linux-kernel - Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp

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Date:   Wed, 21 Jun 2023 10:42:16 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Yair Podemsky <ypodemsk@...hat.com>, mtosatti@...hat.com,
        ppandit@...hat.com, david@...hat.com, linux@...linux.org.uk,
        mpe@...erman.id.au, npiggin@...il.com, christophe.leroy@...roup.eu,
        hca@...ux.ibm.com, gor@...ux.ibm.com, agordeev@...ux.ibm.com,
        borntraeger@...ux.ibm.com, svens@...ux.ibm.com,
        davem@...emloft.net, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        keescook@...omium.org, paulmck@...nel.org, frederic@...nel.org,
        will@...nel.org, peterz@...radead.org, ardb@...nel.org,
        samitolvanen@...gle.com, juerg.haefliger@...onical.com,
        arnd@...db.de, rmk+kernel@...linux.org.uk, geert+renesas@...der.be,
        linus.walleij@...aro.org, akpm@...ux-foundation.org,
        sebastian.reichel@...labora.com, rppt@...nel.org,
        aneesh.kumar@...ux.ibm.com, x86@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to MM CPUs

On 6/20/23 07:46, Yair Podemsky wrote:
> -void tlb_remove_table_sync_one(void)
> +#ifdef CONFIG_ARCH_HAS_CPUMASK_BITS
> +#define REMOVE_TABLE_IPI_MASK mm_cpumask(mm)
> +#else
> +#define REMOVE_TABLE_IPI_MASK cpu_online_mask
> +#endif /* CONFIG_ARCH_HAS_CPUMASK_BITS */
> +
> +void tlb_remove_table_sync_one(struct mm_struct *mm)
>  {
>  	/*
>  	 * This isn't an RCU grace period and hence the page-tables cannot be
> @@ -200,7 +206,8 @@ void tlb_remove_table_sync_one(void)
>  	 * It is however sufficient for software page-table walkers that rely on
>  	 * IRQ disabling.
>  	 */
> -	smp_call_function(tlb_remove_table_smp_sync, NULL, 1);
> +	on_each_cpu_mask(REMOVE_TABLE_IPI_MASK, tlb_remove_table_smp_sync,
> +			NULL, true);
>  }

That "REMOVE_TABLE_IPI_MASK" thing is pretty confusing.  It *looks* like
a constant.  It does *NOT* look at all like it consumes 'mm'.  Worst
case, just create a local variable:

	if (IS_ENABLED(CONFIG_ARCH_HAS_CPUMASK_BITS))
		ipi_mask = mm_cpumask(mm);
	else
		ipi_mask = cpu_online_mask;

	on_each_cpu_mask(ipi_mask, ...);

That's a billion times more clear and it'll compile down to the same thing.

I do think the CONFIG_ARCH_HAS_CPUMASK_BITS naming is also pretty
confusing, but I don't have any better suggestions.  Maybe something
with "MM_CPUMASK" in it?