linux-kernel - Re: [PATCH v3 3/3] RISC-V: Do not invoke SBI call if cpumask is empty

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Message-ID: <20190822015113.GC11922@lst.de>
Date:   Thu, 22 Aug 2019 03:51:13 +0200
From:   Christoph Hellwig <hch@....de>
To:     Atish Patra <atish.patra@....com>
Cc:     linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Christoph Hellwig <hch@....de>,
        Anup Patel <Anup.Patel@....com>,
        Andreas Schwab <schwab@...ux-m68k.org>
Subject: Re: [PATCH v3 3/3] RISC-V: Do not invoke SBI call if cpumask is
 empty

On Wed, Aug 21, 2019 at 05:46:44PM -0700, Atish Patra wrote:
> SBI calls are expensive. If cpumask is empty, there is no need to
> trap via SBI as no remote tlb flushing is required.
> 
> Signed-off-by: Atish Patra <atish.patra@....com>
> ---
>  arch/riscv/mm/tlbflush.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index 9f58b3790baa..2bd3c418d769 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -21,6 +21,9 @@ static void __sbi_tlb_flush_range(struct cpumask *cmask, unsigned long start,
>  		goto issue_sfence;
>  	}
>  
> +	if (cpumask_empty(cmask))
> +		goto done;

I think this can even be done before the get_cpu to optimize it a little
further.