linux-kernel - Re: [PATCHv2 3/4] arm64: do not use dummy vcpu_is

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Date:   Mon, 12 Jul 2021 16:47:03 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Will Deacon <will@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCHv2 3/4] arm64: do not use dummy vcpu_is_preempted()

On Fri, 09 Jul 2021 05:37:12 +0100,
Sergey Senozhatsky <senozhatsky@...omium.org> wrote:
> 
> vcpu_is_preempted() now can represent the actual state of
> the VCPU, so the scheduler can make better decisions when
> it picks the idle CPU to enqueue a task on.
> 
> Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
> ---
>  arch/arm64/include/asm/spinlock.h | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/spinlock.h b/arch/arm64/include/asm/spinlock.h
> index 0525c0b089ed..1d579497e1b8 100644
> --- a/arch/arm64/include/asm/spinlock.h
> +++ b/arch/arm64/include/asm/spinlock.h
> @@ -7,21 +7,23 @@
>  
>  #include <asm/qspinlock.h>
>  #include <asm/qrwlock.h>
> +#include <asm/paravirt.h>
>  
>  /* See include/linux/spinlock.h */
>  #define smp_mb__after_spinlock()	smp_mb()
>  
> -/*
> - * Changing this will break osq_lock() thanks to the call inside
> - * smp_cond_load_relaxed().
> - *
> - * See:
> - * https://lore.kernel.org/lkml/20200110100612.GC2827@hirez.programming.kicks-ass.net
> - */

Why are you deleting this? Please explain your reasoning in the commit
message. It seems to me that it still makes complete sense when
CONFIG_PARAVIRT is not defined.

>  #define vcpu_is_preempted vcpu_is_preempted
> -static inline bool vcpu_is_preempted(int cpu)
> +
> +#ifdef CONFIG_PARAVIRT
> +static inline bool vcpu_is_preempted(unsigned int cpu)
> +{
> +	return paravirt_vcpu_is_preempted(cpu);
> +}
> +#else
> +static inline bool vcpu_is_preempted(unsigned int cpu)
>  {
>  	return false;
>  }
> +#endif /* CONFIG_PARAVIRT */
>  
>  #endif /* __ASM_SPINLOCK_H */

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.