linux-kernel - Re: [PATCH v4 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

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Message-ID: <168eac93-a6ee-0b2e-12bb-4222eff24561@arm.com>
Date:   Tue, 27 Sep 2022 11:46:38 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Yicong Yang <yangyicong@...wei.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, catalin.marinas@....com, will@...nel.org,
        linux-doc@...r.kernel.org
Cc:     corbet@....net, peterz@...radead.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, darren@...amperecomputing.com,
        yangyicong@...ilicon.com, huzhanyuan@...o.com, lipeifeng@...o.com,
        zhangshiming@...o.com, guojian@...o.com, realmz6@...il.com,
        linux-mips@...r.kernel.org, openrisc@...ts.librecores.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, Barry Song <21cnbao@...il.com>,
        wangkefeng.wang@...wei.com, xhao@...ux.alibaba.com,
        prime.zeng@...ilicon.com, Barry Song <v-songbaohua@...o.com>,
        Nadav Amit <namit@...are.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH v4 2/2] arm64: support batched/deferred tlb shootdown
 during page reclamation

[...]

On 9/21/22 14:13, Yicong Yang wrote:
> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> +{
> +	/* for small systems with small number of CPUs, TLB shootdown is cheap */
> +	if (num_online_cpus() <= 4)

It would be great to have some more inputs from others, whether 4 (which should
to be codified into a macro e.g ARM64_NR_CPU_DEFERRED_TLB, or something similar)
is optimal for an wide range of arm64 platforms.

> +		return false;> +
> +#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
> +	if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
> +		return false;
> +#endif
> +
> +	return true;
> +}
> +

[...]