linux-kernel - Re: [PATCH v2 0/7] Implement inline static calls on PPC32

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Message-ID: <47772b9ae10be6dbe5b0cfcd0bc14efc8fb22c0c.camel@linux.ibm.com>
Date:   Thu, 08 Sep 2022 10:13:57 +1000
From:   Benjamin Gray <bgray@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     X86 ML <x86@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
        Chen Zhongjin <chenzhongjin@...wei.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Jason Baron <jbaron@...mai.com>,
        Ingo Molnar <mingo@...hat.com>,
        "sv@...ux.ibm.com" <sv@...ux.ibm.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        "agust@...x.de" <agust@...x.de>,
        "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" 
        <linuxppc-dev@...ts.ozlabs.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v2 0/7] Implement inline static calls on PPC32 - v2

On Thu, 2022-09-01 at 16:46 +0000, Christophe Leroy wrote:
> Surprisingly, I get worst performance with inline static call than
> with 
> out of line static call:

I'm not sure what hackbench is doing, but when microbenchmarking 64 bit
out-of-line calls in a loop I saw a similar thing where adding more
indirection improved the performance despite doing more work. The cause
seemed to be a combination of using older hardware and the target being
too short (just an integer increment). Moving to a newer machine and
adding a lot of NOPs to the target made the performance make sense.

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