linux-kernel - Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.

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Message-ID: <20190822042717.GA14076@lst.de>
Date:   Thu, 22 Aug 2019 06:27:17 +0200
From:   "hch@....de" <hch@....de>
To:     Atish Patra <Atish.Patra@....com>
Cc:     "hch@....de" <hch@....de>,
        "paul.walmsley@...ive.com" <paul.walmsley@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "schwab@...ux-m68k.org" <schwab@...ux-m68k.org>,
        Anup Patel <Anup.Patel@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "palmer@...ive.com" <palmer@...ive.com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>
Subject: Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.

On Thu, Aug 22, 2019 at 04:01:24AM +0000, Atish Patra wrote:
> The downside of this is that for every !cmask case in true SMP (more
> common probably) it will execute 2 extra cpumask instructions. As
> tlbflush path is in performance critical path, I think we should favor
> more common case (SMP with more than 1 core).

Actually, looking at both the current mainline code, and the code from my
cleanups tree I don't think remote_sfence_vma / __sbi_tlb_flush_range
can ever be called with  NULL cpumask, as we always have a valid mm.

So this is a bit of a moot point, and we can drop andling that case
entirely.  With that we can also use a simple if / else for the local
cpu only vs remote case.  Btw, what was the reason you didn't like
using cpumask_any_but like x86, which should be more efficient than
cpumask_test_cpu + hweigt?