linux-kernel - Re: [v2 PATCH] RISC-V: Optimize tlb flush path.

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Message-ID: <20190820030641.GA24946@infradead.org>
Date:   Mon, 19 Aug 2019 20:06:41 -0700
From:   "hch@...radead.org" <hch@...radead.org>
To:     Atish Patra <atish.patra@....com>
Cc:     linux-kernel@...r.kernel.org, Albert Ou <aou@...s.berkeley.edu>,
        Allison Randal <allison@...utok.net>,
        Anup Patel <anup@...infault.org>,
        linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Andreas Schwab <schwab@...ux-m68k.org>,
        "hch@...radead.org" <hch@...radead.org>
Subject: Re: [v2 PATCH] RISC-V: Optimize tlb flush path.

On Mon, Aug 19, 2019 at 05:47:35PM -0700, Atish Patra wrote:
> In RISC-V, tlb flush happens via SBI which is expensive.
> If the target cpumask contains a local hartid, some cost
> can be saved by issuing a local tlb flush as we do that
> in OpenSBI anyways. There is also no need of SBI call if
> cpumask is empty.
> 
> Do a local flush first if current cpu is present in cpumask.
> Invoke SBI call only if target cpumask contains any cpus
> other than local cpu.

Btw, you can use up your 70-ish chars per line for commit logs..

> +	if (cpumask_test_cpu(cpuid, cmask)) {
> +		/* Save trap cost by issuing a local tlb flush here */
> +		if ((start == 0 && size == -1) || (size > PAGE_SIZE))
> +			local_flush_tlb_all();
> +		else if (size == PAGE_SIZE)
> +			local_flush_tlb_page(start);
> +	}

This looks a little odd to m and assumes we never pass a size smaller
than PAGE_SIZE.  Whule that is probably true, why not something like:

	if (size < PAGE_SIZE && size != -1)
		local_flush_tlb_page(start);
	else
		local_flush_tlb_all();

?