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

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Message-ID: <6142d5abf18206d6ad7db9d89a3385651649b4a6.camel@wdc.com>
Date:   Thu, 22 Aug 2019 05:39:39 +0000
From:   Atish Patra <Atish.Patra@....com>
To:     "hch@....de" <hch@....de>
CC:     "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>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Anup Patel <Anup.Patel@....com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "palmer@...ive.com" <palmer@...ive.com>
Subject: Re: [PATCH v3 1/3] RISC-V: Issue a local tlbflush if possible.

On Thu, 2019-08-22 at 06:27 +0200, hch@....de wrote:
> 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.
> 

Yes. You are correct.

As both cpumask functions here will crash if cpumask is null, we should
probably leave a harmless comment to warn the consequeunce of cpumask
being null.

> 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. 

Done.

>  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?

I had it in v2 patch but removed as it can potentially return garbage
value if cpumask is empty. 

However, we are already checking empty cpumask before the local cpu
check. I will replace cpumask_test_cpu + hweight with
cpumask_any_but().

-- 
Regards,
Atish