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Date:   Thu, 25 Apr 2019 13:17:37 -0700 (PDT)
Subject:     Re: [1/3] x86: Update DEBUG_TLBFLUSH options description.
In-Reply-To: <20190410224449.10877-2-atish.patra@wdc.com>
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From:   Palmer Dabbelt <palmer@sifive.com>
To:     atish.patra@wdc.com
Message-ID: <mhng-4767a1c0-439a-4092-ba98-ec388d714a10@palmer-si-x1e>
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On Wed, 10 Apr 2019 15:44:47 PDT (-0700), atish.patra@wdc.com wrote:
> CONFIG_DEBUG_TLBFLUSH was added in 'commit 3df3212f9722 ("x86/tlb: add
> tlb_flushall_shift knob into debugfs")' to support tlb_flushall_shift
> knob. The knob was removed in 'commit e9f4e0a9fe27 ("x86/mm: Rip out
> complicated, out-of-date, buggy TLB flushing")'.  However, the debug
> option was never removed from Kconfig. It was reused in commit
> '9824cf9753ec ("mm: vmstats: tlb flush counters")' but the commit text was
> never updated accordingly.
>
> Update the Kconfig option description as per its current usage.
>
> Signed-off-by: Atish Patra <atish.patra@wdc.com>
> ---
>  arch/x86/Kconfig.debug | 15 +++------------
>  1 file changed, 3 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
> index 15d0fbe27872..c1a48d4ffebb 100644
> --- a/arch/x86/Kconfig.debug
> +++ b/arch/x86/Kconfig.debug
> @@ -125,21 +125,12 @@ config DOUBLEFAULT
>  	  hair.
>
>  config DEBUG_TLBFLUSH
> -	bool "Set upper limit of TLB entries to flush one-by-one"
> +	bool "Save tlb flush statstics to vmstat"
>  	depends on DEBUG_KERNEL
>  	---help---
>
> -	X86-only for now.
> -
> -	This option allows the user to tune the amount of TLB entries the
> -	kernel flushes one-by-one instead of doing a full TLB flush. In
> -	certain situations, the former is cheaper. This is controlled by the
> -	tlb_flushall_shift knob under /sys/kernel/debug/x86. If you set it
> -	to -1, the code flushes the whole TLB unconditionally. Otherwise,
> -	for positive values of it, the kernel will use single TLB entry
> -	invalidating instructions according to the following formula:
> -
> -	flush_entries <= active_tlb_entries / 2^tlb_flushall_shift
> +	Add tlbflush statstics to vmstat. It is really helpful understand tlbflush
> +	performance and behavior. It should be enabled only for debugging purpose.
>
>  	If in doubt, say "N".

Reviewed-by: Palmer Dabbelt <palmer@sifivee.com>

I'm not going to take this via my tree, but I'll look into the next two.