linux-kernel - Re: [PATCH] sched: Optimize in_task() and in

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Date:   Fri, 14 Jul 2023 14:02:53 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Finn Thain <fthain@...ux-m68k.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Optimize in_task() and in_interrupt() a bit

On Fri, Jul 14, 2023 at 07:18:31PM +1000, Finn Thain wrote:
> Except on x86, preempt_count is always accessed with READ_ONCE.
> Repeated invocations in macros like irq_count() produce repeated loads.
> These redundant instructions appear in various fast paths. In the one
> shown below, for example, irq_count() is evaluated during kernel entry
> if !tick_nohz_full_cpu(smp_processor_id()).
> 
> 0001ed0a <irq_enter_rcu>:
>    1ed0a:       4e56 0000       linkw %fp,#0
>    1ed0e:       200f            movel %sp,%d0
>    1ed10:       0280 ffff e000  andil #-8192,%d0
>    1ed16:       2040            moveal %d0,%a0
>    1ed18:       2028 0008       movel %a0@(8),%d0
>    1ed1c:       0680 0001 0000  addil #65536,%d0
>    1ed22:       2140 0008       movel %d0,%a0@(8)
>    1ed26:       082a 0001 000f  btst #1,%a2@(15)
>    1ed2c:       670c            beqs 1ed3a <irq_enter_rcu+0x30>
>    1ed2e:       2028 0008       movel %a0@(8),%d0
>    1ed32:       2028 0008       movel %a0@(8),%d0
>    1ed36:       2028 0008       movel %a0@(8),%d0
>    1ed3a:       4e5e            unlk %fp
>    1ed3c:       4e75            rts
> 
> This patch doesn't prevent the pointless btst and beqs instructions
> above, but it does eliminate 2 of the 3 pointless move instructions
> here and elsewhere.
> 
> On x86, preempt_count is per-cpu data and the problem does not arise
> perhaps because the compiler is free to perform similar optimizations.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Fixes: 15115830c887 ("preempt: Cleanup the macro maze a bit")

Does this optimization really deserves a "Fixes:" tag?

> Signed-off-by: Finn Thain <fthain@...ux-m68k.org>
> ---
> This patch was tested on m68k and x86. I was expecting no changes
> to object code for x86 and mostly that's what I saw. However, there
> were a few places where code generation was perturbed for some reason.
> ---
>  include/linux/preempt.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 0df425bf9bd7..953358e40291 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -102,10 +102,11 @@ static __always_inline unsigned char interrupt_context_level(void)
>  #define hardirq_count()	(preempt_count() & HARDIRQ_MASK)
>  #ifdef CONFIG_PREEMPT_RT
>  # define softirq_count()	(current->softirq_disable_cnt & SOFTIRQ_MASK)
> +# define irq_count()		((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count())
>  #else
>  # define softirq_count()	(preempt_count() & SOFTIRQ_MASK)
> +# define irq_count()		(preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK))
>  #endif
> -#define irq_count()	(nmi_count() | hardirq_count() | softirq_count())

Perhaps add a comment as to why you're making these two versions (ie: because
that avoids three consecutive reads), otherwise people may be tempted to roll
that back again in the future to make the code shorter.

>  
>  /*
>   * Macros to retrieve the current execution context:
> @@ -118,7 +119,11 @@ static __always_inline unsigned char interrupt_context_level(void)
>  #define in_nmi()		(nmi_count())
>  #define in_hardirq()		(hardirq_count())
>  #define in_serving_softirq()	(softirq_count() & SOFTIRQ_OFFSET)
> -#define in_task()		(!(in_nmi() | in_hardirq() | in_serving_softirq()))
> +#ifdef CONFIG_PREEMPT_RT
> +# define in_task()		(!((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | in_serving_softirq()))
> +#else
> +# define in_task()		(!(preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
> +#endif

Same here, thanks!

>  
>  /*
>   * The following macros are deprecated and should not be used in new code:
> -- 
> 2.39.3
>