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Message-ID: <680fadba-9104-3914-5175-e207fd3d9246@intel.com>
Date:   Wed, 21 Jun 2023 10:42:16 -0700
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Subject: Re: [PATCH v2 2/2] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to MM CPUs
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To:     Yair Podemsky <ypodemsk@redhat.com>, mtosatti@redhat.com,
        ppandit@redhat.com, david@redhat.com, linux@armlinux.org.uk,
        mpe@ellerman.id.au, npiggin@gmail.com, christophe.leroy@csgroup.eu,
        hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com,
        borntraeger@linux.ibm.com, svens@linux.ibm.com,
        davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com,
        keescook@chromium.org, paulmck@kernel.org, frederic@kernel.org,
        will@kernel.org, peterz@infradead.org, ardb@kernel.org,
        samitolvanen@google.com, juerg.haefliger@canonical.com,
        arnd@arndb.de, rmk+kernel@armlinux.org.uk, geert+renesas@glider.be,
        linus.walleij@linaro.org, akpm@linux-foundation.org,
        sebastian.reichel@collabora.com, rppt@kernel.org,
        aneesh.kumar@linux.ibm.com, x86@kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        sparclinux@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20230620144618.125703-1-ypodemsk@redhat.com>
 <20230620144618.125703-3-ypodemsk@redhat.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20230620144618.125703-3-ypodemsk@redhat.com>
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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?