linux-kernel - Re: [PATCH v5 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

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Message-ID: <86fbdc8c-0dcb-9b8f-d843-63460d8b1d6a@arm.com>
Date:   Mon, 14 Nov 2022 08:59:11 +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,
        punit.agrawal@...edance.com, 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 v5 2/2] arm64: support batched/deferred tlb shootdown
 during page reclamation



On 10/28/22 13:42, Yicong Yang wrote:
> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> +{
> +	/*
> +	 * TLB batched flush is proved to be beneficial for systems with large
> +	 * number of CPUs, especially system with more than 8 CPUs. TLB shutdown
> +	 * is cheap on small systems which may not need this feature. So use
> +	 * a threshold for enabling this to avoid potential side effects on
> +	 * these platforms.
> +	 */
> +	if (num_online_cpus() <= CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB)
> +		return false;
> +
> +#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
> +	if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
> +		return false;
> +#endif

should_defer_flush() is immediately followed by set_tlb_ubc_flush_pending() which calls
arch_tlbbatch_add_mm(), triggering the actual TLBI flush via __flush_tlb_page_nosync().
It should be okay to check capability with this_cpu_has_cap() as the entire call chain
here is executed on the same cpu. But just wondering if cpus_have_const_cap() would be
simpler, consistent, and also cost effective ?

Regardless, a comment is needed before the #ifdef block explaining why it does not make
sense to defer/batch when __tlbi()/__tlbi_user() implementation will execute 'dsb(ish)'
between two TLBI instructions to workaround the errata.

> +
> +	return true;
> +}
> +
> +static inline void arch_tlbbatch_add_mm(struct arch_tlbflush_unmap_batch *batch,
> +					struct mm_struct *mm,
> +					unsigned long uaddr)
> +{
> +	__flush_tlb_page_nosync(mm, uaddr);
> +}
> +
> +static inline void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)
> +{
> +	dsb(ish);
> +}