linux-kernel - Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier

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Date:   Fri, 29 Jun 2018 10:05:15 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Rik van Riel <riel@...riel.com>, linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, luto@...nel.org, mingo@...nel.org,
        kernel-team@...com, tglx@...utronix.de, efault@....de,
        songliubraving@...com, hpa@...or.com
Subject: Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier

On 06/29/2018 07:29 AM, Rik van Riel wrote:
> +		/*
> +		 * If the CPU is not in lazy TLB mode, we are just switching
> +		 * from one thread in a process to another thread in the same
> +		 * process. No TLB flush required.
> +		 */
> +		if (!was_lazy)
> +			return;
> +
> +		/*
> +		 * Read the tlb_gen to check whether a flush is needed.
> +		 * If the TLB is up to date, just use it.
> +		 * The barrier synchronizes with the tlb_gen increment in
> +		 * the TLB shootdown code.
> +		 */
> +		smp_mb();
> +		next_tlb_gen = atomic64_read(&next->context.tlb_gen);
> +		if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) ==
> +				next_tlb_gen)
> +			return;

Nit: it might be nice to have this hunk in a helper like
cpu_tlbstate_needs_flush(next) to keep this nice and readable.