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

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Message-ID: <20230703135734.GK4253@hirez.programming.kicks-ass.net>
Date:   Mon, 3 Jul 2023 15:57:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yair Podemsky <ypodemsk@...hat.com>
Cc:     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, 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 Tue, Jun 20, 2023 at 05:46:18PM +0300, Yair Podemsky wrote:
> @@ -191,7 +191,13 @@ static void tlb_remove_table_smp_sync(void *arg)
>  	/* Simply deliver the interrupt */
>  }
>  
> -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);

Aside from what Dave said about the REMOVE_TABLE_IPI_MASK thing, this
isn't right.

on_each_cpu_mask() includes the current cpu, while smp_call_function()
explicitly does not.

Yes, they all end up in smp_call_function_many_cond(), but the
on_each_cpu*() family will have SCF_RUN_LOCAL set, while the
smp_call_function*() family will not.