linux-kernel - Re: [PATCH v6 0/4] riscv: tlb flush improvements

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Date:   Mon, 06 Nov 2023 23:00:03 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     nadav.amit@...il.com
CC:     alexghiti@...osinc.com, Will Deacon <will@...nel.org>,
        aneesh.kumar@...ux.ibm.com, akpm@...ux-foundation.org,
        npiggin@...il.com, peterz@...radead.org, mchitale@...tanamicro.com,
        vincent.chen@...ive.com, Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, samuel@...lland.org,
        prabhakar.csengg@...il.com
Subject:     Re: [PATCH v6 0/4] riscv: tlb flush improvements

On Mon, 30 Oct 2023 07:01:48 PDT (-0700), nadav.amit@...il.com wrote:
>
>> On Oct 30, 2023, at 3:30 PM, Alexandre Ghiti <alexghiti@...osinc.com> wrote:
>> 
>> +			on_each_cpu_mask(cmask,
>> +					 __ipi_flush_tlb_range_asid,
>> +					 &ftd, 1);
>> 
>
> Unrelated, but having fed

Do you mean `ftd`?

If so I'm not all that convinced that's a problem: sure it's 4x`long`, 
so we pass it on the stack instead of registers, but otherwise we'd need 
another `on_each_cpu_mask()` callback to shim stuff through via 
registers.

> on the stack might cause it to be unaligned to
> the cacheline, which in x86 we have seen introduces some overhead.

We have 128-bit stack alignment on RISC-V, so the elements are at least 
aligned.  Since they're just being loaded up as scalars for the next 
function call I'm not sure the alignment is all that exciting here.

> Actually, it is best not to put it on the stack, if possible to reduce
> cache traffic.

Sorry if I'm just missing something, but I'm not convinced this is a 
measurable performance problem.