On Wed, Feb 01, 2023 at 11:10:20AM +0000, Mark Rutland wrote:

On Tue, Jan 31, 2023 at 08:38:32AM -0800, Josh Poimboeuf wrote:

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On Tue, Jan 31, 2023 at 10:22:09AM +0000, Mark Rutland wrote:

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Hm, it might be nice if our out-of-line static call implementation would
automatically do a static key check as part of static_call_cond() for
NULL-type static calls.

But the best answer is probably to just add inline static calls to
arm64.  Is the lack of objtool the only thing blocking that?

The major issues were branch range limitations (and needing the linker to add
PLTs),

Does the compiler do the right thing (e.g., force PLT) if the branch
target is outside the translation unit?  I'm wondering if we could for
example use objtool to help enforce such rules at the call site.

It's the linker (rather than the compiler) that'll generate the PLT if the
caller and callee are out of range at link time. There are a few other issues
too (e.g. no guarnatee that the PLT isn't used by multiple distinct callers,
CMODX patching requirements), so we'd have to generate a pseudo-PLT ourselves
at build time with a patching-friendly code sequence. Ard had a prototype for
that:

  https://lore.kernel.org/linux-arm-kernel/20211105145917.2828911-1-ardb@kernel.org/ (local)

... but that was sufficiently painful that we went with the current static key
approach:

  https://lore.kernel.org/all/20211109172408.49641-1-mark.rutland@arm.com/ (local)

Thanks for the background.

Was there a reason for putting it out-of-line rather than directly in
_cond_resched()?

If it were inline then it wouldn't be that much different from the
static called version and I wonder if we could simplify by just using
the static key for all PREEMPT_DYNAMIC configs.

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If we knew each call-site would only call a particular function or skip the
call, then we could do better (and would probably need something like objtool
to NOP that out at compile time), but since we don't know the callee at build
time we can't ensure we have a PLT in range when necessary.

Unfortunately most static calls have multiple destinations.

Sure, but here we're just enabling/disabling a call, which we could treat
differently, or wrap at a different level within the scheduler code. I'm happy
to take a look at that.

I can try to emulate what you did for PREEMPT_DYNAMIC.  I'll Cc you on
my actual patch to come soon-ish.

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And most don't have the option of being NULL.

Oh, I was under the impression that all could be disabled/skipped, which is
what a NULL target implied.

I guess what I was trying to say is that if the target can be NULL, the
call site has to use static_call_cond() to not break the
!HAVE_STATIC_CALL case.  But most call sites use static_call().

-- 
Josh