linux-kernel - Re: [RFC PATCH] powerpc: Investigate static

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Message-ID: <20210827160436.GQ1583@gate.crashing.org>
Date:   Fri, 27 Aug 2021 11:04:36 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Jason Baron <jbaron@...mai.com>,
        Paul Mackerras <paulus@...ba.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linuxppc-dev@...ts.ozlabs.org, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [RFC PATCH] powerpc: Investigate static_call concept

On Fri, Aug 27, 2021 at 04:18:47PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 27, 2021 at 09:45:37AM +0000, Christophe Leroy wrote:
> > This RFC is to validate the concept of static_call on powerpc.
> > 
> > Highly copied from x86.
> > 
> > It replaces ppc_md.get_irq() which is called at every IRQ, by
> > a static call.
> 
> The code looks saner, but does it actually improve performance? I'm
> thinking the double branch also isn't free.

It isn't, but it is very cheap, while the branch-to-count is not, even
*if* it is correctly predicted.

> The paranoid in me would've made it:
> 
> 	BUG_ON(patch_branch(...));
> 
> just to make sure to notice the target not fitting. Ohh, patch_branch()
> doesn't return the create_branch() error, perhaps that wants to be
> fixed?

Should that be allowed to fail ever?  I.e., should a failure be a fatal
error?  Sounds very fragile otherwise.


Segher