linux-kernel - Re: [PATCH 2/3] powerpc: support dynamic preemption

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Message-ID: <5089fd16-bc8c-4231-a89b-2658445e04b7@csgroup.eu>
Date: Wed, 27 Nov 2024 07:44:00 +0100
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>, mpe@...erman.id.au,
 linuxppc-dev@...ts.ozlabs.org
Cc: npiggin@...il.com, maddy@...ux.ibm.com, bigeasy@...utronix.de,
 ankur.a.arora@...cle.com, linux-kernel@...r.kernel.org,
 mark.rutland@....com, vschneid@...hat.com, peterz@...radead.org
Subject: Re: [PATCH 2/3] powerpc: support dynamic preemption



Le 25/11/2024 à 05:22, Shrikanth Hegde a écrit :
> Once the lazy preemption is supported, it would be desirable to change
> the preemption models at runtime. So this change adds support for dynamic
> preemption using DYNAMIC_KEY.
> 
> In irq-exit to kernel path, use preempt_model_preemptible for decision.
> Other way would be using static key based decision. Keeping it
> simpler since key based change didn't show performance improvement.

What about static_call, wouldn't it improve performance ?

> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 6d6bbd93abab..01c58f5258c9 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -270,6 +270,7 @@ config PPC
>   	select HAVE_PERF_EVENTS_NMI		if PPC64
>   	select HAVE_PERF_REGS
>   	select HAVE_PERF_USER_STACK_DUMP
> +	select HAVE_PREEMPT_DYNAMIC_KEY

Can you use HAVE_PREEPT_DYNAMIC_CALL instead ? That should be more 
performant.

I know static calls are not in for PPC64 yet, you can restart from 
http://patchwork.ozlabs.org/project/linuxppc-dev/cover/20221010002957.128276-1-bgray@linux.ibm.com/ 
and https://github.com/linuxppc/issues/issues/416

Christophe