linux-kernel - Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.

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Message-ID: <20190822014642.GA11922@lst.de>
Date:   Thu, 22 Aug 2019 03:46:42 +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 1/3] RISC-V: Issue a local tlbflush if possible.

On Wed, Aug 21, 2019 at 05:46:42PM -0700, Atish Patra wrote:
> In RISC-V, tlb flush happens via SBI which is expensive. If the local
> cpu is the only cpu in cpumask, there is no need to invoke a SBI call.
> 
> Just do a local flush and return.
> 
> Signed-off-by: Atish Patra <atish.patra@....com>
> ---
>  arch/riscv/mm/tlbflush.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> index df93b26f1b9d..36430ee3bed9 100644
> --- a/arch/riscv/mm/tlbflush.c
> +++ b/arch/riscv/mm/tlbflush.c
> @@ -2,6 +2,7 @@
>  
>  #include <linux/mm.h>
>  #include <linux/smp.h>
> +#include <linux/sched.h>
>  #include <asm/sbi.h>
>  
>  void flush_tlb_all(void)
> @@ -13,9 +14,23 @@ static void __sbi_tlb_flush_range(struct cpumask *cmask, unsigned long start,
>  		unsigned long size)
>  {
>  	struct cpumask hmask;
> +	unsigned int cpuid = get_cpu();
>  
> +	if (!cmask) {
> +		riscv_cpuid_to_hartid_mask(cpu_online_mask, &hmask);
> +		goto issue_sfence;
> +	}
> +
> +	if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) == 1) {
> +		local_flush_tlb_all();
> +		goto done;
> +	}

I think a single core on a SMP kernel is a valid enough use case given
how litte distros still have UP kernels.  So Maybe this shiuld rather be:

	if (!cmask)
		cmask = cpu_online_mask;

	if (cpumask_test_cpu(cpuid, cmask) && cpumask_weight(cmask) == 1) {
		local_flush_tlb_all();
	} else {
	 	riscv_cpuid_to_hartid_mask(cmask, &hmask);
	  	sbi_remote_sfence_vma(hmask.bits, start, size);
	}