linux-kernel - Re: [PATCH v5 3/8] x86/mm/tlb: Open-code on_each_cpu_cond_mask() for tlb_is_not

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Message-ID: <20210218081624.GA337128@infradead.org>
Date:   Thu, 18 Feb 2021 08:16:24 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v5 3/8] x86/mm/tlb: Open-code on_each_cpu_cond_mask() for
 tlb_is_not_lazy()

On Tue, Feb 09, 2021 at 02:16:48PM -0800, Nadav Amit wrote:
> +		/*
> +		 * Although we could have used on_each_cpu_cond_mask(),
> +		 * open-coding it has performance advantages, as it eliminates
> +		 * the need for indirect calls or retpolines. In addition, it
> +		 * allows to use a designated cpumask for evaluating the
> +		 * condition, instead of allocating one.
> +		 *
> +		 * This code works under the assumption that there are no nested
> +		 * TLB flushes, an assumption that is already made in
> +		 * flush_tlb_mm_range().
> +		 *
> +		 * cond_cpumask is logically a stack-local variable, but it is
> +		 * more efficient to have it off the stack and not to allocate
> +		 * it on demand. Preemption is disabled and this code is
> +		 * non-reentrant.
> +		 */
> +		struct cpumask *cond_cpumask = this_cpu_ptr(&flush_tlb_mask);
> +		int cpu;
> +
> +		cpumask_clear(cond_cpumask);
> +
> +		for_each_cpu(cpu, cpumask) {
> +			if (tlb_is_not_lazy(cpu))
> +				__cpumask_set_cpu(cpu, cond_cpumask);
> +		}
> +		smp_call_function_many(cond_cpumask, flush_tlb_func, (void *)info, 1);

No need for the cast here, which would also avoid the pointlessly
overly long line.