linux-kernel - Re: [PATCH v2 3/5] lazy tlb: shoot lazies, a non-refcounting lazy tlb option

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Message-ID: <de068125-7d95-b3f0-2de9-71923e3c3651@infradead.org>
Date:   Sun, 13 Dec 2020 23:04:55 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>, linux-kernel@...r.kernel.org
Cc:     linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, Anton Blanchard <anton@...abs.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2 3/5] lazy tlb: shoot lazies, a non-refcounting lazy tlb
 option

On 12/13/20 10:53 PM, Nicholas Piggin wrote:
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 84faaba66364..e69c974369cc 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -443,9 +443,22 @@ config MMU_LAZY_TLB
>  config MMU_LAZY_TLB_REFCOUNT
>  	def_bool y
>  	depends on MMU_LAZY_TLB
> +	depends on !MMU_LAZY_TLB_SHOOTDOWN
>  	help
> -	  This must be enabled if MMU_LAZY_TLB is enabled until the next
> -	  patch.
> +	  This refcounts the mm that is used as the lazy TLB mm when switching
> +	  switching to a kernel thread.

duplicate "switching".

> +
> +config MMU_LAZY_TLB_SHOOTDOWN
> +	bool
> +	depends on MMU_LAZY_TLB
> +	help
> +	  Instead of refcounting the "lazy tlb" mm struct, which can cause
> +	  contention with multi-threaded apps on large multiprocessor systems,
> +	  this option causes __mmdrop to IPI all CPUs in the mm_cpumask and
> +	  switch to init_mm if they were using the to-be-freed mm as the lazy
> +	  tlb. To implement this, architectures must use _lazy_tlb variants of
> +	  mm refcounting, and mm_cpumask must include at least all possible
> +	  CPUs in which mm might be lazy.
>  
>  config ARCH_HAVE_NMI_SAFE_CMPXCHG
>  	bool


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