I did a "remove all
the flip buffer code" in commit 2accfdb7eff6 ("profiling: attempt to
remove per-cpu profile flip buffer").

If somebody ends up having a real use-case for this old horrid code,
we may just have to remove it.

But it might also be the case that somebody actually does want the
boot-time profiling, and then the runtime overhead is annoying just
because they are on a multi-socket machine and the profiling just
keeps going - even after better profilers are available.

So it might be that nobody wants to actually re-instate the flip
buffer thing, but instead just turn the thing off entirely.

Technically you can do that by writing to /proc/profile with a
"profiling multiplier" that effectively turns it off, but very few
architectures actually support that (see "setup_profiling_timer()").

End result: maybe we should add a way to just say "I'm done profiling
now" if somebody reports that it causes performance issues after boot.

But I hope (and think it's very possible) that nobody will ever notice
any other way than from following this LKML discussion.

              Linus