linux-kernel - Re: [PATCH 5/7] powerpc, arm64: Mark __switch_to() as _

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Message-ID: <202110220914.11A7C074AF@keescook>
Date:   Fri, 22 Oct 2021 09:15:31 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, akpm@...ux-foundation.org,
        mark.rutland@....com, zhengqi.arch@...edance.com,
        linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
        mpe@...erman.id.au, paul.walmsley@...ive.com, palmer@...belt.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com, borntraeger@...ibm.com,
        linux-arch@...r.kernel.org, ardb@...nel.org
Subject: Re: [PATCH 5/7] powerpc, arm64: Mark __switch_to() as __sched

On Fri, Oct 22, 2021 at 05:09:38PM +0200, Peter Zijlstra wrote:
> Unlike most of the other architectures, PowerPC and ARM64 have
> __switch_to() as a C function which remains on the stack. Their
> respective __get_wchan() skips one stack frame unconditionally,
> without testing is_sched_functions().
> 
> Mark them __sched such that we can forgo that special case.

I wonder if this change will improve any benchmarks? (i.e. this will
move __switch_to into the scheduler section, maybe improving icache?)

Reviewed-by: Kees Cook <keescook@...omium.org>

> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/arm64/kernel/process.c   |    4 ++--
>  arch/powerpc/kernel/process.c |    4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -490,8 +490,8 @@ void update_sctlr_el1(u64 sctlr)
>  /*
>   * Thread switching.
>   */
> -__notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
> -				struct task_struct *next)
> +__notrace_funcgraph __sched
> +struct task_struct *__switch_to(struct task_struct *prev, struct task_struct *next)
>  {
>  	struct task_struct *last;
>  
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1201,8 +1201,8 @@ static inline void restore_sprs(struct t
>  
>  }
>  
> -struct task_struct *__switch_to(struct task_struct *prev,
> -	struct task_struct *new)
> +__sched struct task_struct *__switch_to(struct task_struct *prev,
> +					struct task_struct *new)
>  {
>  	struct thread_struct *new_thread, *old_thread;
>  	struct task_struct *last;
> 
> 

-- 
Kees Cook