linux-kernel - Re: [PATCH v3] powerpc/32: Add support for out-of-line static calls

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Date:   Wed, 1 Sep 2021 10:52:36 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v3] powerpc/32: Add support for out-of-line static calls

On Wed, Sep 01, 2021 at 08:30:21AM +0000, Christophe Leroy wrote:
> Add support for out-of-line static calls on PPC32. This change
> improve performance of calls to global function pointers by
> using direct calls instead of indirect calls.
> 
> The trampoline is initialy populated with a 'blr' or branch to target,
> followed by an unreachable long jump sequence.
> 
> In order to cater with parallele execution, the trampoline needs to
> be updated in a way that ensures it remains consistent at all time.
> This means we can't use the traditional lis/addi to load r12 with
> the target address, otherwise there would be a window during which
> the first instruction contains the upper part of the new target
> address while the second instruction still contains the lower part of
> the old target address. To avoid that the target address is stored
> just after the 'bctr' and loaded from there with a single instruction.
> 
> Then, depending on the target distance, arch_static_call_transform()
> will either replace the first instruction by a direct 'bl <target>' or
> 'nop' in order to have the trampoline fall through the long jump
> sequence.
> 
> For the special case of __static_call_return0(), to avoid the risk of
> a far branch, a version of it is inlined at the end of the trampoline.

(also, it's in the same line, so it avoids another cachemiss and it
nicely fills the hole you had in your 32byte chunk)

> Performancewise the long jump sequence is probably not better than
> the indirect calls set by GCC when we don't use static calls, but
> such calls are unlikely to be required on powerpc32: With most
> configurations the kernel size is far below 32 Mbytes so only
> modules may happen to be too far. And even modules are likely to
> be close enough as they are allocated below the kernel core and
> as close as possible of the kernel text.
> 
> static_call selftest is running successfully with this change.

Nice!, I'd ask if you'd tried PREEMPT_DYNAMIC, since that should really
stress the thing, but I see that also requires GENERIC_ENTRY and you
don't have that. Alas.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>