Re: [PATCH] kbuild: AutoFDO: Enable -salvage-stale-profile

[Nick Desaulniers]

On Thu, Nov 5, 2026 at 3:17 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> On Thu, Nov 5, 2026 at 1:30 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> >
> > > While I haven't looked into this specific bug yet and am unsure if it
> > > is unique to FS-AFDO, I recommend using -fno-sample-profile-use-profi
> > > to disable Profi for now.
> >
> > Interesting. I can suggest that to the team digging into it and see if
> > it solves the problem. My AI agent says it will though. It claims that
> > the default flipped recently and "-fno-sample-profile-use-profi" will
> > restore the old (tested) behavior.
> >
> > Should I send a patch adding that flag?
>
> I went ahead and posted the patch:
>
> https://lore.kernel.org/r/20261105151159.1.I0b27923381ef34cc99cdf8b65fea2e1bafce6370@changeid
>
> Obviously that patch and this patch conflict because they touch the
> same lines of code. If folks think we should still move forward with
> both patches, I'm happy to rebase.
>
>
> > > Regarding the "--mllvm=-salvage-stale-profile" flag, I also have
> > > reservations about adding it for the following reasons:
> > > 1.  It is heuristic-based and not enabled by default. I am unsure how
> > > thoroughly it has been tested, so we should assess it further before
> > > enabling it.
> >
> > FWIW, we did find that Android seems to enable this globally across
> > its userspace, which suggests it has been fairly well tested. You can
> > see the change here, which landed about 1.5 years ago.
> >
> > https://android-review.git.corp.google.com/c/platform/build/soong/+/3524291
> >
> > A few notes:
> >
> > * That change _also_ adds
> > "--salvage-stale-profile-max-callsites=2000". Internally, there has
> > been debate about whether adding that would be a good idea, too.
>
> The conclusion of the internal debate is that, yes, we should add

After Doug sent this email, him, I, Yabin, Rong, and a few others met
and decided we probably want

-fno-sample-profile-use-profi

...but then I stepped out of that meeting and ran into Yi who
extensively tested AutoFDO for Android userspace and suggested _the
opposite_.

i.e. Rong suggested:
yes: -fno-sample-profile-use-profi
no: --mllvm=-salvage-stale-profile

Yi suggested:
no: -fno-sample-profile-use-profi
yes: --mllvm=-salvage-stale-profile

Yi, can you share with the list why you hold that opinion?

I suspect we'll want to disable "profi" for now, but likely will end
up revisiting these pairs of flags in the near future.

> `--salvage-stale-profile-max-callsites=2000` too. So if we decide to
> move forward with this patch, I'll send a v2 that adds that flag as
> well. For now I'll hold off until there is consensus. :-)
>
> -Doug



-- 
Thanks,
~Nick Desaulniers